Global environmental issues - پایگاه مهندسی بهداشت محیط ایران · 2012. 12. 5. · Global environmental issues This book provides a balanced account of the

Global environmental issues This book provides a balanced account of the global environmental issues which threaten our societyand which we neglect at our peril Analysing both societal and environmental components of theissuesmdashglobal warming ozone depletion acid rain and droughtmdashthe book offers a valuableintegrative approach At a time when the technical level of publications on environmental issues isrising this introductory text expresses sophisticated scientific ideas in a clear non-technical manner

Emphasising the climatological dimension common to all environmental issues GlobalEnvironmental Issues recognises the multi-faceted nature of the issues their common causes andthe possibility of common solutions Assessment of socio-economic cultural and political factorsprovides a balanced introduction to both the dangers and advantages of human interference withthe environment What have we done to deserve our current environmental crisis Can we solve ourcurrent environmental problems or is it too late

This new edition of a best-selling text is completely updated and expanded to include greaterdetail and new material such as a new section on atmospheric modelling A glossary has been addedtogether with a bibliography for further reading at the end of each chapter allowing readers todevelop their interest in specific topics This interdisciplinary text will prove invaluable to studentsin geography environmental studies and other courses in which the environmental approach isemphasised

David DKemp is Professor of Geography at Lakehead University Canada and has over twenty-fiveyears of experience in teaching climatology and environmental studies

To Susan Heather Qais Qarma Beisan Alisdair Colin Mairi Eilidh Euan

Deirdre Neil Ross Andrew Heather Lynn and all of their generation whose

future requires that we find solutions to our current environmental problems

Global environmental issuesA climatological approach

SECOND EDITION

DAVID DKEMP

London and New York

British Library Cataloguing in Publication DataA catalogue record for this book is available fromthe British Library

Library of Congress Cataloguing in Publication DataKemp David D

Global environmental issues a climatologicalapproachDavid DKemp

p cmIncludes bibliographical references and index1 Environmental policy 2 Climatologymdash

Environmental aspects 3 Social ecology4 Atmospheric physics I TitleGE170K46 19943637ndashdc20 93ndash49500

ISBN 0-203-42530-8 Master e-book ISBN

ISBN 0-203-73354-1 (Adobe eReader Format)ISBN 0-415-10309-6 (hbk)ISBN 0-415-10310-X(pbk)

First published 1990 Second edition published 1994by Routledge11 New Fetter Lane London EC4P 4EE

Simultaneously published in the USA and Canadaby Routledge29 West 35th Street New York NY 10001

This edition published in the Taylor amp Francise-Library 2004

Routledge is an International ThomsonPublishing company I P

copy 1990 and 1994 David DKemp

All rights reserved No part of this book may bereprinted or reproduced or utilised in anyform or by any electronic mechanical orother means now known or hereafterinvented including photocopying andrecording or in any information storage orretrieval system without permission inwriting from the publishers

Contents

List of figures viList of tables ixPreface xiAcknowledgements xiiiList of acronyms xv

1 SETTING THE SCENE 1

2 THE ATMOSPHERE 14

3 DROUGHT FAMINE AND DESERTIFICATION 40

4 ACID RAIN 70

5 ATMOSPHERIC TURBIDITY 100

6 THE THREAT TO THE OZONE LAYER 121

7 THE GREENHOUSE EFFECT AND GLOBAL WARMING 144

8 PRESENT PROBLEMS FUTURE PROSPECTS 173

Glossary 186Bibliography 203Index 217

Figures

11 World population growth and significant technological developments 512 Schematic diagram of the earthatmosphere system 613a Schematic diagram of a natural subsystemmdasha lake basin 713b Schematic diagram of a subsystem of anthropogenic originmdasha thermal electric

power plant 721 Spectral distribution of solar and terrestrial radiation 1522 The hydrologic cycle 1723 Energy transfer during the change in state of water 1824 The vertical structure of the atmosphere and its associated temperature profile 2025 The earthrsquos energy budget 2226 The latitudinal imbalance in solar and terrestrial radiation 2327 The circulation of the oceans 2428 The Southern Oscillation as represented by the Tahiti minus Darwin atmospheric

pressure anomaly 1968ndash82 2529 Sea surface temperature (SST) anomalies in the Pacific Ocean December 1982 26210a Simple convective circulation on a uniform non-rotating earth heated at the

equator and cooled at the poles 27210b Three-cell model of the atmospheric circulation on a uniform rotating earth

heated at the equator and cooled at the poles 27211 The index cycle associated with the meandering of the mid-latitude westerlies

in the northern hemisphere 28212 Schematic diagram of the vertical circulation of the atmosphere and the location

of the major jet streams in the northern hemisphere 29213 Global wind and pressure patterns for January and July 31214 Schematic illustration of the components of the coupled

atmospheremdashoceanmdashicemdashland climatic system 3631 Rainfall fluctuations in five regions of Africa 1901ndash87 4232a The deserts and arid lands of the world 4332b Areas at risk from desertification 4333 Variability of precipitation at selected stations in Africa 4434 Sample climatic water budget for a mid-latitude station in North America or

Europe based on the Thornthwaite model 4635 The distribution of drought famine and desertification in Africa 4936 Seasonal changes in the position of the ITCZ in Africa 50

FIGURES vii

37 North-South section across West Africa showing bands of weather associatedwith the ITCZ 51

38 Precipitation regimes in West Africa 5239 August rainfall in the Sahel 1964ndash84 53310 The Great Plains of North America 55311 Graph of temperature and precipitation for Topeka Kansas 55312 The causes and development of desertification 58313 Drought and sunspot cycles in western North America 66314 Variations in the northward penetration of the monsoon rains in the Sahel 1950ndash72 6641 The pH scale showing the pH level of acid rain in comparison to that of other

common substances 7142 Schematic representation of the formation distribution and impact of acid rain 7243 The geography of acid rain in Europe and North America 7444 Sulphur dioxide emissions in selected countries 1970ndash89 7545 The geography of acid rain showing areas with pH below 50 7646 Annual emissions of sulphur dioxide (106 tonnes) in regions of the northern

hemisphere that influence the Arctic 7747 Decrease in lake pH from pre-industrial times to 198085 8048 Aluminium concentration in relation to pH for 20 lakes near Sudbury Ontario 8149 The impact of acid rain on aquatic organisms 82410 The age-class composition of samples of the white sucker populations in lakes

Skidway (pH 50) and Bentshoe (pH 59) 83411 The impact of acid rain on the terrestrial environment 85412 Reduction of sulphur dioxide through emission controls 91413 Technologies and trade-offs in power station emission control 94414 Emissions of SO2 and NO2 in Canada and the United States 1970ndash89 97415 The balance of trade in sulphur dioxide in 1980 9851 A sample of the different approaches used in the classification of atmospheric aerosols 10152 Diagrammatic representation of the sources and types of atmospheric aerosols 10253 A comparison of the size-range of common aerosols with radiation wavelength 10254 Cumulative DVI for the northern hemisphere 10755 Monthly mean global temperature anomalies obtained using microwave sounding

units (MSU) 10956 Mean anomalies of surface air temperature (degC) for the period 2 to 3 years

after volcanic eruptions with volcanic explosivity power approximately equal to theKrakatoa eruption 110

57 A comparison of natural and anthropogenic sources of particulate matter 11158 Emissions of particulate matter from selected nations 1980ndash89 11259 An estimate of the mean vertical profile of the concentration of anthropogenic

aerosol mass in the high Arctic during March and April 113510 The development of nuclear winter (a) the conflict (b) post-conflict fires (c) nuclear

winter (d) the after-effects 115511 The impact of nuclear winter on the natural environment 11761 Schematic representation of the formation of stratospheric ozone 12362 Diagrammatic representation of the sources of natural and anthropogenic

ozone-destroyers 12463 The destruction of ozone by HOx 125

FIGURESviii

64 The destruction of ozone by NOx 12665 The break-down of a chlorofluorocarbon molecule (CFCl3) and its effect on ozone 13166 Changing ozone levels at the South Pole (1964ndash85) 13367 Incidence of melanoma in Scotland 1979ndash89 13868 Schematic representation of the radiation and temperature changes accompanying

the depletion of stratosphere ozone 13971 Measured globally-averaged (ie land and ocean) surface air temperatures for this

century 14472 Schematic representation of the storage and flow of carbon in the earthatmosphere

system 14773 CO2 emissions percentage share by region 14774 Rising levels of atmospheric CO2 at Mauna Loa Hawaii 14875 Net regional release of carbon to the atmosphere as a result of deforestation during

the 1980s (teragrams of carbon) 14976 Projected changes in atmospheric CO2 concentration 15477 Changing greenhouse gas levels comparison of the lsquobusiness-as-usualrsquo scenario

with one involving major emission controls 15578 Change in global surface temperature following a doubling of CO2 15779 Change in global soil moisture levels following a doubling of CO2 158710 Greenhouse gas contributions to global warming 161711 Changing vegetation patterns in Canada following a doubling of CO2 163712 Environmental and economic changes expected in central and eastern Canada

following a doubling of atmospheric CO2 levels 167713 Changing global annual surface temperature 17081 Changing levels of interest in environmental problems 17482 GCMs and climate change 184

Tables

11 Treaties signed at the world environment meetings in Rio de JaneiromdashJune 1992 212 Energy use technological development and the environment 421 Average gaseous composition of dry air in the troposphere 1522 Estimated impacts of different radiative forcing agents in watts per square metre 2323 El Nintildeo events of various intensities by century 1525ndash1987 2524 Some commonly used general circulation models 3731 Wet and dry spells in Africa south of the Sahara 4032 Examples of drought and aridity indices 4133 Action required for the prevention and reversal of desertification 6241 Commitment of selected nations to the reduction of sulphur dioxide emissions 9561 Different forms of ultraviolet radiation 12162 Some common anthropogenically produced ozone destroying chemicals 13071 Sources of greenhouse gas emissions 14572 A condensation of the IPCC Executive Summary on Climate Change 1990 152

The study of global environmental issues is verymuch a growth industry at the present time andthe amount of new material which has appearedsince this book was first published in 1990 is littleshort of phenomenal As the study of the issueshas intensified the technical level of theassociated scientific reports has tended to increasealso To accommodate this the text in the presentvolume has been expanded and updated toinclude greater detail on the topics covered andthe number of tables and figures has beenincreased by almost 70 per cent It remains anintroductory text however multidisciplinary inits content and designed for students ingeography and environmental studiesprogrammes but also appropriate for courses inother disciplines where the environmentalapproach is followed To retain the broadreadership that this implies a glossary has beenadded to this edition providing succinctdefinitions of the broad range of terms used inthe book for those who require additionalinformation beyond that readily available in thetext The inclusion of a brief bibliography forfurther reading at the end of each chapter servesa similar function but also allows readers todevelop their interests in specific topics For thoseinterested and able to pursue the topics to a moreadvanced level the main bibliography includesthe appropriate scientific and academicreferences

The topics covered in the second editionremain the same as in the first but with someimportant changes in emphasis With the end ofthe lsquoCold Warrsquo and the disintegration of the

Preface

Soviet Union nuclear war has ceased to be apressing concern As a result nuclear winter isnow considered to be irrelevant by manyresearchers and has received little attention inrecent years It is therefore no longer consideredin a separate chapter but has been condensedinto the chapter on atmospheric turbiditywhere it logically belongs since the predictedonset of nuclear winter is initiated by a rapidincrease in turbidity

Among the other issues ozone depletion andglobal warming have emerged as the topicseliciting the greatest level of concern in recentyears As a result there is a large volume of newmaterial in these issues and that is reflected inthe changes made in the chapters that deal withthem

Much of the recent work on environmentalissues has involved the use of computerizedatmospheric circulation models In recognitionof this a new section outlining the developmentand characteristics of such models has beenadded to the chapter on the atmosphere Thebulk of the chapter continues to providebackground on the various atmosphericelements involved in the creation andintensification of current environmentalproblems and references a number of well-established introductory climatology textswhich should be consulted by students whorequire to develop renew or broaden theirexperience in atmospheric studies

An encouraging development since thepublication of the first edition of this book hasbeen the (somewhat) slow but steady progress

PREFACExii

from investigation to decision-making Controlor abatement programmes have beenintroduced to deal with the problems of acidrain atmospheric turbidity and ozonedepletion and are at the discussion stage forglobal warming Successful implementation ofthese programmes will require the cooperationof all levels of society and community supportis most likely to come from a public well-informed about the nature and extent of the

problems This book will I hope contributetowards that end by providing a balanced andreadable account of the global environmentalissues which threaten our society and which weneglect at our peril

David DKempThunder Bay

The preparation of the original edition of thisbook involved the work of many peopleAlthough the experience gained then made thingseasier for me this time this second editionmdashlikethe firstmdashrepresents the combined efforts of avariety of individuals who helped me in manyways Tristan Palmer of Routledge was the idealeditor providing appropriate guidance andencouragement as required In the Departmentof Geography at Lakehead Gwen Henry dealtwith the revisions and additions in her usualefficient manner and Cathy Chapin redrew manyof the maps and diagrams for this editionCatherine Fear provided invaluable editorialservices and Jane Mayger guided me through theproduction process Their help is muchappreciated The reviewers who commented onthe first edition and my colleagues and studentswho drew my attention to new material have mythanks also I am again grateful to my wife anddaughters for their patience encouragement andunderstanding while living under the shadow oflsquothe bookrsquo My thanks are also due to thefollowing for allowing me to reproduce copyrightmaterialFigures 28 and 29 From Rasmusson EM and

Hall JM (1983) lsquoEl Nintildeorsquo Weatherwise36166ndash75 Reprinted with permission of theHelen Dwight Reid Educational FoundationPublished by Heldref Publications 1319Eighteenth St NW Washington DC 20036ndash1802 Copyrightcopy 1983

Figure 31 Reprinted with the permission of theRoyal Meteorological Society from Nicholson

SE (1989) lsquoLong-term changes in Africanrainfallrsquo Weather 4447ndash56

Figure 35 Reprinted with permission fromForum Africa 1985 Canadian InternationalDevelopment Agency

Figure 39 This first appeared in the New Scientistmagazine London the weekly review ofscience and technology in Cross M (1985a)lsquoAfricarsquos drought may last many yearsrsquo NewScientist 1059

Figure 314 Reprinted with permission fromBryson RA and Murray TJ (1977) Climatesof Hunger Madison University of WisconsinPress

Figures 45 and 414 Reprinted with permissionfrom Park CC (1987) Acid Rain Rhetoricand Reality London Methuen

Figures 46 and 59 Reprinted from Barrie LA(1986) lsquoArctic air pollution An overview ofcurrent knowledgersquo AtmosphericEnvironment 20643ndash63 Copyright 1986with kind permission from Pergamon PressLtd Headington Hill Hall Oxford OX30BW UK

Figure 47 Reprinted with permission fromCharles DF Battarbee RW Reuberg IVanDam H and Smot JP (1990)lsquoPalaeoecological analysis of lake acidificationtrends in North America and Europe usingdiatoms and clirysophytesrsquo in SANortonSELindberg and ALPage (eds) AcidicPrecipitation Volume 4 Soils AquaticProcesses and Lake Acidification New YorkSpringer-Verlag

Figures 48 and 410 Reprinted with permission

Acknowledgements

ACKNOWLEDGEMENTSxiv

from Harvey HH (1989) lsquoEffects of acidicprecipitation on lake ecosystemsrsquo in DCAdriano and AHJohnson (eds) AcidicPrecipitation Volume 2 Biological andEcological Effects New York Springer-Verlag

Figure 413 This first appeared in New Scientistmagazine London the weekly review ofscience and technology in Ridley M (1993)lsquoCleaning up with cheap technologyrsquo NewScientist 13716ndash17

Figure 54 Reprinted with permission fromBradley RS and Jones PD (1992) Climatesince AD 1500 London Routledge

Figure 67 Reprinted with permission fromMackie R Hunter JAA Aitchison TCHole D McLaren K Rankin R BlessingK Evans AT Hutcheon AW Jones DHSoutar DS Watson ACH Cornbleet MAand Smith JF (1992) lsquoCutaneous malignantmelanoma Scotland 1979ndash89rsquo The Lancet339971ndash5 Copyright by The Lancet Ltd(1992)

Figure 71 Reprinted with permission from JonesMDH and Henderson-Sellers A (1990)lsquoHistory of the greenhouse effectrsquo Progress inPhysical Geography 141ndash18 CopyrightEdward Arnold (1990)

Figures 74 and 76 Reprinted with permissionfrom SCOPE 29 Bolin B Boos BR JagarJ and Warrick RA (1986) The GreenhouseEffect Climatic Change and Ecosystems NewYork John Wiley and Sons

Figures 75 and 710 Every effort has been madeto contact the copyright holder of these figuresfrom Mintzer IM (1992) lsquoNational energystrategies and the greenhouse problemrsquo inLRosen and RGlasser (eds) Climate Changeand Energy Policy New York AmericanInstitute of Physics

Figure 77 Reprinted with permission from IPCC(1990) Climate Change The IPCC ScientificAssessment Eds JTHoughton GIJenkinsand JJEphraums Cambridge UniversityPress UK 365 pp

Figure 711 Reprinted with permission fromHengeveld HG (1991) UnderstandingAtmospheric Change SOE Report 91ndash2Ottawa Environment Canada

Figure 82 Reprinted with permission fromHenderson-Sellers A (1991) lsquoGlobal climatechange the difficulties of assessing impactsrsquoAustralian Geographical Studies 29202ndash25

Acronyms

AGGG Advisory Group on Greenhouse GasesAOSIS Alliance of Small Islands StatesCEGB Central Electricity Generating BoardCFC ChlorofluorocarbonCIAP Climatic Impact Assessment ProgramcT Continental tropical (air)DMS Dimethyl sulphideDVI Dust veil indexEASOE European Arctic Stratospheric Ozone ExperimentENSO El NintildeoSouthern OscillationENUWAR Environmental consequences of nuclear warEPA Environmental Protection Agency (USA)FBC Fluidized bed combustionFGD Flue gas desulphurizationGVI Glaciological volcanic indexHCFC HydrochlorofluorocarbonICSU International Council of Scientific UnionsIIASA International Institute for Applied Systems AnalysisINCO International Nickle CompanyIPCC Intergovernmental Panel on Climate ChangeITCZ Intertropical Convergence ZoneLIMB Lime injection multi-stage burningLRTAP Long range transportation of atmospheric pollutionMSA Methane sulphonic acidmT Maritime tropical (air)NAWAPA North American Water and Power AllianceNCGCC National Coordinating Group on Climate Change (China)NGO Nongovernmental organizationOECD Organization for Economic Cooperation and DevelopmentOPEC Organization of Petroleum Exporting Countriesppbv parts per billion by volumeppmv parts per million by volumepptv parts per trillion by volumeSCEP Study of Critical Environmental ProblemsSCOPE Scientific Committee on Problems of the EnvironmentSCR Selective catalytic reductionSMD Soil moisture deficit

ACRONYMSxvi

SMIC Study of Manrsquos Impact on ClimateSST Sea-surface temperatureSST Supersonic transport (aircraft)TTAPS Turco Toon Ackerman Pollack Sagan (nuclear winter)UNCED United Nations Conference on Environment and DevelopmentUNCOD United Nations Conference on DesertificationUNECE United Nations Economic Commission for EuropeUNEP United Nations Environment ProgramUSAID United States Agency for International DevelopmentUV-A Ultraviolet AUV-B Ultraviolet BUV-C Ultraviolet CVEI Volcanic explosivity indexWMO World Meteorological Organization

The nations of the world came together in Riode Janeiro in June 1992 at the United NationsConference on Environment and Development(UNCED)mdashdubbed the Earth Summitmdashto try toreach consensus on the best way to slow downhalt and eventually reverse ongoingenvironmental deterioration The Summitrepresented the culmination of two decades ofdevelopment in the study of environmental issuesinitiated at the United Nations Conference onthe Human Environment held in Stockholm in1972 Stockholm was the first conference to drawworldwide public attention to the immensity ofenvironmental problems and because of that ithas been credited with ushering in the modernera in environmental studies (Haas et al 1992)

The immediate impact of the Stockholmconference was not sustained for long Writingin 1972 the climatologist Wilfrid Bach expressedconcern that public interest in environmentalproblems had peaked and was already waningHis concern appeared justified as theenvironmental movement declined in theremaining years of the decade pushed out of thelimelight in part by growing fears of the impactof the energy crisis Membership inenvironmental organizationsmdashsuch as the SierraClub or the Wilderness Societymdashwhich hadincreased rapidly in the 1960s declined slowlyand by the late 1970s the environment was seenby many as a dead issue (Smith 1992) In the1980s however there was a remarkableresurgence of interest in environmental issuesparticularly those involving the atmosphericenvironment The interest was broad embracingall levels of society and held the attention of the

general public plus a wide spectrum of academicgovernment and public-interest groups

Most of the issues were not entirely new Somesuch as acid rain the enhanced greenhouse effectatmospheric turbidity and ozone depletion hadtheir immediate roots in the environmentalconcerns of the 1960s although the first two hadalready been recognized as potential problemsin the nineteenth century Drought and faminewere problems of even longer standingContrasting with all of these was nuclearwintermdasha product of the Cold Warmdashwhichremained an entirely theoretical problem butpotentially no less deadly because of that

Diverse as these issues were they had anumber of features in common They were forexample global or at least hemispheric inmagnitude large scale compared to the local orregional environmental problems of earlier yearsAll involved human interference in theatmospheric component of the earthatmospheresystem and this was perhaps the most importantelement they shared They reflected societyrsquos ever-increasing ability to disrupt environmentalsystems on a large scale

These issues are an integral part of the newenvironmentalism which has emerged in the early1990s It is characterized by a broad globaloutlook increased politicizationmdashmarkedparticularly by the emergence of the so-calledGreen Parties in Europemdashand a growingenvironmental consciousness which takes theform of waste reduction prudent use of resourcesand the development of environmentally safeproducts (Marcus and Rands 1992) It is also amuch more aggresive environmentalism with

Setting the scene

GLOBAL ENVIRONMENTAL ISSUES2

certain organizationsmdashGreen-peace and EarthFirst for examplemdashusing direct action in additionto debate and discussion to draw attention tothe issues (Smith 1992)

Another characteristic of this newenvironmentalism is a growing appreciation ofthe economic and political components inenvironmental issues particularly as they applyto the problems arising out of the economicdisparity between the rich and poor nationsThe latter need economic development tocombat the poverty famine and disease that areendemic in many Third World countries but do

not have the capacity to deal with theenvironmental pressures which developmentbrings The situation is complicated by theperception among the developing nations thatimposition of the environmental protectionstrategies proposed by the industrial nations isnot only forcing them to pay for something theydid not create but is also likely to retard theirdevelopment

Development issues were included in theStockholm conference in 1972 but they wereclearly of secondary importance at that time wellbehind all of the environmental issues discussed

Table 11 Treaties signed at the world environment meetings in Rio de JaneiromdashJune 1992

Source Compiled from information in Parson et al (1992)

SETTING THE SCENE 3

Fifteen years later the report of the WorldCommission on Environment andDevelopmentmdashcommonly called the BrundtlandCommission after its chairwoman Gro HarlemBrundtlandmdashfirmly combined economy andenvironment through its promotion oflsquosustainable developmentrsquo a concept whichrequired development to be both economicallyand environmentally sound so that the needs ofthe worldrsquos current population could be metwithout jeopardizing those of future generationsThe Brundtland Commission also proposed amajor international conference to deal with suchissues This led directly to the Earth Summit inRio de Janeiro in 1992 and its parallel conferenceof non-governmental organizations (NGOs)mdashtheGlobal Forum

The theme of economically andenvironmentally sound development was carriedthrough the Summit to the final Rio Declarationof global principles and to Agenda 21 the majordocument produced by the conference andbasically a blueprint for sustainable developmentinto the twenty-first century That theme alsoappeared in other documents signed at Rioincluding a Framework Convention on ClimateChange brought on by concern over globalwarming a Biodiversity Convention whichcombined the preservation of natural biologicaldiversity with sustainable development ofbiological resources and a Statement of ForestPrinciples aimed at balancing the exploitationand conservation of forests Discussions andsubsequent agreements at the Global Forumincluded an equally wide range of concerns (seeTable 11)

There can be no assurance that these effortswill be successful Even if they are it will be sometimemdashprobably decadesmdashbefore the resultsbecome apparent and success may have beenlimited already by the very nature of the varioustreaties and conventions For example both theRio Declaration and Agenda 21 were the resultof compromise among some 150 nationsAttaining sufficient common ground to make thispossible inevitably weakened the language andcontent of the documents leaving them open to

interpretation and therefore less likely to beeffective (Pearce 1992b) The failure of thedeveloped nations to commit new money to allowthe proposals contained in the documents to goahead is also a major concern (Pearce 1992a)Perhaps this lack of financial commitment is areflection of the recessionary conditions of theearly 1990s but without future injections offunds from the developed nations the necessaryenvironmental strategies will not be implementedand Third World economic growth will continueto be retarded (Miller 1992) Other documentsare little more than statements of concern withno legal backing The forestry agreement forexample is particularly weak largely as a resultof the unwillingness of the developing nations toaccept international monitoring and supervisionof their forests (Pearce 1992a) The end productremains no more than a general statementacknowledging the need to balance theexploitation of the forests with theirconservation Similarly the FrameworkConvention on Climate Change signed only aftermuch conflict among the participants was muchweaker than had been hopedmdashlacking evenspecific emission reduction targets and deadlines(Warrick 1993)

Faced with such obstructions to progressfrom the outset the Rio Summit is unlikely tohave much direct impact in the near futureWhen change does come it is unlikely to comethrough such wide-ranging internationalconferences where rhetoric often exceedscommitment It is more likely to be achievedinitially by way of issue-specific organizationsand the Earth Summit contributed to progressin that area by establishing a number of newinstitutionsmdashthe Sustainable DevelopmentCommission for examplemdashand newinformation networks By bringing politiciansnon-governmental organizations and a widerange of scientists together and publicizingtheir activities by way of more than 8000journalists the Summit also added momentumto the growing concern over globalenvironmental issues wiithout which futureprogress will not be possible

THE IMPACT OF SOCIETY ON THEENVIRONMENT

Societyrsquos ability to cause significant disruptionin the environment is a recent phenomenonstrongly influenced by demography and

technological development Primitive peoples forexample being few in number and operating atlow energy levels with only basic tools did verylittle to alter their environment Thecharacteristically low natural growth rates of the

Table 12 Energy use technological development and the environment

Source Compiled from data in Biswas (1974) Kleinbach and Salvagin (1986)

SETTING THE SCENE 5

hunting and gathering groups who inhabited theearth in prehistoric times ensured thatpopulations remained small This combined withtheir nomadic lifestyle and the absence of anymechanism other than human muscle by whichthey could utilize the energy available to themlimited their impact on the environment In truththey were almost entirely dominated by it Whenit was benign survival was assured When it wasmalevolent survival was threatened Populationtotals changed little for thousands of years butslowly and in only a few areas at first thedominance of the environment began to bechallenged Central to that challenge was thedevelopment of technology which allowed themore efficient use of energy (see Table 12) Itwas the ability to concentrate and then expendlarger and larger amounts of energy that madethe earthrsquos human population uniquely able toalter the environment The ever-growing demandfor energy to maintain that ability is at the rootof many modern environmental problems(Biswas 1974)

The level of human intervention in theenvironment increased only slowly overthousands of years punctuated by significantevents which helped to accelerate the process(see Figure 11) Agriculture replaced huntingand gathering in some areas methods forconverting the energy in wind and falling waterwere discovered and coal became the first of the

fossil fuels to be used in any quantity As late asthe mid-eighteenth century however theenvironmental impact of human activitiesseldom extended beyond the local or regionallevel A global impact only became possible withthe major developments in technology and thepopulation increase which accompanied the so-called Industrial Revolution Since thenmdashwiththe introduction of such devices as the steamengine the electric generator and the internalcombustion enginemdashenergy consumption hasincreased sixfold and world population is nowfive times greater than it was in 1800 The exactrelationship between population growth andtechnology remains a matter of controversy butthere can be no denying that in combinationthese two elements were responsible for theincreasingly rapid environmental change whichbegan in the mid-eighteenth century At presentchange is often equated with deterioration butthen technological advancement promised sucha degree of mastery over the environment that itseemed such problems as famine and diseasewhich had plagued mankind for centurieswould be overcome and the quality of life of theworldrsquos rapidly expanding population would beimproved infinitely That promise was fulfilledto some extent but paradoxically the sametechnology which had solved some of the oldproblems exacerbated others and ultimatelycreated new ones

Figure 11 World population growth and significant technological developments

THE RESPONSE OF THE ENVIRONMENT

TO HUMAN INTERFERENCE

The impact of society on the environmentdepends not only on the nature of society butalso on the nature of the environment Althoughit is common to refer to lsquothersquo environment thereare in fact many environments and thereforemany possible responses to human interferenceIndividually these environments vary in scale andcomplexity but they are intimately linked andin combination constitute the whole earthatmosphere system (see Figure 12) Like allsystems the earthatmosphere system consists ofa series of inter-related components orsubsystems (see Figure 13) ranging in scale frommicroscopic to continental and working togetherfor the benefit of all Human interference hasprogressively disrupted this beneficialrelationship In the past disruption was mainlyat the sub-system levelmdashcontamination of a riverbasin or pollution of an urban environment forexamplemdashbut the integrated nature of thesystem coupled with the growing scale ofinterference caused the impact to extend beyond

individual environments to encompass the entiresystem

In material terms the earthatmosphere is aclosed system since it receives no matter (otherthan the occasional meteorite) from beyond itsboundaries The closed nature of the system is ofconsiderable significance since it means that thetotal amount of matter in the system is fixedExisting resources are finite once they are usedup they cannot be replaced Some elementsmdashforexample water carbon nitrogen sulphurmdashcanbe used more than once because of the existenceof efficient natural recycling processes whichclean repair or reconstitute them Even they areno longer immune to disruption however Globalwarming is in large part a reflection of theinability of the carbon cycle to cope with theadditional carbon dioxide introduced into thesystem by human activities

The recycling processes and all of the othersub-systems are powered by the flow of energythrough the system In energy terms the earthatmosphere system is an open system receivingenergy from the sun and returning it to spaceafter use When the flow is even and the rates ofinput and output of energy are equal the system

Figure 12 Schematic diagram of the earthatmosphere system

SETTING THE SCENE 7

Figure 13a Schematic diagram of a natural subsystemmdasha lake basin

Figure 13b Schematic diagram of a subsystem of anthropogenic originmdasha thermal electric power plant

is said to be in a steady state Because of thecomplexity of the earthatmosphere system thegreat number of pathways available to the energyand the variety of short and long-term storagefacilities that exist it may never actually reachthat condition Major excursions away from asteady state in the past may be represented bysuch events as the Ice Ages but in most cases theresponses to change are less obvious Changes inone element in the system tending to produceinstability are countered by changes in otherelements which attempt to restore balance Thistendency for the components of the environmentto achieve some degree of balance has long beenrecognized by geographers and referred to asenvironmental equilibrium The balance is nevercomplete however Rather it is a dynamicequilibrium which involves a continuing seriesof mutual adjustments among the elements thatmake up the environment The rate nature andextent of the adjustments required will vary withthe amount of disequilibrium introduced into thesystem but in every environment there will beperiods when relative stability can be maintainedwith only minor adjustments This inherentstability of the environment tends to dampen theimpact of changes even as they happen and anydetrimental effects that they produce may gounnoticed At other times the equilibrium is sodisturbed that stability is lost and majorresponses are required to restore the balanceMany environmentalists view the presentenvironmental deterioration as the result ofhuman interference in the system at a level whichhas pushed the stabilizing mechanisms to theirlimits and perhaps beyond

Running contrary to this is the much lesspessimistic point of view expressed by JamesLovelock and his various collaborators in theGaia hypothesis (Lovelock 1972 Lovelock andMargulis 1973) First developed in 1972 andnamed after an ancient Greek earth-goddess thehypothesis views the earth as a single organismin which the individual elements exist togetherin a symbiotic relationship Internal controlmechanisms maintain an appropriate level ofstability Thus it has much in common with the

concept of environmental equilibrium It goesfurther however in presenting the biocentric viewthat the living components of the environmentare capable of working actively together toprovide and retain optimum conditions for theirown survival Animals for example take upoxygen during respiration and return carbondioxide to the atmosphere The process isreversed in plants carbon dioxide being absorbedand oxygen released Thus the waste productfrom each group becomes a resource for the otherWorking together over hundreds of thousandsof years these living organisms have combinedto maintain oxygen and carbon dioxide at levelscapable of supporting their particular forms oflife This is one of the more controversial aspectsof Gaia flying in the face of majority scientificopinionmdashwhich since at least the time ofDarwin has seen life responding toenvironmental conditions rather than initiatingthemmdashand inviting some interesting and possiblydangerous corollaries It would seem to followfor example that existing environmentalproblems which threaten lifemdashozone depletionfor examplemdashare transitory and will eventuallybe brought under control again by theenvironment itself To accept that would be toaccept the efficacy of regulatory systems whichare as yet unproven particularly in their abilityto deal with large scale human interference Suchacceptance would be irresponsible and has beenreferred to by Schneider (1986) as lsquoenvironmentalbrinksmanshiprsquo

Lovelock himself has allowed that Gaiarsquosregulatory mechanisms may well have beenweakened by human activity (Lovelock andEpton 1975 Lovelock 1986) Systems cope withchange most effectively when they have a numberof options by which they can take appropriateaction and this was considered one of the mainstrengths of Gaia It is possible however thatthe earthrsquos growing population has created somuch stress in the environment that the optionsare much reduced and the regulatorymechanisms may no longer be able to nullify thethreats to balance in the system This reductionin the variety of responses available to Gaia may

SETTING THE SCENE 9

even have cumulative effects which couldthreaten the survival of human species Althoughthe idea of the earth as a living organism is abasic concept in Gaia the hypothesis is notanthropocentric Humans are simply one of themany forms of life in the biosphere and whateverhappens life will continue to exist but it maynot be human life For example Gaia includesmechanisms capable of bringing about theextinction of those organisms which adverselyaffect the system (Lovelock 1986) Since thehuman species is currently the source of mostenvironmental deterioration the partial orcomplete removal of mankind might be Gaiarsquosnatural answer to the earthrsquos current problems

The need for coexistence between people andthe other elements of the environment now beingadvocated by Lovelock as a result of his researchinto Gaia has been accepted by severalgenerations of geographers but historicallysociety has tended to view itself as being inconflict with the environment Many primitivegroups may have enjoyed a considerable rapportwith their environment but for the most partthe relationship was an antagonistic one(Murphey 1973 Smith 1992) The environmentwas viewed as hostile and successful growth ordevelopment depended upon fighting it andwinning In the beginning human inputs wererelatively minor and the results of victory couldeasily be accommodated in the system Graduallythrough technological advancement andsometimes sheer weight of numbers societybecame increasingly able to challenge itsenvironment and eventually to dominate itNatural vegetation was replaced by cultivatedcrops rivers were dammed or diverted naturalresources were dug from the earth in suchquantity that people began to rivalgeomorphological processes as agents oflandscape change and to meet the need forshelter nature was replaced with the builtenvironment created by urbanization

The environment can no longer be consideredpredominantly natural in most of Europe andNorth America Technological innovations sincethe mid-eighteenth century have ensured that In

the less-developed nations of Asia Africa andSouth Americamdashwhere the impact of technologyis less strongmdashpressure from large and rapidlygrowing populations has placed an obviouslyhuman imprint on the landscape Extensive as itis however dominance is far from completeEven after 200 years of technologicaldevelopment and the exponential growth ofpopulation in recent years some geographicalregions continue to resist human domination Thefrozen reaches of the Arctic and Antarctic arenot unaffected by human activity but they arelargely unpopulated while habitation in theworldrsquos hot deserts is in all senses marginal

HUMAN ACTIVITIES AND THEATMOSPHERIC ENVIRONMENT

Certain elements in the environment remainuntamed uncontrollable and imperfectlyunderstood Nowhere is this more evident thanin the realm of weather and climate Neithernineteenth nor twentieth century technologycould prevent cyclones from devastating theshores of the Bay of Bengal or hurricanes fromlaying waste the Caribbean The Saheliandrought spread uncontrollably even as the firstastronauts were landing on the moon Thedeveloped nations where societyrsquos dominance ofthe environment is furthest advanced continueto suffer the depredation of tornadoes andblizzards as well as the effects of less spectacularweather events such as frost drought heatwavesand electrical storms which even today aredifficult to forecast No one is immune

It is scarcely surprising therefore that weatherand climate are universal topics of interest at alllevels of society In most cases concern centreson the impact of short-lived local weather eventson individuals and their activities It is very muchone-sided normally ignoring the potential thatmankind has to alter its atmosphericenvironment There is a growing appreciationhowever that the nature and extent of theclimatological component in many current globalissues is strongly influenced by humaninterference in the earthatmosphere system The

evolution of this awareness has been traced byWilliam Kellogg (1987) who points out that aslong ago as the late nineteenth century the firsttentative links between fossil fuels atmosphericcarbon dioxide and world climate had beenexplored The results failed to elicit much interestin the scientific community however andremained generally unknown to the public atlarge Such a situation prevailed until the mid-1960s From that time on the cumulative effectsof a number of high-level national andinternational conferences culminating in theStudy of Critical Environmental Problems (SCEP)in 1970 produced a growing awareness of globalenvironmental issues The impact of humanactivities on regional and global climates wasconsidered in the SCEP and when it became clearthat the issue was of sufficient magnitude towarrant further investigation a follow-upconference was convened It focused oninadvertent climate modification and in 1971produced a report entitled The Study of ManrsquosImpact on Climate (SMIC) The report wasrecognized as an authoritative assessment of allaspects of human-induced climatic change andit might even be considered as the finalcontribution to the lsquocritical massrsquo necessary toinitiate the numerous and increasingly detailedstudies which characterized the next two decadesThe pace quickened in the late 1980s withinternational conferences on various aspects ofclimate and the changing atmosphere being heldin Montreal Toronto Hamburg London andGeneva (see Chapter 8) A significant stepforward was the creation in 1988 of theIntergovernmental Panel on Climate Change Setup to evaluate global climate trendsmdashparticularlyglobal warmingmdashit provided major input for thediscussions leading up to the signing of theconvention on climate change at the UnitedNations Conference on Environment andDevelopment in Rio de Janeiro in 1992

One of the results of all that activity was thepositive identification of human interference asa common element in many of the majorproblems of the atmospheric environment Theinformation gathered during the studies is quite

variable in content and approach It has beenpresented in highly technical scientific reportsas well as in simple basic articles prepared forpopular consumption The latter are particularlyimportant as a means of disseminatinginformation to the wider audience which pastexperience suggests must be educated beforeprogress can be made in dealing withenvironmental problems Because of the time andspace constraints and the marketing requirementsof modern journalism however the issues areoften treated with much less intellectual rigourthan they deserve In addition the topics are oftenrepresented as being new or modern when infact most have existed in the past Drought acidprecipitation and the greenhouse effect all resultfrom natural processes and were part of theearthatmosphere system even before the humanspecies came on the scene Their current statushowever is largely the result of humanintervention particularly over the last 200 yearsand it is the growing appreciation of the impactof this intervention that has given the issues theirpresent high profile One topic that can beclassified as new is nuclear winter Unlike theothers which exist at present and havedeveloped gradually as the accumulated resultsof a variety of relatively minor inputs nuclearwinter remains in the future with an impact thatcan only become reality following the majorcatastrophic inputs of nuclear war For many inthe mid-1980s it was seen as the ultimateintervention the ultimate blow to environmentalequilibrium Despite this nuclear winter is nolonger considered a major environmental issueby most observers Events such as the ending ofthe Cold War the break-up of the USSR and thesigning of a variety of arms control agreementshave reduced the potential for inter-continentalnuclear war and as a result interest in nuclearwinter has declined almost to zero

Present concerns may be seen to some extentas the most recent elements in a continuum Inthe 1960s and early 1970s the mainenvironmental issues were those associated withpollution in its various forms Pollution is thecontamination of the components of the earth

SETTING THE SCENE 11

atmosphere system to such an extent that normalenvironmental processes are adversely affectedContamination is not always serious howeverThe environment has a considerable capacity forridding itself of pollutants and problems onlybegin to arise when the input of contaminantsexceeds the ability of the environment to dealwith them In modern times this situation hasbecome common as a result of the tremendousamount of waste generated by human activitiesand deposited in the environment In the 1960sand 1970s the most pressing popular concernswere often local in origin dealing with suchproblems as urban air pollution or reduced waterquality in rivers and lakes although considerationwas given to the environment as a whole in theacademic and scientific community (eg Detwyler1971) Along with increased concern there wasalso increased understanding of the environmentbrought about by the development of educationalprogrammes at all levels from elementary schoolto university and by judicious use of the mediaby environmental groups such as the Sierra ClubFriends of the Earth Pollution Probe andGreenpeace The high level at which publicinterest in environmental affairs was sustainedduring the years when improvement wasmarginal is in no small measure attributable tothese groups

Public pressure forced the political andindustrial establishment to reassess its positionon environmental quality Oil companies theforest products industry and even automobilemanufacturers began to express concern forpollution abatement and the conservation ofresources Similar topics began to appear onpolitical platforms particularly in NorthAmerica and although this increased interest wasregarded with suspicion and viewed as a publicrelations exercise in some quarters legislationwas gradually introduced to alleviate some of theproblems By the early 1970s some degree ofcontrol seemed to be emerging While this mayhave helped to reduce anxiety over environmentalconcerns progress was slow and some observersattribute the decline in interest in all thingsenvironmental at about this time to

disenchantment rather than recognition that theproblems were being solved (Bach 1972)

Whatever the reason the level of concern hadpeaked by then and when the oil crisis struck in1973 energy quickly replaced environmentalissues in the minds of the politicians academicsand the public at large In the first half of the1990s the energy situation is perceived as lesscritical the dire predictions of the economistsand energy futurists have not come to pass andthe waning of interest in energy topics has beenmatched by a resurgence of environmentalconcern particularly for problems involving theatmosphere The new issues are global in scaleand at first sight may appear different fromthose of earlier years in fact they share the sameroots Current topics such as acid rain and globalwarming are linked to the sulphurous urbansmogs of two or three decades ago by societyrsquoscontinuing dependence on fossil fuels to meet itsseemingly insatiable demand for more energyPopulation pressures on land of limited carryingcapacity contribute to famine and desertificationmuch as they did in the past The depletion ofthe ozone layer associated with modern chemicaland industrial technology might be consideredas only the most recent result of mankindrsquoscontinual and seemingly inherent desire toimprove its lotmdashall the while acting in ignoranceof the environmental consequences

Many of the problems currently of concernhave causes which can be traced to ignorance ofthe workings of the atmosphere This isparticularly true when the impact of air pollutionon climate is considered Almost all humanactivities produce waste products and some ofthese are introduced into the atmosphere Thispresented no great problem when populationswere small and technological levels were lowfor the atmosphere includes mechanisms designedto keep such emissions in check For every processadding material to the atmosphere there isanother which works to remove or reduce theexcess either by neutralizing it or by returning itto the earthrsquos surface Gases for example maybe absorbed by vegetation neutralized byoxidation or dissolved in water and precipitated

Particulate matter falls out of the atmosphere asa result of gravity or is washed out byprecipitation These processes have removedextraneous gases and aerosols from theatmosphere usually quite effectively for millionsof years

Ongoing physical and biological activitiesmdashsuch as volcanic eruptions soil erosion and thecombustion or decay of vegetable mattermdashensurethat the cleansing process is never complete andthat in itself is a natural part of the system Thereare indications for example that a minimumlevel of extraneous material is essential for theworking of such atmospheric processes ascondensation and precipitation Thus acompletely clean atmosphere may not bedesirable (Barry and Chorley 1992) Desirableor not it is unlikely to be achieved given thepresent rates of gaseous and particulateemissions

In the past the main pollutants were naturalin origin and sources such as the oceansvolcanoes plants and decaying organic materialcontinue to provide about 90 per cent of the totalglobal aerosol content (Bach 1979) Events suchas the eruption of Mount Pinatubo indicate thecontinued capability of nature to provide massivevolumes of pollutants but anthropogenic sourcesare now paramount in many areas Humanactivities provide pollutants in such amounts andwith such continuity that the atmosphericcleansing processes have been all butoverwhelmed and a full recovery may not bepossible even after large-scale attempts to reduceemission levels

Air pollution was one of the elements whichelicited a high level of concern during the heydayof the environmental movement in the late 1960sand early 1970s It was mainly an urban problemat that time most common in large cities whichhad high seasonal heating requirements wereheavily industrialized had large volumes ofvehicular traffic or experienced combinations ofall three Even then however existing airpollution control ordinances were beginning tohave an effect on the problem In Pittsburgh theintroduction of smokeless fuel and the

establishment of emission controls on the ironand steel industry brought a steady reduction inair pollution between 1945 and 1965 (Thackrey1971) Similar improvements were achieved inLondon England where sunshine levels in thecity centre increased significantly following theClean Air Act of 1956 (Jenkins 1969) Thereplacement of coal by natural gas as the mainheating fuel on the Canadian Prairies in the1950s and 1960s allowed urban sunshine totalsto increase there also (Catchpole and Milton1976) Success was achieved mainly by reducingthe atmospheric aerosol content Little was doneto reduce the gaseous component of pollutionexcept in California where in 1952 gaseousemissions from the statersquos millions of cars werescientifically proven to be the main source ofphotochemical smog (Leighton 1966) Preventionof pollution was far from complete but theobvious improvements in visibility and sunshinetotals coupled with the publicity whichaccompanied the introduction of new air qualityand emission controls in the 1970s caused thelevel of concern over urban air pollution todecline markedly by the end of the decade

The relationship between pollution andweather or climate is a complex one Sometimesclimatic conditions will influence the nature andextent of a pollution episode while at other timesthe linkages are reversed allowing the pollutantsto instigate or magnify variations in climate Theproblem of acid rain illustrates quite well theimpact of atmospheric processes on the operationand distribution of a particular group ofpollutants whereas the issues of increasedatmospheric turbidity and the depletion of theozone layer illustrate the other relationship inwhich pollutants cause sufficient change in theatmosphere to initiate climatic change

The full complexity of the earthatmospheresystem is only now beginning to be appreciatedbut in recent years knowledge of the impact ofhuman social and technological development onthe atmospheric environment has grown quitedramatically Government sponsored studies onsuch topics as the greenhouse effect and acid rainalong with reports by respected scientists and

academics on nuclear winter and the depletionof the ozone layer have become availableAlthough often quite technical they havecontained material of sufficient general interestthat it could be abstracted and disseminatedwidely by the media Other issues have beendeveloped at the popular level from the outsetThe interest in African drought for example wasto a large extent an emotional response totelevision coverage of events in Ethiopia and thesuccess of the Live Aid concerts of 1985 althoughexcellent academic studies of the problem havebeen carried out (Bryson and Murray 1977Glantz 1977)

As more information becomes available onthese global environmental issues it is clear thatthe present problems have existed undetected forsome time It is also clear that the activities whichproduced them were entered into with the bestof intentionsmdashto improve the quality of humanlifemdashand society might even be seen as sufferingfrom its own success That of course doesnothing to reduce the seriousness of the problemsbut it indicates the need for extreme caution wheninitiating schemes which promise majoradvantages The linkages in the earthatmosphere system are such that even local orregional changes can be amplified until theirimpact is felt system-wide and in the modernworld schemes which might conceivably alter theenvironment whether immediately or ultimatelycannot be entered into lightly Unfortunately evenin the present era of high technology predictingthe eventual reaction of the environment to aspecific input is seldom possible and changesalready initiated may well be expanding andintensifying undetected to provide the makingsof some future problem

The global environmental topics to beconsidered in the following chapters are thosewhich currently enjoy a high profile They includeglobal warming and ozone depletionmdashpresentlythe leading recipients of research fundingmdash

together with acid rain and atmospheric turbiditywhich now receive less attention than they did5ndash10 years ago but remain significantenvironmental issues In keeping with its recentlyreduced status nuclear winter is incorporated inthe section on atmospheric turbidity where it isconsidered as an example of the consequencesof macro-scale air pollution In contrast to thesemodern high-tech problems there is drought aproblem which has plagued mankind forcenturies causing millions of deaths and largescale environmental degradation yet remainsessentially unsolved It deserves consideration inits own right but as a well-established recurringproblem it also provides a useful contrast withthose of more recent origin

Since society experiences the impact of theseelements or induces change in them by way ofthe atmosphere an understanding of theworkings of that medium is important alsoAlthough many processes are involved in anydiscussion of global environmental issuesquestions of the composition of the atmosphereits general circulation and its role in the globalenergy budget appear with some regularity Thesetopics will therefore be considered as a necessaryintroduction to the issues

SUGGESTIONS FOR FURTHER READING

Kumar R and Murck B (1992) On CommonGround Managing HumanmdashPlanetRelationships Toronto John Wiley and Sons

Mungall C and McLaren DJ (1990) Planetunder Stress Toronto Oxford UniversityPress

Nisbet EG (1991) Leaving Eden To Protectand Manage the Earth CambridgeCambridge University Press

Starke L (1990) Signs of Hope Workingtowards our Common Future OxfordOxford University Press

The atmosphere is a thick blanket of gasescontaining suspended liquid and solid particleswhich completely envelops the earth andtogether with the earth forms an integratedenvironmental system As part of this system itperforms several functions which have allowedmankind to survive and develop almost anywhereon the earthrsquos surface First it provides andmaintains the supply of oxygen required for lifeitself Second it controls the earthrsquos energybudget through such elements as the ozone layerand the greenhouse effect andmdashby means of itsinternal circulationmdashdistributes heat andmoisture across the earthrsquos surface Third it hasthe capacity to dispose of waste material orpollutants generated by natural or humanactivity Society has interfered with all of theseelements and through ignorance of themechanisms involved or lack of concern for theconsequences of its action has created orintensified problems which are now causingconcern on a global scale

THE ATMOSPHERIC GASES

The constituents of the atmosphere arecollectively referred to as air although air itselfis not a specific gaseous element rather it is amixture of individual gases each of which retainsits own particular properties Although traces ofatmospheric gases have been detected well outinto space 99 per cent of the mass of theatmosphere lies within 30 km of the earthrsquossurface and 50 per cent is concentrated in thelowest 5 km Most of the worldrsquos weather

develops in these lower layers but certainelements in the upper reaches of the atmosphereare also involved and some appear as importantcomponents in the global environmental issuesto be examined here

Oxygen and nitrogen

Ignoring for the moment the liquids and solidsalways present the gaseous mixture which makesup the atmosphere has a remarkably uniformcomposition in the troposphere where most ofthe air is located Two gases oxygen andnitrogen account for 99 per cent of the total byvolume (see Table 21) Oxygen (21 per cent byvolume) participates readily in chemicalreactions and provides one of the necessities oflife It is also capable of absorbing solar radiationIn contrast nitrogen (78 per cent by volume) isbasically inert seldom becoming directly involvedin atmospheric chemical or biological processesexcept under extraordinary circumstancesDuring thunderstorms for example theenormous energy flow in a lightning stroke maycause nitrogen to combine with oxygen toproduce oxides of nitrogen On a less spectacularbut ultimately more important level certain soilbacteriamdashsuch as Clostridium and Azobactermdashalong with those found in the root nodules ofleguminous plants are capable of fixing theatmospheric nitrogen essential for the creationof the complex nitrogen compounds found in allforms of life on earth (Steila 1976)

Efficient recycling processes maintain thevolume of both gases and turbulent mixing

The atmosphere

THE ATMOSPHERE 15

ensures that they are evenly distributed There isno evidence that the relative levels of oxygen andnitrogen are changing significantly althoughthere have been measurable changes in theproportions of other gases in the atmosphereChanges in the nature of oxygen however areinvolved in the depletion of the ozone layer nowrecognised as one of the worldrsquos majorenvironmental issues Oxygen can exist in theatmosphere as atomic diatomic or triatomicoxygen depending upon the number of atoms ina molecule The most common form is diatomicoxygen (O2) but the triatomic form called ozone(O3) created through the combination of theother two types is present in the upperatmosphere Ozone is also found close to the

Table 21 Average gaseous composition of dry air inthe troposphere

Figure 21 S pectral distribution of solar and terrestrial radiation Solar radiation is represented by a curve fora black body at 6000degK and terrestrial by a black body at 300degK A black body is a perfect radiator orabsorber of energy

surface on occasionmdashusually as a product of airpollutionmdashbut its main location is in the upperatmosphere where it effectively filters out short-wave solar radiation at the ultraviolet end of thespectrum Any change in ozone levels allowingan increase or decrease in the transmission ofradiation would therefore cause disruption ofthe earthrsquos energy budget and lead to alterationsin temperature levels and distribution patternsIt is also estimated that a reduction in ozone inthe upper atmosphere would allow an increasein the incidence of skin cancer in humans as wellas genetic mutation in lower level organisms asa result of the increase in the proportion ofultraviolet radiation reaching the earthrsquos surface(Dotto and Schiff 1978)

The minor gases and the greenhouse effect

Although gases other than oxygen or nitrogenaccount for only about 1 per cent of theatmospheric total they have an influence quiteout of proportion to their volume The mostabundant of these is argon at 093 per cent byvolume but it is inert Another of these minorgases carbon dioxide has a much more activerole in environmental processes It comprises only003 per cent by volume yet it makes a significantcontribution to the heating of the atmosphereand is a major participant in the process ofphotosynthesis by which sugars starches andother complex organic compounds are producedin plants

The atmosphere is quite selective in itsresponse to solar radiation It is transparent tohigh energy short-wave radiation such as thatfrom the sun but partially opaque to the lowerenergy long-wave radiation emanating from theearthrsquos surface For example a major proportionof the radiation in the visible range of thespectrum between 03 and 07 micrometres (microm)is transmitted through to the surface withoutlosing its high energy content (see Figure 21)Once it arrives it is absorbed the surface heatsup and begins to emit terrestrial long-waveradiation back into the atmosphere Thisradiation from the infrared end of the

spectrummdashwith wavelengths between 1ndash30micrommdashis captured and the temperature of theatmosphere rises The capture of the outgoingterrestrial radiation is effected largely by watervapour and carbon dioxide along with methaneand traces of about twenty other gases whichtogether are called the greenhouse gases Thewhole process was labelled the greenhouse effectsince the gases by trapping the heat appearedto work in much the same way as the glass in agreenhouse The name remains in common usealthough it is now generally accepted that theprocesses involved are not exactly the same Forexample the glass in the greenhouse acts as aphysical barrier to the transfer of energy Thereis no such barrier in the atmosphere Whateverthe accuracy of the analogy the selective natureof the atmosphere in its response to radiation isof supreme importance to the earthrsquos energybudget

Since the greenhouse effect depends uponcarbon dioxide and the other gases in theatmosphere it follows that any change in thesegases including their relative concentration willhave an effect on the intensity of the greenhouseeffect Changes in greenhouse gas levels in thepast were brought about by natural processesbut since the middle of the nineteenth centuryhuman activities have had a major role inincreasing the intensity of the greenhouse effectthrough the production of higher volumes ofcarbon dioxide methane and a number of othergreenhouse gases Concern over the impact ofsuch changes has promoted the intensificationof the greenhouse effect to its present position asa significant environmental issue

Oxides of sulphur and nitrogen

There are many other gases which from time totime become constituents of the atmosphereThese include sulphur dioxide oxides of nitrogenhydrogen sulphide and carbon monoxide alongwith a variety of more exotic hydrocarbonswhich even in small quantities can be harmful tothe environment All of these gases are naturalconstituents of the atmosphere released as a

THE ATMOSPHERE 17

result of biological activity created duringvolcanic eruptions or produced by natural woodand grass fires Increasingly however theirpresence is associated with pollution fromindustrial or vehicular sources In recent yearsconcern has centred on the widespreaddissemination and detrimental environmentalimpact of some of these gases Increasingindustrial activity and the continued reliance onfossil fuels as energy sources has caused agradual but steady growth in the proportion ofsulphur and nitrogen oxides in the atmosphereover the past 2ndash3 decades In combination withatmospheric water these gasesmdashwhether naturalor anthropogenic in originmdashare the mainingredients of acid rain Anthropogenicallyproduced acid rain is commonly many times moreacid than its natural conterpart however andhas been identified as a major cause of damageto aquatic and terrestrial ecosystems in NorthAmerica and Europe Although it has receivedless attention in recent years it remains asignificant environmental problem in the

industrialized nations of the world and there isgrowing evidence that areas currently littleaffectedmdashthe southern hemisphere forexamplemdashmay not always be immune

WATER IN THE ATMOSPHERE

The creation of acid rain would not be possiblewithout water another of the major naturalconstituents of the atmosphere Lists of theprincipal gases in the atmospheremdashsuch as Table21mdashcommonly refer to dry air but theatmosphere is never completely dry Theproportion of water vapour in the atmospherein the humid tropics may be as much as 4 percent by volume and even above the worldrsquos driestdeserts there is water present if only in fractionalamounts At any one time the total volume ofwater in the atmosphere is relatively small andif precipitated completely and evenly across theearthrsquos surface would produce the equivalent ofno more than 25 mm of rainfall (Barry andChorley 1992) In reality the distribution is very

Figure 22 The hydrologic cycle

uneven as a result of regional variability in thedynamic processes which produce precipitationIntense thunderstorms can yield 25 mm ofprecipitation in a matter of minutes whereas thesame amount may take several months or evenyears to accumulate under more stableatmospheric conditions Variations such as theseaccount for annual precipitation totals whichrange from virtually nothing in some of theworldrsquos deserts to as much as 4000 mm in themonsoon lands of the tropics Precipitation inexcess of the quantities normally in the air is madepossible by the hydrologic cycle which circulateswater through the earthatmosphere system andregularly replenishes the atmospheric reservoir(see Figure 22)

Water is unique among the constituents of theatmosphere in that it is capable of existing assolid liquid or gas and of changing readily fromone state to another It becomes involved inenergy transfer as a result of these changes Forexample energy absorbed during the conversionof liquid water to water vapour is retained bythe latter in the form of latent heat until theprocess is reversed The stored energy is then

released (see Figure 23) The water vapour maytravel over great distances in the atmosphere inthe period between the absorption and re-releaseof the energy and in this way energy absorbedin one location is transported elsewhere in thesystem

Water is also involved in the earthrsquos energybudget through its ability to absorb and reflectradiation As vapour it contributes to thegreenhouse effect by absorbing terrestrialradiation whereas in its liquid and solid forms itcan be highly reflective As clouds in the air orsnow on the ground it may reflect as much as90 per cent of the solar radiation it interceptsThat radiation reflected back into space makesno contribution to the energy requirements ofthe system In contrast clouds can also help toretain terrestrial radiation by reflecting it backto the surface

Water is an integral part of many globalenvironmental issues such as droughtdesertification and acid rain In addition becauseof its role in the earthrsquos energy budget any changein the distribution of water in the earthatmosphere system might well augment or

Figure 23 Energy transfer during the change in state of water

THE ATMOSPHERE 19

diminish the impact of other elements such asthe greenhouse effect or ozone depletion whichin whole or in part make their presence feltthrough that budget

ATMOSPHERIC AEROSOLS

In addition to the gaseous components of theatmosphere and the water in its various formsthere are also solid or liquid particles dispersedin the air These are called aerosols and includedust soot salt crystals spores bacteria virusesand a variety of other microscopic particlesCollectively they are often regarded asequivalent to air pollution although many ofthe materials involved are produced naturallyby volcanic activity forest and grass firesevaporation local atmospheric turbulence andbiological processes The proportion ofparticulate matter in the atmosphere hasincreased from time to time in the pastsometimes dramatically but in most cases theatmospherersquos built-in cleansing mechanismswere able to react to the changes and theoverall impact was limited in extent andduration When the island of Krakatoaexploded in 1883 for example it threw severalcubic kilometres of volcanic dust into theatmosphere Almost all of it is thought to havereturned to the earthrsquos surface in less than fiveyears as a result of particle coagulation drysedimentation and wash-out by precipitation(Ponte 1976) The lsquored-rainrsquo which occasionallyfalls in northern Europe is a manifestation ofthis cleansing process being caused when dustfrom the Sahara is carried up into theatmosphere by turbulence over the desert andwashed out by precipitation in more northerlylatitudes (Tullett 1984) Thus the atmospherecan normally cope with the introduction ofaerosols by natural processes Cleansing is nevercomplete however There is always a globalbackground level of atmospheric aerosols whichreflects a dynamic balance between the outputfrom natural processes and the efficiency of thecleansing mechanisms Data collected over thepast several decades suggest that the

background level is rising as a result of theincreasing volume of aerosols of anthropogenicorigin although the evidence is sometimescontradictory (Bach 1979)

Measurements since the 1930smdashin locationsas far apart as Mauna Loa in Hawaii Davos inSwitzerland and the Russian Caucasusmdashshow asharp rise in the atmospherersquos aerosol contentor turbidity as it is called Results from suchstations located at high altitudes and relativelyremote from the worldrsquos main industrial areasare considered representative of globalbackground aerosol levels Recent observationsof increasing cold season atmospheric pollutionin high latitudesmdashthe so-called lsquoArctic hazersquomdashare also considered indicative of rising globallevels (Environment Canada 1987) Volcanicactivity may also have provided some naturalenhancement in recent years but the closecorrespondence between elevated turbidity levelsand such indicators of human development asindustrialization and energy use suggests thatanthropogenic sources are major contributorsSome studies claim however that theobservations are insufficient to allow the humancontribution to increased turbidity to beidentified (Bach 1979)

Any increase in the turbidity of the atmosphereshould cause global temperatures to decline asthe proportion of solar radiation reaching theearthrsquos surface is reduced by scattering andabsorption In addition the condensation ofwater vapour around atmospheric aerosolswould lead to increased cloudiness and a furtherreduction in the transmission of incomingradiation This approach has been used to explainthe decline in global average temperatures whichoccurred between 1940 and 1960 and in the1970s it was seen by some as the mechanism bywhich a new ice age would be initiated (Ponte1976) Such thinking is also central to the conceptof nuclear winter which would be caused by arapid temperature decline following the injectionof large volumes of aerosols into the atmosphere(Bach 1986)

Providing a dissenting opinion are those whoclaim that an increase in atmospheric aerosols

would have less serious results The reduction ininsolation is accepted but it is also consideredthat there would be a concomitant reduction inthe amount of terrestrial radiation escaping intospace which would offset the cooling andperhaps result in some warming of the loweratmosphere The overall effects would dependvery much on the altitude and distribution of theaerosols (Mitchell 1975)

Contradictory conclusions such as thesemdashdrawn from the same basic informationmdashare tobe expected in climatological studies They reflectthe inadequacy of existing knowledge of theworkings of the earthatmosphere system andalthough research and technological developmentis changing that situation it remains a majorelement in restricting societyrsquos response to manyglobal issues

THE VERTICAL STRUCTURE OF THEATMOSPHERE

Although its gaseous constituents are quite evenlymixed the atmosphere is not physically uniformthroughout Variations in such elements as

temperature and air pressure provide form andstructure in what would otherwise be anamorphous medium The commonly accepteddelineation of the atmosphere into a series oflayers for example is temperature based (seeFigure 24) The lowest layer is the troposphereIt ranges in thickness from about 8 km at thepoles to 16 km at the equator mainly as a resultof the difference in energy budgets at the twolocations and temperatures characteristicallydecrease with altitude at a rate of 65degC perkilometre within itmdashthe tropospheric lapse rateTemperatures at the upper edge of thetroposphere average between -50 and -60degC butin equatorial regions where it reaches its greatestaltitude values may be as low as -80degC Thetropospheric lapse rate is in fact quite variableparticularly close to the surface Such variationsregularly produce instability in the system andhelp to make the troposphere the most turbulentof the atmospheric layers

The troposphere contains as much as 75 percent of the gaseous mass of the atmosphere plusalmost all of the water vapour and otheraerosols (Barry and Chorley 1992) It is also the

Figure 24 The verticalstructure of the atmosphereand its associated temperatureprofile

THE ATMOSPHERE 21

zone in which most weather systems developand run their course These factors togetherwith the high level of human intervention in thispart of the atmosphere ensure that many of theglobal environmental problems of currentconcernmdashincluding acid rain atmosphericturbidity and global warmingmdashhave theirorigins or reach their fullest extent in thetroposphere

The tropopause marks the upper limit of thetroposphere Beyond it in the stratosphereisothermal conditions prevail temperaturesremain constant at or about the value reachedat the tropopause up to an altitude of about 20km Above that level the temperature begins torise again reaching a maximum some 50 kmabove the surface at the stratopause wheretemperatures close to or even slightly above 0degCare common This is caused by the presence ofozone which absorbs ultraviolet radiation fromthe sun and warms the middle and upper levelsof the stratosphere creating a temperatureinversion The combination of that inversionwith the isothermal layer in the lowerstratosphere creates very stable conditions andthe stratosphere has none of the turbulenceassociated with the troposphere This hasimportant implications for the environmentAny foreign material introduced into thestratosphere tends to persist there much longerthan it would if it had remained below thetropopause Environmental problems such asozone depletion and atmospheric turbidity areaggravated by this situation Much of theimpact of nuclear winter would result from thepenetration of the tropopause by the originalexplosions and the consequent introduction oflarge volumes of pollutants into thestratosphere

Temperatures again decrease with heightabove the stratopause and into the mesospherefalling as low as -100degC at the mesopause some80 km above the surface The thermospherestretches above this altitude with no obviousouter limit In this layer temperatures mayexceed 1000degC but such values are notdirectly comparable to temperatures in the

stratosphere and troposphere because of therarified nature of the atmosphere at very highaltitudes Knowledge of the nature of thethermosphere and its internal processes is farfrom complete and linkages between the upperand lower layers of the atmosphere remainspeculative For the climatologist andenvironmentalist the most important structuralelements of the atmosphere are the troposphereand stratosphere The main conversion andtransfer of energy in the earthatmospheresystem takes place within these two layers ofthe lower atmosphere and interference withthe mechanisms involved has contributed to thecreation and intensification of currentproblems in the atmospheric environment

THE EARTHrsquoS ENERGY BUDGET

Virtually all of the earthrsquos energy is received fromthe sun in the form of short-wave solar radiationand balancing this inflow is an equivalent amountof energy returned to space as long-waveterrestrial radiation The concept is a useful onebut it applies only to the earth as a whole overan extended time scale of several years it is notapplicable to any specific area over a short periodof time This balance between incoming andoutgoing radiation is referred to as the earthrsquosenergy budget

The earth intercepts only a small proportionof the total energy given out by the sunmdashperhaps as little as one five-billionth (Critchfield1983)mdashand not all of that reaches the earthrsquossurface The estimates differ in detail but it isgenerally accepted that only about 50 per centof the solar radiation arriving at the outer edgeof the atmosphere is absorbed at the surface(Lutgens and Tarbuck 1982) That proportion issplit almost equally between direct solarradiation and diffuse radiation which has beenscattered by water vapour dust and otheraerosols in its passage through the atmosphere(see Figure 25) Of the other 50 per cent some30 per cent returns to space as a result ofreflection from the land and sea reflection fromclouds or scattering by atmospheric aerosols

This 30 per cent of incoming radiation bouncedback unaltered into space is a measure of theearthrsquos reflectivity or albedo The remaining 20per cent of the original incoming radiation isabsorbed mainly by oxygen ozone and watervapour in the atmosphere Thus of every 100units of solar energy arriving at the outer edgeof the atmosphere 70 are absorbed into theearth atmosphere system and 30 are returned tospace having made no contribution to thesystem Most of the 50 units absorbed by theearth are reradiated as long-wave terrestrialradiation Some energy is also transferred intothe atmosphere by convective and evaporativeprocesses at the earthrsquos surface The greenhousegases trap the bulk of the outgoing terrestrial

energy but the atmosphere is transparent towavelengths between 8 microm and 11 microm and thisallows 5 units of radiation to escape directly tospace through the so-called atmosphericwindow Some of the terrestrial radiationabsorbed by the atmosphere is emitted to spacealso but sufficient accumulates to allow 95units to be reradiated back to the surface Theexchange of energy between the atmosphereand the earthrsquos surface involves amountsapparently in excess of that provided by solarradiation This is a direct function of thegreenhouse effect and its ability to retain energyin the lower atmosphere Eventually all of thelong-wave radiation passes out into space alsobut not before it has provided the energy

Figure 25 The earthrsquos energy budget

THE ATMOSPHERE 23

necessary to allow the various atmospheric andterrestrial processes to function

The energy balance of the earthatmospheresystem is subject to stress from both natural andanthropogenic sources Any factor capable ofdisturbing that balance is called a radiativeforcing agent (Shine et al 1990) In the past themost common sources of forcing were naturaland involved changes in such elements as solarradiation the planetary albedo and atmosphericaerosol concentrations which individually andin combination altered the flow of energy intoand out of the system These natural forcingagents continue to disrupt the balance butcurrent concern is with anthropogenic forcingagents and the climate change that they are likelyto produce

The depletion of the ozone layer resulting fromhuman interference disturbs the balance byallowing additional radiation to reach thesurface and changes in atmospheric turbiditydisrupt both incoming and outgoing radiationIn the immediate future however the greatestchange in radiative forcing is expected to comeabout as a result of the rising levels of greenhousegases (see Table 22) It is estimated that theireffect on the radiative balance of the earthradiative forcing agent natural or anthropogenic(Shine et al 1990)

The concept of balance in the earthrsquos heatbudget is a useful one but it provides only aglobal picture and cannot be applied to specificareas There is a definite latitudinal imbalancein the budget Annually the equator receivesabout five times the amount of solar radiationreaching the poles and those areas equatorwardsof 35 degrees of latitude receive more energy thanis returned to space (Figure 26) The excess ofoutgoing radiation over incoming poleward of35 degrees of latitude creates a radiation deficitin higher latitudes (Trewartha and Horn 1980)In theory such an imbalance could lead to higherlatitudes becoming infinitely colder andequatorial latitudes infinitely warmer In realityas soon as the latitudinal differences develop theyinitiate circulation patterns in the atmosphere andin the oceans which combine to transfer heat

Figure 26 The latitudinal imbalance in solar andterrestrial radiation

Table 22 Estimated impacts of different radiativeforcing agents in watts per square metre (1990ndash2000)

Source Based on data in Shine et al (1990)

from the tropics towards the poles and in sodoing serve to counteract the imbalance

OCEANIC AND ATMOSPHERICCIRCULATION PATTERNS

The circulation of the oceans

More than half of the solar radiation reachingthe earthrsquos surface is absorbed by the oceanswhere it is stored and redistributed before beingreleased back into the atmosphere Ocean andatmosphere are quite intimately linked Theprevailing winds in the atmospheric circulationfor example drive water across the ocean surfaceat speeds of less than 5 km per hour in the formof broad relatively shallow drifts In some cases

they carry warm water polewards in others theycarry cooler water into lower latitudes (see Figure27) In addition density differences in partthermally induced cause horizontal and verticalmovement of water within the oceans All of theseprocesses help to transfer excess heat fromequatorial regions towards the poles This isillustrated particularly well in the North Atlanticwhere the warm waters of the Gulf Stream Driftensure that areas as far north as the Arctic Circleare anomalously mild during the winter monthsThe amount and rate of energy transfer variedin the past producing significant climatologicaleffects Changes in the North Atlantic circulationfor example may have contributed to the IceAges Current estimates of poleward energytransfer in the northern hemisphere indicate thatocean transfer exceeds atmospheric transfer in

Figure 27 The circulation of the oceans

THE ATMOSPHERE 25

low latitudes whereas the latter is moreimportant in mid to high latitudes On averageoceanic transport accounts for 40 per cent of thetotal energy transfer and atmospheric transportfor 60 per cent (Trewartha and Horn 1980)

One ocean current which does not appear aspart of the well-established pattern illustrated inFigure 27 but which makes a major contributionto energy transfer is El Nintildeo This is a flow ofanomalously warm surface water which hasappeared with some frequency over the centuriesin the equatorial regions of the eastern Pacific

(see Table 23) The name originally referred toa warm current which appeared off the coast ofPeru close to Christmasmdashhence El Nintildeo the(Christ) Childmdashbut now it is applied to a largerscale phenomenon (Lockwood 1984) It owes itsdevelopment to the Southern Oscillation aperiodic fluctuation in atmospheric pressure inthe southern Pacific first recognized in the 1920sby Sir Gilbert Walker as he sought to developmethods for forecasting rainfall in the Indianmonsoon The term ENSO is commonly used torefer to the combination of El Nintildeo and theSouthern Oscillation

An indication of the Southern Oscillation isobtained by comparing barometric pressuredifferences between Tahiti in the easternPacific and Darwin in northern AustraliaPressure at these two stations is negativelycorrelated high pressure over Tahiti normallybeing accompanied by low pressure overDarwin for example In contrast low pressureat Tahiti is matched by high pressure at DarwinThese pressure differences induce a stronglatitudinal circulation in the equatorialatmospheremdashreferred to as the Walkercirculation Periodically the regional pressurepatterns reverse and it is this phenomenonwhich is referred to as the Southern Oscillation

Table 23 El Nintildeo events of various intensities bycentury 1525ndash1987

Source Quinn and Neal (1992) p 637Notes M=moderate S=strong VS=very strong M+ =significantly exceeding the general characteristics of MS+= significantly exceeding the general characteristicsof S

Figure 28 The Southern Oscillation as represented by the Tahiti minus Darwin atmosphere pressure anomaly1968ndash82

Source Adapted from Rasmusson and Hall (1983)

(see Figure 28) It has a periodicity of one tofive years and in its wake it brings changes inwind fields sea surface temperatures and oceancirculation patterns When pressure is high overTahiti and low over Darwin the general windand surface water flow is from east to west andthere is a tendency for relatively warm water topond up at the western end of the Pacific OceanThe removal of the warm surface water fromthe eastern Pacific allows the upwelling ofrelatively cold water from below in that areaThese conditions are reversed following theOscillation Easterly winds are replaced by thewesterlies as the atmospheric pressure changesand warm water begins to flow east again toreplace the cold (see Figure 29) It is thisphenomenon which has come to be called ElNintildeo When it is strongly developed it maykeep areas in the eastern Pacific warmer thannormal for periods of up to a year (Rasmussonand Hall 1983) but its influence extends farbeyond the immediate area (see Chapter 3)

During some yearsmdashwhen the differencebetween the high pressure over Tahiti and lowpressure over Darwin is particularly well-markedfor examplemdashthe equatorial easterly winds arestronger than normal and push the cold water

upwelling off the South American coast far acrossthe Pacific This creates a cold current flowingeast to west which has been given the name LaNintildea Like El Nintildeo it appears to have an effectoutside the region but as a recently identifiedphenomenon (1986) its full climatologicalsignificance is as yet uncertain (Hidore and Oliver1993)

In addition to its direct role in energytransfer the oceanic circulation also contributesto the earthrsquos climate through its participationin the global carbon cycle The oceans containclose to 80 per cent of the earthrsquos total carbon atany one time It is held in active storage and istransferred between the oceanic andatmospheric reservoirs in the form of carbondioxide Horizontal and vertical mixing withinthe oceans helps to control the rate of thatexchange which impacts on the greenhouseeffect and therefore on the earthrsquos energy budget(Taylor 1992)

The circulation of the atmosphere

George Hadley developed the classic model ofthe general circulation of the atmosphere some250 years ago It was a simple convective system

Figure 29 Sea surface temperature (SST) anomalies in the Pacific Ocean December 1982

Source After Rasmusson and Hall (1983)

THE ATMOSPHERE 27

Figure 210a Simple convective circulation on a uniform non-rotating earth heated at the equator and cooledat the poles

Figure 210b Three-cell model of the atmospheric circulation on a uniform rotating earth heated at theequator and cooled at the poles

based on the concept of a non-rotating earth witha uniform surface which was warm at theequator and cold at the poles (see Figure 210a)The warmth caused the surface equatorial air tobecome buoyant and rise vertically into theatmosphere As it rose away from its source ofheat it cooled and became less buoyant but wasunable to sink back to the surface because of thewarm air rising behind it Instead it spread northand south away from the equator eventuallyreturning to the surface at the poles From thereit flowed back towards the equator to close thecirculation The air rising at the equator andspreading polewards carried energy with ithelping to reduce the energy imbalance betweenthe equator and the poles This type of energytransfer initiated by differential heating is calledconvection and the closed circulation whichresults is a convection cell Hadleyrsquos originalmodel with its single convection cell in eachhemisphere was eventually replaced by a three-cell model as technology advanced and additionalinformation became available but hiscontribution was recognized in the naming of thetropical cell (see Figure 210b) The three-cellmodel continued to assume a uniform surfacebut the rotation of the earth was introduced andwith it the Coriolis effect which causes movingobjects to swing to the right in the northernhemisphere and to the left in the southern Thusthe winds became westerly or easterly in this newmodel rather than blowing north or south as inthe one cell version The three cells and theCoriolis effect in combination producedalternating bands of high and low pressureseparated by wind belts which were easterly inequatorial and polar regions and westerly in mid-latitudes Although only theoretical elements ofthis model can be recognized in existing globalwind and pressure patterns particularly in thesouthern hemisphere where the greater expanseof ocean more closely resembles the uniformsurface of the model

In the late 1940s and 1950s as knowledge ofthe workings of the atmosphere improved itbecame increasingly evident that the three-cellmodel oversimplified the general circulation

The main problems arose with the mid-latitudecell According to the model the upper airflowin mid-latitudes should have been easterly butobservations indicated that it waspredominantly westerly The winds followed acircular path centred on the pole which led totheir description as circumpolar westerliesObservations also indicated that most energytransfer in mid-latitudes was accomplished byhorizontal cells rather than the vertical cellindicated by the model The mechanismsinvolved included travelling high and lowpressure systems at the surface plus wavepatterns in the upper atmosphere called Rossbywaves (Starr 1956) Modern interpretations ofthe general circulation of the atmosphere retainthe tropical Hadley cell but horizontal eddies

Figure 211 The index cycle associated with themeandering of the mid-latitude westerlies in thenorthern hemisphere

THE ATMOSPHERE 29

have come to dominate mid latitudes and haveeven replaced the simple thermal cell of polarlatitudes (Barry and Chorley 1992)

The flow pattern adopted by the Rossby wavesis quite variable and that variability makes amajor contribution to energy transfer When thewesterlies follow a latitudinal path from westto east they are said to be zonal and the strengthof the latitudinal flow is indicated by the zonalindex If the westerly flow is strong the zonalindex is high In contrast as the amplitude ofthe Rossby waves increases the flow becomesless zonal and more meridional (ie it follows anorth-south or longitudinal path) The zonalindex is then said to be low Changes in the wavepatterns occur as indicated in Figure 211 andthe entire sequence from high-zonal indexthrough low-zonal index and back to high iscalled the index cycle It has an important role inthe atmospheric energy exchange process As theamplitude of the Rossby waves increases andthe westerlies loop southwards they carry coldair into lower latitudes Conversely as they loopnorthwards they introduce warmer air into higherlatitudes These loops are often short-circuitedleaving pools of abnormally cool air in lowerlatitudes and abnormally warm air in high

latitudes The net result is significant latitudinalenergy transfer Such developments are notcompletely random but neither are theypredictable The cycle occurs over a period of 3to 8 weeks and is repeated with some frequencyyet it lacks the regularity necessary forforecasting

The jet streams

The modification of the three-cell model in the1940s and 1950s was made possible in largepart by improved knowledge of conditions inthe upper atmosphere The upper atmosphericcirculation is quite complex in detail but ingeneral terms it is characterized by an easterlyflow in the tropics and a westerly flowassociated with the Rossby waves in mid to highlatitudes Within these broad airflows there arerelatively narrow bands of rapidly moving aircalled jet streams in which wind speeds average125ndash130 km per hour in places The jet streamsare usually located at the tropopause and areassociated with zones in which steeptemperature gradients exist and where inconsequence the pressure gradient is also steepThe most persistent jets are found in the sub-

Figure 212 Schematic diagram of the vertical circulation of the atmosphere and the location of the major jetstreams in the northern hemisphere

tropics at the poleward edge of the tropicalHadley cell and in mid-latitudes at the polarfront (see Figure 212) Both of these jetsgenerally flow from west to east In addition anArctic jet associated with the long polar nighthas been identified (Hare and Thomas 1979)and an intermittent but recognizable easterlyjet is a feature of the upper atmosphericcirculation in equatorial regions (Barry andChorley 1992)

The northern hemisphere polar front jetstream the one most commonly encountered asheadwinds or tail winds during trans-continentalor trans-oceanic flights is the best known of allthe jet streams It circles the earth in midlatitudesfollowing a meandering track from west to eastat speeds averaging perhaps 100 km per hourbut with a maximum recorded speed of almost500 km per hour (Eagleman 1985) During thewinter it follows a more southerly track closeto 35degN and has an average velocity of 130 kmper hour whereas in the summer it is locatedcloser to 50degN and its velocity decreases to about65 km per hour

The influence of the polar front jet extends tothe lower atmosphere through its control overthe various systems which produce the surfaceweather conditions For example the differencebetween a mild winter and a cold one in theinterior of North America is often determinedby the location of the polar front jet stream Amore southerly track allows the cold polar airon the north side of the jet to penetrate intolower latitudes whereas a more northerly trackallows the continent to remain bathed in thewarmer southern air The jet also exerts itsinfluence on moisture regimesmdashthrough itscontrol over the tracks followed by mid-latitudelow pressure systemsmdashand it has beenimplicated in the tornado outbreaks whichoccur in North America every spring North-south thermal contrasts are strong at that timeand the jet is therefore particularly vigorous(Eagleman 1985)

The importance of the jet stream and theassociated upper westerlies from anenvironmental point of view lies in their ability

to transport pollutants over great distancesthrough the upper atmosphere Smoke volcanicdebris and acid particles are all spread by suchtransportation and as a result the problemsthey represent are global in scale When above-ground atomic tests were being carried out inthe USSR and China during the 1950s and early1960s radioactive fallout was carried overnorthern Canada in the jet stream (Hare 1973)A similar mechanism spread the products of theChernobyl nuclear accident Any future nuclearwar would cause great quantities of debris to bethrown into the upper atmosphere where the jetstreams would ensure a hemisphericdistribution and contribute to the rapid onset ofnuclear winter

The effects of surface conditions and seasonalvariations

Modern representations of the general circulationof the atmosphere take into account the non-uniform nature of the earthrsquos surface with itsmixture of land and water and includeconsideration of seasonal variations in energyflow (see Figure 213)

Land and water respond differently to thesame energy inputs because of differences in theirphysical properties Land tends to heat up andcool down more rapidly than water andtemperatures over land exhibit a greater rangediurnally and seasonally than those over waterThese temperature differences in turn have animpact on atmospheric pressure particularly inthe northern hemisphere with its juxtapositionof oceans and major land masses During thenorthern summer for example highertemperatures promote lower pressure over thecontinents whereas the adjacent seas are coolerand pressure remains high in the cells whichrepresent the sub-tropical high pressure belt overthe North Atlantic and Pacific oceans By alteringthe regional airflow such pressure differencescause disruption of the theoretical globalcirculation patterns

The permanent or semi-permanent features ofthe circulation also vary in extent and intensity

THE ATMOSPHERE 31

by the season or by the year and in some casesover much shorter time scales as patterns ofenergy flow change There is a general southwardshift of the systems during the northern winterfor example The Intertropical Convergence Zone(ITCZ)mdashthe belt of low pressure produced by

the combination of equatorial heating and theconvergence of trade windsmdashmigratessouthwards in sympathy with the southwardmovement of the zone of maximum energyreceipt Behind it the sub-tropical anticyclonesalso move south contracting as they do so

Figure 213 Global wind and pressure patterns for January and July

allowing the mid-latitude westerlies and theirassociated travelling highs and lows to extendtheir influence into lower latitudes

In the southern hemisphere at this time thesub-tropical high pressure systems expand andextend polewards over the oceans The increasedsolar radiation of the summer season contributesto the formation of thermal lows over SouthAmerica Africa and Australia The absence ofmajor landmasses polewards of 45ndash50degS allowsthe westerly wind belt to stretch as a continuousband around the earth

The patterns change during the northernsummer as the ITCZ moves northwards againand the other elements of the system respond tochanging regional energy budgets Although suchchanges are repeated year after year themovements of the systems are not completelyreliable In the tropics for example the ITCZmigrates at different rates and over differentdistances from one year to the next This inherentvariability contributes to the problem of droughtIt also adds complexity to the impact ofenvironmental problemsmdashsuch as the enhancedgreenhouse effect or atmospheric turbiditymdashwhich cause changes in the earthrsquos energy budgetand therefore have the potential to altercirculation patterns

AUTOVARIATIONS AND FEEDBACK

It is convenient to consider the various elementsof the earthatmosphere system as separateentities as has been done here They are howeverquite intimately linked and understanding thenature of these links is importantmdashnot only inthe pure scientific study of the atmosphere itselfbut also in the applied interdisciplinary approachrequired for the study of modern globalenvironmental problems The elements andprocesses incorporated in the earthrsquos energybudget and the atmospheric circulation forexample are parts of a dynamic system in whichthe components are sufficiently integrated thatone change will automatically produce othersSuch changes produced through the activities ofinternal processes are called autovariations

(Trewartha and Horn 1980) and in combinationwith feedback mechanisms they may augmentor dampen the effects of a particular change inthe system Lower temperatures at the earthrsquossurface for example would allow the persistenceof snow cover beyond the normal season Thisin turn would increase the amount of solarradiation reflected back into space causingsurface temperatures to fall even more andencourage the snow to remain even longer Sucha progression in which the original impact ismagnified illustrates the concept of positivefeedback Rising temperatures may initiatenegative feedback One of the initial effects ofhigher temperatures would be an increase in therate of evaporation from the earthrsquos surfaceSubsequent condensation of the water vapour inthe atmosphere would increase cloudiness andreduce the amount of solar radiation reachingthe surface As a result temperatures would fallagain and the initial impact of risingtemperatures would be diminished Ultimatelythese changes in the earthrsquos energy budget wouldbe reflected in the general circulation of theatmosphere Many current global issuesmdashsuchas the intensification of the greenhouse effectincreased atmospheric turbidity anddesertificationmdashinvolve autovariations andinclude positive or negative feedback in theirdevelopment

MODELLING THE ATMOSPHERE

Atmospheric modelling has a long tradition inclimatology stretching back to Hadley and hisbasic representation of the earthrsquos wind andpressure belts That classic model of theatmospheric circulation along with the manyvariants which were subsequently developed wasa representative model It showed thecharacteristic distribution of wind and pressureacross the earthrsquos surface in a simplified formwith no attempt to predict change in the systemChange was first dealt with at the local orregional level when attempts were made toforecast future weather conditions and it is toweather forecasting that most modern predictive

THE ATMOSPHERE 33

models can trace their origins The earliestweather forecasts depended very much on theinterpretation of atmospheric conditions by anexperienced observer who used his accumulatedknowledge of past events to predict future short-term changes in weather conditions Particularskills in that field were often attributed to sailorsfarmers and others who were regularly exposedto the vagaries of the weather in their work Thissubjective approach continued well into moderntimes but by the end of World War I the first ofthe modern predictive models was beingdeveloped This was the mid-latitude frontalmodel which grew out of the work of theNorwegian school of meteorology With itscombination of air masses anticyclones andfrontal depressions it was the mainstay ofweather forecasting in mid-latitudes for half acentury Past experience with such systemscoupled with observations of their on-goingdevelopment allowed local weather conditionsto be predicted twelve to twenty-four hoursahead As knowledge of the working of theatmosphere grew in the 1940s and 1950s itbecame clear that such models over-simplifiedthe complex dynamics of the atmospheric systemand new methods of prediction were sought

In these new models atmospheric physicalprocesses were represented by a series offundamental equations Three of the equationswere derived from the laws of conservation ofmomentum mass and energy which dealt withmotion in the atmosphere In addition the modelsincluded an equation of state derived from thegas laws of classical physicsmdashwhich relatedatmospheric pressure density and temperaturemdashplus a moisture equation (Washington andParkinson 1986) When solved repeatedly for aseries of small but incremental changes atselected grid points across the study area theseequations provided a forecast of the future stateof the atmosphere LFRichardson a Britishmeteorologist was a pioneer in this fieldpublishing the results of his work in 1922 in hisbook Weather Prediction by Numerical Process(Ashford 1981) At that time his methods werenot practicable The complexity of the

computations and the existence of onlyrudimentary methods of mechanical calculationmeant that the predictions could not keep aheadof changing weather conditions As a result noreal forecast could be made The inadequaciesof existing upper-air observations alsocontributed to the failure of this first attempt atnumerical forecasting (Ashford 1992)

Following this initial setback little was doneto develop numerical prediction further until the1950s and 1960s when advances in computertechnology and improved methods of observationled to the development of models capable ofpredicting changes in the essential meteorologicalelements across the globe Since then thesemethods have been widely adopted andnumerical modelling is currently the mostcommon method of weather forecasting forperiods of several hours to 5 or 6 days aheadProblems remain however despite the growingsophistication of the systems used Gaps exist inthe data required to run the models for exampleOcean coverage is thin and the number ofobserving stations in high latitudes is smallProblems of scale may also arise depending uponthe horizontal and vertical resolution of themodel A coarse resolution for example will missthe development of small scale phenomena suchas the shower cells associated with localconvective activity A fine resolution model willprovide greater accuracy but only at a cost Eventhe simplest weather models require a billionmathematical operations before they can producea single dayrsquos forecast (Levenson 1989) In settingup the grid reference points used in thecalculations therefore a compromise has to bestruck between the accuracy required and the costof running the program The UK MeteorologicalOffice for example has developed two variantsof its forecasting model The global version hasa horizontal grid with a resolution of 15deg latitudeby 1875deg longitude and includes fifteen levelsstretching from the surface into the stratosphereIn contrast a finer grid with a resolution of 075deglatitude and 09375deg longitude is only availablefor an area covering the North Atlantic andWestern Europe (Barry and Chorley 1992)

Atmospheric models designed to simulateclimate change also depend upon numericalprediction but they differ in a number ofimportant ways from the weather forecastingmodels For example over the short time periods(4ndash5 days) covered by the weather forecastingmodels environmental elements such asvegetation oceans ice sheets and glaciers can beconsidered as static or unchanging and thereforecontributing little to atmospheric change Overthe longer time scales (10ndash50 years) involved inin the climate models however these elementswould be expected to change and that changemust be incorporated in the models This maybe simple if the change remains linear butdifficulties arise when response to change initiatesfeedback in the earthatmosphere system Manyof the uncertainties in the results of climatemodelling can be traced to the inability of themodels to deal adequately with climate feedbackmechanisms

The spatial resolution of climate models alsotends to be coarser than that of weather modelsModern computer systems are remarkablysophisticated but their use is at times constrainedby such factors as operational speed memorycapacity and cost For example the additionalcalculations introduced to accommodate thelonger time scale and greater number ofenvironmental variables incorporated in theclimate models increases the computer memoryand time required This in turn increases the costof running the model A reduction in the numberof grid points at which the calculations are madehelps to keep these elements at manageable levelsbut it also produces a coarser resolution in themodel Typical climate models have a resolutionwhich is three to six times coarser than that ofweather forecasting models Although this stillallows adequate representation of macro-scaleclimate features it gives only limited results atthe regional level (Cubasch and Cess 1990) Inan attempt to improve regional or meso-scaleresolution Australian scientists have developeda method in which a meso-scale model is lsquonestedrsquoor lsquoembeddedrsquo within a macro-scale globalclimate model This nested model driven by the

global model which surrounds it is programmedto provide much finer detail at the regional levelthan is possible with the standard model aloneSince the extra information is required for onlya limited area and the running of the globalmodel is unaffected both the additionalcomputer time needed and the costs remainmanageable Tests suggest that this is the bestmethod currently available for achievingincreased spatial resolution although it requiresfurther development before it can be used forclimate prediction (Henderson-Sellers 1991)

Several important climatic processes take placeat regional scales and are therefore missed bythe macro-scale grids of most climate modelsHowever since they affect the predictionsproduced by the models they must be includedThis is done through a process ofparameterization in which statisticalrelationships are established between the smallscale processes and the grid scale variables Sincethe latter can be calculated by the models thevalues of the small scale processes can then beestimated (Hengeveld 1991) For exampleexisting models cannot deal with individualclouds but average cloudiness in a particulargrid-box can be predicted using temperature andhumidity values calculated by the model(Schneider 1987) Other variables that requireparameterization include radiation evaporationand land surface processes

Climate models take various forms andinvolve various levels of complexity dependingupon the application for which they aredesigned A simple model for example mayprovide only one value such as the averagetemperature of the earth Increasing levels ofsophistication produce one- and two-dimensional models and the complex three-dimensional general circulation models whichdepend upon the full use of numerical predictionto produce results

One dimensional (1-D) models provideinformation on change along a vertical linestretching from the earthrsquos surface up into theatmosphere The main inputs into these modelsare incoming solar radiation and returning

THE ATMOSPHERE 35

terrestrial radiation and they can be used toestimate such elements as temperature changesinitiated by changing atmospheric aerosollevels They treat the earth as a uniform surfacewith no geography and no seasons As a resultthey are inadequate to deal with the unevensurface energy distribution associated with thedifferences in heat capacity between land andocean One dimensional models have been usedfrequently to estimate the impact of volcaniceruptions on climate and a 1-D radiativeconvective model was used to establish theTTAPS scenario of nuclear winter (Turco et al1983) At best 1-D models are useful for thepreliminary investigation of global scaleradiative and convective processes at differentlevels in the atmosphere However they cannotdeal with seasonal or regional scale featuresand require so many assumptions that theirability to provide accurate predictions islimited

Two-dimensional (2-D) models add ameridional or latitudinal component to thealtitudinal component of the 1-D models Theycan consider variations in climate along avertical cross-section from pole to pole forexample or along a specific line of latitude Thisallows the horizontal redistribution of suchelements as energy or particulate matter to beexamined Two-dimensional models cantherefore include consideration of differences inheat capacity between land and ocean but theirability to deal with the evolving dynamics of theatmosphere once change has been initiatedremains limited Like the 1-D models theycontributed to the early development of theconcept of nuclear winter but they were quicklysuperseded by more sophisticated three-dimensional models

Three-dimensional (3-D) models provide fullspatial analysis of the atmosphere Theyincorporate major atmospheric processes pluslocal climate features predicted through theprocess of parameterization The simulations ofcurrent and future climates provided by thesemodels require powerful computers capable ofprocessing as many as 200000 equations at tens

of thousands of points in a three-dimensionalgrid covering the earthrsquos surface and reachingthrough two to fifteen levels as high as 30 kminto the atmosphere (Hengeveld 1991) Inaddition to these grid-point models spectralmodels have been developed In these theemphasis is on the representation ofatmospheric disturbances or waves by a finitenumber of mathematical functions Many of themore advanced models incorporate thisapproach (Cubasch and Cess 1990)

Climate models of this type are known asgeneral circulation models (GCMs) They can beprogrammed to recognize the role of land andsea in the development of global climates Theircomplex representation of atmospheric processesallows the inclusion of the important feedbackmechanisms missing from 1-D and 2-D modelsand they can deal with the progressive changeset in motion when one or more of thecomponents of the atmosphere is altered Withtypical integration times for GCMs ranging fromseveral decades to 100 years this ability to dealwith the evolving dynamics of the atmosphere isimportant

In an attempt to emulate the integratednature of the earthatmosphere systematmospheric GCMs have been coupled withother environmental models (see Figure 214)Recognizing the major contribution of theoceans to world climatology the most commoncoupling is with ocean models In theory suchmodels combining the atmospheric and oceaniccirculations should provide a more accuraterepresentation of the earthrsquos climate This isnot always the case however The coupling ofthe models leads to the coupling of any errorsincluded in the individual models The so-called lsquomodel driftrsquo which occurs can betreated but it remains a constraint for coupledmodels (Cubasch and Cess 1990) Anothermajor problem is the difference in time scalesover which atmospheric and oceanicphenomena develop and respond to changeThe atmosphere generally responds withindays weeks or months while parts of theoceansmdashthe ocean deeps for examplemdashmay

take centuries or even millenia to respond(Washington and Parkinson 1986) As a resultrunning a completely interactive coupledatmosphere-ocean model until all elementsreach equilibrium is time-consuming andcostly Because of this the oceanic element inmost coupled models is much lesscomprehensive than the atmospheric elementThe ocean is commonly modelled as a slabwhich represents only the uppermost layer ofwater in which the temperature is relativelyuniform with depth Oceanic heat storage iscalculated only for the chosen depth of thelayer and other elements such as oceanic heattransport and exchanges with the deeper partsof the ocean are neglected or calculated

indirectly (Cubasch and Cess 1990) Thusalthough the coupling of the atmospheric andoceanic circulations should improve theaccuracy of the modelling process since itmore closely emulates the real environmentthe results are often no better than thoseobtained from the individual uncoupledmodels (Washington and Parkinson 1986)

Sea-ice models carbon cycle models andchemical models have also been recognized ashaving the potential to contribute to climatesimulation when coupled to existing GCMsSea-ice has been incorporated in some oceanmodels but separate sea-ice simulation modelshave also been created Carbon cycle models

Figure 214 Schematic illustration of the components of the coupled atmosphere-ocean-ice-land climatic systemThe full arrows are examples of external processes and the open arrows are examples of internal processes inclimatic change

Source From Houghton et al (1990)

THE ATMOSPHERE 37

Table 24 Some commonly used general circulation models

particularly important in studies of globalwarming have already been coupled to oceanmodels and chemical models are beingdeveloped to investigate the influence of othertrace gases on the general circulation of theatmosphere (Cubasch and Cess 1990)

Global circulation models are being developedor refined at about a dozen universitiesgovernment laboratories and research institutesaround the world (see Table 24) Most of the

effort has gone into producing equilibriummodels In these change is introduced into amodel which represents existing climateconditions and the model is then allowed to rununtil a new equilibrium is reached The newmodel climate can then be compared with theoriginal to establish the overall impact of thechange The numerous GCMs used to study theimpact of a doubling of atmospheric carbondioxide on world climates are of this type They

Table 24 continued

Source Compiled from data in Cubasch and Cess (1990) All models are global with realistic geographyand a seasonal cycle of insolationResolution is defined either by latitude and longitude (eg 4degtimes5deg) in grid point models or by the numberof waves (moving disturbances) that can be accommodated in spectral models (eg R15 or T32)

THE ATMOSPHERE 39

make no attempt to estimate changing conditionsduring the transient phase of the model runalthough these conditions may well haveimportant environmental impacts long beforeequilibrium is reached The development oftransient or time-dependent models which wouldprovide the interim information currently lagsbehind that of the equilibrium models The fewexisting transient models are of coarse resolutionbut yield results which are broadly consistentwith those from the more common equilibriummodels (Bretherton et al 1990) They incorporatea coupled ocean-atmosphere system with fullocean dynamics and require increased computerpower and improved ocean observation databefore they can make a greater contribution tothe study of future climate change

Despite the increasing complexity of currentGCMs the fact remains that like all models theyrepresent a compromise between the complexitiesof the earthatmosphere system and theconstraints imposed by such factors as dataavailability computer size and speed and the costof model development and operation which limitthe accuracy of the final result The quality ofspecific simulations can be tested by comparingmodel predictions with the results obtained bydirect measurement or observation Thisapproach shows for example that GCMs cansimulate short-term changes such as seasonalcycles remarkably well (Schneider 1987) andtheir portrayal of atmospheric responses to seasurface temperature anomalies is usuallyconsidered as satisfactory Simulations ofHolocene climates which provide an indicationof the long-term capabilities of the models havegiven results supported by palaeo-environmentalindicators such as fossil pollen distribution andformer lake levels revealed in lake and oceansediment cores (Gates et al 1990) These testsindicate that most simulations can represent thelarge scale characteristics of climate quite wellbut significant errors remain at regional scalesas a result of the coarse resolution common to

most models The accuracy of coupled models isalso restricted by inadequate data on which tobase the oceanic component More powerfulcomputers and improved parameterization willtake care of some of these problems but thegeneral consensus is that the gap between climatesimulation and reality will remain for some timeto come

SUMMARY

Changes in such elements as the composition ofthe atmosphere global circulation patterns andthe earthrsquos energy budget are now widelyrecognized as components of most current globalenvironmental issues The relationships involvedare complex and remain imperfectly understooddespite major advances in the acquisition andanalysis of atmospheric data Global climatemodels have been developed to investigate theprocesses further and provide forecasts of futuredevelopments These investigations and forecastswill allow a more effective response to thechanges with the ultimate aim of minimizing theirenvironmental impact

Ahrens CD (1993) Essentials of MeteorologyMinneapolisSt Paul West PublishingCompany

Barry RG and Chorley RJ (1992)Atmosphere Weather and Climate LondonRoutledge

Briggs D Smithson P and Ball T (1993)Fundamentals of Physical GeographyMississauga Ontario Copp Clark Pitman

Hidore JJ and Oliver JE (1993) ClimatologyAn Atmospheric Science New YorkMacmillan

Levenson T (1989) Ice Time Climate Scienceand Life on Earth New York Harper andRow

In recent years the worldrsquos attention has beendrawn time and again to the Third World nationsof Africa by the plight of millions of people whoare unable to provide themselves with food waterand the other necessities of life The immediacyof television with its disturbing images of dull-eyed pot-bellied malnourished childrenskeletons of cattle in dried-up water courses anddesert sands relentlessly encroaching upon onceproductive land raised public awareness tounexpected heights culminating in themagnanimous response to the Live Aid concertsof 1985 Not unexpectedly given therequirements of modern popular journalism andbroadcasting coverage of the situation has often

been narrow highly focused and shallow lackingthe broader deeper investigation necessary toplace the events in a geographical orenvironmental framework For example thepresent problems are often treated as a modernphenomenon when in fact they are in manyways indigenous to the areas involved Theinhabitants of sub-Saharan Africa have sufferedthe effects of drought and famine for hundredsof years (see Table 31) It is part of the pricethat has to be paid for living in a potentiallyunstable environment

Current episodes seem particularlycatastrophic but they are only the most recentin a continuing series In dealing with thesituation the media have tended to concentrateon the problems of famine and its consequenceswhile most of the aid being supplied has ofnecessity been aimed at alleviating hungerHowever while famine may be the direct causeof the present suffering and hardship it is inreality a symptom of more fundamentalproblems Elements of a cultural socio-economicand political nature may contribute to theintensity and duration of the famine but in areassuch as the Sahel the ultimate causes are to befound in the environmental problems associatedwith drought and desertification The formerthrough its impact on plants and animalsdestroys the food supply and initiates the faminethe latter with its associated environmentalchanges causes the productive land to becomebarren and ensures that the famine will persistor at least recur with some frequency Togetherdrought famine and desertification have been

Drought famine and desertification

Table 31 Wet and dry spells in Africa south of theSahara

Source After Nicholson (1989)

DROUGHT FAMINE AND DESERTIFICATION 41

implicated in some of the major humancatastrophes of the past and present and willundoubtedly contribute to future suffering anddespair

DROUGHT

The problem of definition

Drought is a rather imprecise term with bothpopular and technical usage To some itindicates a long dry spell usually associatedwith lack of precipitation when crops shriveland reservoirs shrink To others it is a complexcombination of meteorological elementsexpressed in some form of moisture index (seeTable 32) There is no widely accepteddefinition of drought It is however very mucha human concept and many current approachesto the study of drought deal with moisturedeficiency in terms of its impact on human

activities particularly those involvingagriculture Agricultural drought is defined interms of the retardation of crop growth ordevelopment by reduced soil moisture levelsThis in turn may lead to economic definitions ofdrought when for example dry conditionsreduce yield or cause crop failure leading to areduction in income It is also possible to definedrought in purely meteorological terms wheremoisture deficiency is measured against normalor average conditions which have beenestablished through long-term observationssuch as those illustrated in Figure 31 (Katz andGlantz 1977)

Aridity and drought

The establishment of normal moisture levels alsoallows a distinction to be made between aridityand drought Aridity is usually considered to be

Table 32 Examples of drought and aridity indices

Source After Landsberg (1986) SymbolsP mean annual precipitation (mm)T mean annual temperature (degC)Pi individual monthly precipitation (mm)ti individual monthly temperature (degC)P^ climatically appropriate water balance for existing conditionsPE annual potential evapotranspirationL loss (mm)R mean monthly recharger number of months

the result of low average rainfall and is apermanent feature of the climatology of a region(see Figure 32a) The deserts of the world forexample are permanently arid with rainfallamounts of less than 100 mm per year Incontrast drought is a temporary featureoccurring when precipitation falls below normalor when near normal rainfall is made less effectiveby other weather conditions such as hightemperature low humidity and strong winds(Felch 1978)

Aridity is not a prerequisite for drought Evenareas normally considered humid may suffer fromtime to time but some of the worst droughts everexperienced have occurred in areas which include

some degree of aridity in their climatologicalmakeup Along the desert margins in Africa forexample annual precipitation is low rangingbetween 100 and 400 mm but under normalconditions this would allow sufficient vegetationgrowth to support pastoral agriculture Somearable activity might also be possible if dry-farming techniques were employed Droughtoccurs with considerable regularity in these areas(Le Houerou 1977) The problem lies not in thesmall amount of precipitation but rather in itsvariability Mean values of 100 to 400 mm arebased on long-term observations and effectivelymask totals in individual years which may rangewell above or below the values quoted (see Figure33) Weather records at Beijing in drought-pronenorthern China show that the city receives closeto 600 mm of precipitation in an average yearHowever the amount falling in the wetter yearscan be 6ndash9 times that of the drier years Only148 mm were recorded in 1891 for exampleand 256 mm in 1921 compared to a maximumof 1405 mm in 1956 (NCGCC 1990) Rainfallvariability is now recognized as a major factorin the occurrence of drought (Oguntoyinbo1986) and a number of writers have questionedthe use of lsquonormalrsquo values in such circumstancesIn areas of major rainfall variability the natureof the environment reflects that variability ratherthan the so-called normal conditions and anyresponse to the problems which arise fromdrought conditions must take that into account(Katz and Glantz 1977)

Some researchers claim that the drought insub-Saharan Africa has been intensifying as aresult of climatic change Bryson (1973) forexample has identified changes in atmosphericcirculation patterns which could intensify andprolong drought in the Sahel There is howeverno conclusive evidence that the current droughtis anything other than a further indication of theinherent unreliability of precipitation in the area(Nicholson 1989)

The human response to drought

Each generation has its images of drought In

Figure 31 Rainfall fluctuations in five regions ofAfrica 1901ndash87 expressed as a per cent departurefrom the long-term mean

the 1990s it is East Africa in the 1980s it wasEthiopia in the 1960s it was the Sahel in the1930s it was the Dustbowl described with suchfeeling by John Steinbeck in The Grapes ofWrath Although these have gripped the popularimagination they are only the more serious

examples of perhaps the most ubiquitousclimatological problem that society has to faceDrought strikes areas as far apart as Australiaand the Canadian Prairies or southern Africaand the China with some regularity often withconsiderable severity and with an impact that is

Figure 32a The deserts and arid lands of the world

Source Compiled from various sources

Figure 32b Areas at risk from desertification

felt beyond the areas directly affected Droughtis expected in such areas but elsewhere it is highlyirregular and as a result all the more seriousSuch was the case in 1976 when the normallyhumid UK was sufficiently dry that thegovernment felt it necessary to appoint a Ministerof Drought

Drought may also have played a significantrole in the historical development of societythrough its impact on past civilizations Forexample changes in wind circulation andincreased aridity in what is now northern Syriafrom about 2200 BC are thought to havedestroyed the agricultural base of theMesopotamian Subir civilization (Weiss et al1993) and the decay of the Harrapan civilizationin the Indus valley has been linked to thedesiccation of that region after 1800 BC (Calder1974) In Europe some 3000 years ago theflourishing Mycenaean civilization of southernGreece went into irreversible decline The rapidityand extent of the decline was such that it wascommonly attributed to the invasion of Mycenae

by Greeks from the regions to the north In 1968however Rhys Carpenter reassessed the availableevidence and suggested that no invasion hadtaken place Instead he postulated that droughtfollowed by starvation social unrest andmigration led to the downfall of the MycenaeansClimatologists at the University of Wisconsin-Madison developed Carpenterrsquos theory furtherand concluded that drought was a majorcontributory factor in the decay of the civilization(Bryson et al 1974 Bryson and Murray 1977)As in many cases where a climatic explanation isinvoked to account for major societal changes inthe distant past the link between the decline ofthe Mycenaean civilization and drought is notconsidered convincing by some researchers (seeeg Parry 1978) The absence of adequate andappropriate data prevents such hypotheses frombeing developed much beyond the speculativestage

Modern views of drought vary with time andplace and with the nature of the event itself Itmay be seen as a technological problem an

Figure 33 Variability of precipitation at selected stations in Africa

Source Compiled from data in Landsberg (1986)

economic problem a political problem a culturalproblem or sometimes a multi-faceted probleminvolving all of these Whatever else it may behowever it is always an environmental problemand basic to any understanding of the situationis the relationship between society andenvironment in drought-prone areas

Over thousands of years certain plants andanimals have adapted to life with limitedmoisture Their needs are met therefore nodrought exists This is the theoretical situationin most arid areas In reality it is much morecomplex for although the flora and fauna mayexist in a state of equilibrium with otherelements in the environment it is a dynamicequilibrium and the balance can be disturbedChanges in weather patterns for examplemight further reduce the already limited amountof precipitation available changing the wholerelationship If the plants and animals can nolonger cope with the reduced water supply theywill suffer the effects of drought Dependingupon the extent of the change plants may diefrom lack of moisture they may be forced out ofthe area as a result of competition with speciesmore suited to the new conditions or they maysurvive but at a reduced level of productivityThe situation is more complex for animals butthe response is often easier In addition torequiring water they also depend upon theplants for food and their fate therefore will beinfluenced by that of the plants They have onemajor advantage over plants however Beingcapable of movement they can respond tochanging conditions by migrating to areaswhere their needs can be met Eventually somedegree of balance will again be attainedalthough certain areasmdashsuch as the worldrsquosdesert marginsmdashcan be in a continual state offlux for long periods of time

The human animal like other species is alsoforced to respond to such changingenvironmental conditions In earlier times thisoften involved migration which was relativelyeasy for small primitive communities living byhunting and gathering in areas where the overallpopulation was small As societies changed

however this response was often no longerpossible In areas of permanent or even semi-permanent agricultural settlement with theirassociated physical and socio-economicstructures migration was certainly not anoptionmdashindeed it was almost a last resort Theestablishment of political boundaries which tookno account of environmental patterns alsorestricted migration in certain areas As a resultin those regions susceptible to drought thetendency perhaps even the necessity to challengethe environment grew If sufficient water was notavailable from precipitation either it had to besupplied in other waysmdashby well and aqueductfor examplemdashor different farming techniques hadto be adopted to reduce the moisture need in thefirst place The success of these approachesdepended very much on such elements as thenature intensity and duration of the droughtplus a variety of human factors which includedthe numbers stage of cultural development andtechnological level of the peoples involved

Types of drought

CWThornthwaite the eminent appliedclimatologist whose pioneering water balancestudies made a major contribution to theunderstanding of aridity recognized four typesof drought defined in terms of agriculturalrequirements (Thornthwaite 1947) These werepermanent seasonal contingent and invisibledrought Agriculture is not normally possible inareas of permanent drought since there isinsufficient moisture for anything but thexerophytic plants which have adapted to the aridenvironment Crops can be produced in suchareas but only at great expense or underexceptional circumstances such as those whichapply to the Israeli activities in the Negev Desertfor example (Berkofsky 1986) On the marginsof the worldrsquos great deserts there are regions ofseasonal drought where arid conditions prevailfor part of the year but which are balanced by adistinct wet season Much of India the Sahel andthe southern parts of Africa experience suchseasonal drought Agriculture is carried out often

very successfully during the wet season and evenduring the dry season if the moisture from thepreceding rainy season can be retained If forsome reason the rainy season is curtailedhowever the potential for drought is great andit is not surprising that areas such as the Saheland the Indian sub-continent have experiencedsome of the worldrsquos most spectacular andcatastrophic droughts The problem is intensifiedin drier years by the irregularity with which therains fall making planning difficult if notimpossible

Irregular and variable precipitation is alsocharacteristic of contingent drought InThornthwaitersquos definition this is experienced inareas which normally have an adequate supplyof moisture to meet crop needs Serious problemsarise because the agricultural system is not set

up to cope with unpredictable and lengthyperiods of inadequate precipitation The interiorplains of North America have suffered fromcontingent drought for hundreds of years andthe droughts of 1975ndash76 and 1988ndash92 in Britainwould fit this category also

The presence of these three types of droughtis indicated by physical changes in the soil andvegetation in the areas affected but there is alsoa fourth type which is less obvious This is theso-called invisible drought which often can beidentified only by sophisticated instrumentationand statistical techniques The crops appear tobe growing well even to the experienced observerand there is no obvious lack of precipitationHowever moisture requirements are not beingmet the crop is not growing at its optimum rateand the potential yield from the land is reduced

Figure 34 Sampleclimatic water budget fora mid-latitude station inNorth-America orEurope based on theThornthwaite model

Invisible drought can be dealt with relativelyeasily by irrigation In eastern Britain forexample supplementary moisture has beensupplied to sugar beet and potato crops since atleast the late 1950s to deal with that problem(Balchin 1964)

Determination of moisture deficit

To establish the existence of a deficit in an areait is necessary to compare incoming and outgoingmoisture totals The former is normallyrepresented by precipitation and the latter byevaporation from the earthrsquos surface plustranspiration by plants commonly combined intoone unit referred to as evapotranspiration In thesimplest relationship if precipitation exceedsevapotranspiration a water surplus will exist awater deficit results when the relationship isreversed Several factors complicate this simplesituation For example all soils have the abilityto store a certain amount of water and thisdisturbs the relationship between precipitationand evapotranspiration If the soil water storageis full the soil is said to be at field capacity andany additional precipitation not evaporated isconsidered as run-off The presence of soil waterwill help to offset any water deficit since evenif the evapotranspiration exceeds precipitationsome or all of the deficit may be made up fromthe soil moisture storage This has the effect ofdelaying the onset of drought until the storagecapacity is exhausted and illustrates one of thedangers of measuring drought by precipitationalone or even by some simple comparison ofprecipitation and evapotranspiration Onceprecipitation is again in excess the soil waterstorage must be recharged before a moisturesurplus exists

A second factor complicating the relationshipbetween precipitation and evapotranspirationand the existence of moisture deficiency involvesthe measurement of evapotranspirationEvapotranspiration will only occur if moistureis available Thus if the moisture directlyavailable from precipitation and in storage in thesoil is completely exhausted no

evapotranspiration can take place Yet even whenno water is available the environment may retainthe ability to cause evapotranspiration throughsuch elements as temperature radiationhumidity and air movement Thornthwaite(1948) developed the concept of potentialevapotranspiration to recognize this situationPotential evapotranspiration may be consideredthe amount of evaporation and transpiration thatwould take place if sufficient moisture wasavailable to fill the environmentrsquos capacity forevapotranspiration In this way the distinctionbetween measurable (actual) and theoretical(potential) amounts of evapotranspiration can berecognized As long as precipitation exceedsevapotranspiration the actual and potentialvalues will be the same but as soon as thesituation is reversed the two values begin todeviate at a rate which depends upon theavailability of soil moisture The differencebetween actual and potential evapotranspirationcan be considered as a measure of the waterdeficit and agricultural areas experiencingmoisture deficiency depend upon this approachto estimate the appropriate amount of moisturerequired to combat drought or to allow crops togrow at their full potential The relationshipsbetween elements such as precipitation actualand potential evapotranspiration water surplusand deficit as identified by Thornthwaite areillustrated in Figure 34

Modern methods of drought evaluation tendto focus on the soil moisture deficit (SMD) ratherthan the basic water deficit represented by theexcess of evapotranspiration over precipitation(see eg Palutikof 1986) Such factors as thenature and stage of development of the crop thestorage capacity of a specific soil and the easewith which the crop can extract moisture fromthe soil are all given greater attention than inThornthwaitersquos original approach to the problemThe SMD in a region will not be a specific valuebut will vary according to crop and soilconditions A consistently high SMD over thegrowing season however will ultimately lead todrought unless the situation is recognized andrectified

Thornthwaitersquos treatment of the measurementand classification of drought is only one of manyIt may not meet all needs but its broadlyclimatological approach lends itself to thegeographical examination of drought-proneenvironments The greatest human impact is feltin those areas which experience seasonal orcontingent drought although the nature of theimpact is different in each case The influence ofthe other two types of drought is limited In areasof permanent drought for example populationsare small and may exist only under specialcircumstances such as those at an oasis wherethe effects of the drought are easily counteredAlthough invisible drought may have importantconsequences for individuals it often passesunrecognized It does not produce the life anddeath concerns that prevail in areas of seasonaldrought nor does it have the dire economicimpacts that may be experienced by theinhabitants of areas of contingent drought

Seasonal drought in the sub-tropics

Seasonal drought is most commonly experienced inthe sub-tropics There the year includes a distinctdry season and a distinct wet season associatedwith the north-south movement of the intertropicalconvergence zone (ITCZ) and its attendant windand pressure belts (see Figure 213)

During the dry season these areas aredominated by air masses originating in the sub-tropical high pressure systemsmdashwhichcharacteristically contain limited moisture andare dynamically unsuited to produce muchprecipitation Anticyclonic subsidence preventsthe vertical cloud development necessary to causerain In contrast the rainy season is made possibleby the migration of the ITCZ behind which thecombination of strong convection and air massconvergence promotes the instability and strongvertical growth which leads to heavy rainfall Thepassage of the ITCZmdashin Africa India SE Asiaand Australiamdashallows the incursion of moistrelatively unstable air from the ocean over theland to initiate the wet season This is the basisof the monsoon circulation in these areas and it

is often the failure of this circulation that sets upthe conditions necessary for drought If forexample the ITCZ fails to move as far polewardsas it normally does those regions which dependupon it to provide the bulk of their yearly supplyof water will remain under the influence of thedry air masses and receive little or no rainfallSimilarly any increase in the stability of theairflow following the passage of the ITCZ willalso cause a reduction in water supply It isdevelopments such as these that have set the stagefor some of the worst droughts ever experienced

DROUGHT AND FAMINE IN AFRICA

Although seasonal drought is experienced in allof the worldrsquos sub-tropical areas in recent yearsthe greatest effects have been felt in sub-SaharanAfrica including the region known as the Sahelwhere drought is much more persistent thanelsewhere on the continent (Nicholson 1989)The Sahel proper is that part of western Africalying to the south of the Sahara Desert and northof the tropical rainforest It comprises six nationsstretching from Senegal Mauretania and Maliin the west through Burkina Faso to Niger andChad in the east This region with its populationof 33 million inhabiting slightly more than 5million sq km of arid or semi-arid land came toprominence between 1968 and 1973 when it wasvisited by major drought starvation and diseaseDespite this prominence it is in fact only partof a more extensive belt of drought-prone landin Africa south of the Sahara Drought pays noheed to political boundaries reaching as it doesto the Sudan Ethiopia and Somalia in the eastand including the northern sections of GhanaNigeria Cameroon the Central AfricanRepublic Uganda and Kenya In the 1980sdrought also extended into MozambiqueZimbabwe and other parts of eastern andsouthern Africa (see Figure 35)

The atmospheric circulation in sub-SaharanAfrica

The supply of moisture in all of these areas is

Figure 35 The distribution of drought famine and desertification in Africa

Source After CIDA (1985)

governed by seasonal fluctuations in the positionof the ITCZ Dry conditions are associated withhot continental tropical (cT) air from the Saharain the west and the Arabian Peninsula in the eastwhich moves in behind the ITCZ as it migratessouthwards during the northern hemispherersquoswinter At its most southerly extent in Januaryor February the ITCZ remains at about 8 degreesnorth of the equator in West Africa but curvessharply southwards across the centre of thecontinent to reach 15ndash20degS in East Africa (seeFigure 36) Apart from East Africa whichreceives some precipitation brought in off theIndian Ocean by the north-east trade winds ofthe winter monsoon most of the northern partof the continent experiences its dry season at thattime All of West Africa beyond a narrow stripsome 200 km wide along the coast is under theinfluence of a north-easterly airflow from thecentral Sahara This is known locally as theHarmattanmdasha hot dry wind which carries withit large volumes of fine dust from the desert On

occasion the dust is so thick that visibility isreduced to less than 1000 m and in combinationwith the break-up of radio transmissions causedby the high concentration of aerosols thisdisrupts local air traffic The scattering of solarradiation by the Harmattan haze also has broaderimplications for such activities as agriculturesolar energy engineering and environmentalplanning (Adetunji et al 1979) At the personallevel the low humidity of the dusty aircontributes to discomfort in the form of dry skinsore throats and cracked lips

The ITCZ moves north again during thenorthern summer and by July and August hasreached its most northerly location at about 20degN(see Figure 36) Hot moist maritime tropical(mT) air flows in behind it bringing the rainyseason In the east the moisture is provided byair masses from the Indian Ocean as part of theAsiatic monsoon system while in the west itarrives in the south westerly flow off the SouthAtlantic Although relatively simple to describe

Figure 36 Seasonal changes in the position of the ITCZ in Africa

in general terms in detail the precipitationpatterns are quite complex In West Africa forexample the existence of four weather zonesaligned east-west in parallel with the ITCZ wasrecognized by Hamilton and Archbold in 1945and these with some modern modifications (egMusk 1983) provide the standard approach tothe regional climatology of the area (see Figure37) Each of the zones is characterized by specificweather conditions which can be identified inthe precipitation regimes of the various stationsin the area (see Figure 38) The precipitation iscaused mainly by convection and convergenceand reaches the ground in a variety of formsranging from light intermittent showers to theheavy downpours associated with violentthunderstorms or line squalls The easterlytropical jet is also active in the upper atmosphereat this time and may well influence the amountintensity and distribution of precipitation(Kamara 1986)

Drought and human activity

Throughout the Sahelian region the peak of therainy season in July and August is also the timeof year when grass and other forage is mostwidely available for the herds of cattle camelsgoats and sheep belonging to the localagriculturalists The original herdsmen lived anomadic existence following the rains north inthe summer and south in the winter to obtainthe food their animals needed There wassufficient moisture available in the southern areasto allow a more permanent lifestyle supportedby basic arable agriculture producing sorghumand millet Drawn south by the rains theherdsmen eventually encroached upon thisfarmed land but instead of the conflict that mighthave been expected in such a situation the twosocieties enjoyed a basic symbiotic relationshipThe nomads exchanged meat and milk for grainthe cattle grazed the stubble and providedfertilizer for the following yearrsquos crop

Figure 37 North-South section across West Africa showing bands of weather associated with the ITCZ

Source After Hamilton and Archbold (1945)

Although the movement of the ITCZ isrepeated season after season it shows someirregularity in its timing and in the latitudinaldistance it covers each year As a result therainfall associated with it is never completelyreliable and this is particularly so in those areasclose to the poleward limits of its travel (Musk1983) In West Africa for example if the ITCZ

fails to reach its normal northern limits at about20degN the regions at that latitude will experiencedrought The greatest problems arise when suchdry years occur in groups The population mightbe able to survive one or two years of droughtbut a longer series has cumulative effects similarto those in the 1960s and 1970s when consecutiveyears of drought led to famine malnutrition

Figure 38 Precipitation regimes in West Africa

disease and death In contrast good years arethose in which the ITCZ with its accompanyingrain moves farther north than normal or remainsat its poleward limits for a few extra days oreven weeks

In the past this variability was very much partof the way of life of the drought-prone areas insub-Saharan Africa The drought of 1968 to 1973in the Sahel was the third major dry spell to hitthe area this century and although the yearsfollowing 1973 were wetter by 1980 drierconditions had returned (see Figure 39) By 1985parts of the region were again experiencing fullyfledged drought (Cross 1985a) A return toaverage rainfall in 1988 provided some respitebut it was short-lived and in 1990 conditionsagain equalled those during the devastatingdroughts of 1972 and 1973 (Pearce 1991b)Much as the population must have suffered inthe past there was little they could do about itand few on the outside showed much concernLike all primitive nomadic populations theinhabitants of the Sahel increased in good yearsand decreased in bad as a result of the checksand balances built into the environment

That situation has changed somewhat inrecent years The introduction of new scientificmedicine limited though it may have been byWestern standards brought with it previously

unknown medical servicesmdashsuch as vaccinationprogrammesmdashand led to improvements in childnutrition and sanitation Together these helpedto lower the death rate and with the birth rateremaining high populations grew rapidlydoubling between 1950 and 1980 (Crawford1985) The population of the Sahel grew at arate of 1 per cent per annum during the 1920sbut by the time of the drought in the late 1960sthe rate was as high as 3 per cent (Ware 1977)Similar values have been calculated for theeastern part of the dry belt in Somalia (Swift1977) and Ethiopia (Mackenzie 1987b) Initiallyeven such a high rate of growth produced noserious problems since in the late 1950s and early1960s a period of heavier more reliable rainfallproduced more fodder and allowed more animalsto be kept Crop yields increased also in the arableareas Other changes with potentially seriousconsequences were taking place at the same timehowever In the southern areas of the Sahel basicsubsistence farming was increasingly replaced bythe cash-cropping of such commodities aspeanuts and cotton The way of life of thenomads had already been changed by theestablishment of political boundaries in thenineteenth century and the introduction of cash-cropping further restricted their ability to moveas the seasons dictated Commercialization hadbeen introduced into the nomadic communityalso and in some areas market influencesencouraged the maintenance of herds larger thanthe carrying capacity of the land This was madepossible to some extent by the drilling or diggingof new wells but as Ware (1977) has pointedout the provision of additional water without aparallel provision of additional pasture onlyserved to aggravate ecological problems All ofthese changes were seen as improvements whenthey were introduced and from a socio-economicpoint of view they undoubtedly wereEcologically however they were suspect andin combination with the dry years of the 1960s1970s and 1980s they contributed to disaster

The grass and other forage dried out duringthe drought reducing the fodder available forthe animals The larger herdsmdashwhich had

Figure 39 August rainfall in the Sahel 1964ndash84

Source After Cross (1985a)

grown up during the years of plentymdashquicklystripped the available vegetation and exposedthe land to soil erosion The water holes andeventually even the rivers dried up and thoseanimals that had not died of starvation died ofthirst When the nomads tried to move theyfound that it was no longer as easy as it hadonce been The farmers who had formerlywelcomed the pastoralists for the meat and milkthey supplied and for the natural fertilizer fromtheir animals no longer wanted herds tramplingfields which with the aid of irrigation werebeing cropped all year round Furthermoregrowing peanuts and cotton and only a littlefood for their own use they had no excess leftfor the starving nomads Eventually as thedrought continued the farmers suffered alsoThere was insufficient water for the irrigationsystems the artificial fertilizers that hadreplaced the animal product was less effective atlow moisture levels and yields were reduceddramatically In a desperate attempt to maintaintheir livelihood they seeded poorer land whichwas soon destroyed by soil erosion in much thesame way as the overgrazed soil of the northhad been

The net result of such developments was thedeath of millions of animalsmdashprobably 5 millioncattle alonemdashand several hundred thousandpeople The latter numbers would have beenhigher but for the outside aid which providedfor 7 million people at the peak of the drought inthe Sahel between 1968 and 1973 (Glantz 1977)Similar numbers were involved in Ethiopiabetween 1983 and 1985 although accuratefigures are difficult to obtain because of civil warin the area (Cross 1985b) Drought and locustsdestroyed crops in the northern Ethiopianprovinces of Tigray and Eritrea again in 1987and the UN World Food Program estimated thatas many as 3 million inhabitants were put at riskof starvation and death (Mackenzie 1987a)Elsewheremdashin Kenya Uganda SudanZimbabwe and Mozambiquemdashsevere droughtcontinued into the 1990s Even in the Sahelwhich had experienced some improvement in thelate 1970s increasing aridity after 1980 was the

precursor of the more intense drought affectingthe area once again The death of the animalsthe destruction of the soil and indeed thedestruction of society has meant that all of sub-Saharan Africamdashfrom Senegal to Somaliamdashremains an impoverished region dependent uponoutside aid and overshadowed by the ever presentpotential for disaster the next time the rains failas fail they will

DROUGHT ON THE GREAT PLAINS

Since all of the nations stricken by drought insub-Saharan Africa are under-developed it mightbe considered that lack of economic andtechnological development contributed to theproblem To some extent it did but it is also quiteclear that economic and technologicaladvancement is no guarantee against droughtThe net effects may be lessened but theenvironmental processes act in essentially thesame way whatever the stage of development

This is well illustrated in the problems facedby farmers on the Great Plains that make up theinterior of North America (see Figure 310)Stretching from western Texas in the southalong the flanks of the Rocky Mountains to theCanadian prairie provinces in the north theyform an extensive area of temperate grasslandwith a semi-arid climate They owe their aridityin part to low rainfall but the situation isaggravated by the timing of the precipitationwhich falls mainly in the summer months whenhigh temperatures cause it to be rapidlyevaporated (see Figure 311) Contingentdrought brought about by the variable andunpredictable nature of the rainfall ischaracteristic of the areamdashconsecutive yearsmay have precipitation 50 per cent abovenormal or 50 per cent below normalmdashand thishas had a major effect on the settlement of theplains Averages have little real meaning undersuch conditions and agricultural planning isnext to impossible The tendency for wet or dryyears to run in series introduces furthercomplexity Strings of dry years during theexploration of the western plains in the

Figure 311 Graph of temperature and precipitation for Topeka Kansas

Figure 310 The Great Plains of North America

nineteenth century for example gave rise to theconcept of the Great American DesertAlthough the concept has been criticized forbeing as much myth as reality it is now evidentthat several expeditions to the western UnitedStates in the early part of the century (Lawsonand Stockton 1981) and some decades later towestern Canada (Spry 1963) encountereddrought conditions which are now known to bequite typical of the area and which have beenrepeated again and again since that time

Historical drought

Local drought is not uncommon on the plainsAlmost every year in the Canadian west thereare areas which experience the limitedprecipitation and high temperatures necessary forincreased aridity (McKay et al 1967) andarchaeological evidence suggests that from theearliest human habitation of the plains it has beenso (Van Royen 1937) The original inhabitantswho were nomadic hunters probably respondedin much the same way as the people of the Sahelwhen drought threatened They migratedfollowing the animals to moister areas At timeseven migration was not enough For exampleduring the particularly intense Pueblo droughtof the thirteenth century the population wasmuch reduced by famine (Fritts 1965) Majordrought episodes extensive in both time andplace are also a recurring feature of the historicalclimatology of the Great Plains (Bark 1978Kemp 1982) with the groups of years centredon 1756 1820 and 1862 being particularlynoteworthy (Meko 1992)

The weather was wetter than normal whenthe first European agricultural settlers moved intothe west in the late 1860s following the Civil WarThe image of the Great American Desert hadpaled and the settlers farmed just as they haddone east of the Mississippi or in the mid-westploughing up the prairie to plant wheat or cornBy the 1880s and 1890s the moist spell had cometo an end and drought once more ravaged theland (Smith 1920) ruining many settlers andforcing them to abandon their farms Ludlum

(1971) has estimated that fully half the settlersin Nebraska and Kansas left the area at that timelike the earlier inhabitants seeking relief inmigration in this case back to the more humideast Some of those who stayed experimentedwith dry-farming but even that requires amodicum of moisture if it is to be successfulOthers allowed the land to revert to its naturalstate and used it as grazing for cattle a use forwhich it was much more suited in the first placeThe lessons of the drought had not been learnedwell however Later in the 1920s the rainsseemed to have returned to stay and a newgeneration of arable farmers moved in Lured byhigh wheat prices they turned most of the plainsover to the plough seemingly unconcerned aboutthe previous drought (Watson 1963) Crops weregood as long as precipitation remained abovenormal By 1931 however the good years wereall but over and the drought of the 1930s incombination with the Depression created suchdisruption of the agricultural and social fabricof the region that the effects have reverberateddown through the decades Even today every dryspell is compared to the benchmark of theDustbowl The only possible response for manyof the drought victims of the 1930s wasmigration as it had been in the past The Okiesof The Grapes of Wrath leaving behind theirparched farms had much in common with theIndians who had experienced the Pueblo droughtsix centuries before The societies were quitedifferent but they felt the same pressures andresponded in much the same way By migratingboth were making the ultimate adjustment to ahostile environment

If the drought of the 1930s brought with ithardship and misery it also produced finally therealization that drought on the plains is anintegral part of the climate of the area Intensedry spells have recurred since thenmdashparticularlyin the mid 1970s and early 1980s (Phillips 1982)and again in 1987 and 1988mdashcausing significantdisruption at the individual farm and local levelBecause of the general acceptance of thelimitations imposed by aridity however theoverall impact was less than it would have been

half a century earlier New agriculturaltechniquesmdashinvolving dry farming as well asirrigationmdashcoupled with a more appropriate useof the land help to offset the worst effects of thearid environment but some problems will alwaysremain

Dry farming and irrigation

Dry farming is based on the preservation ofseveral years of precipitation to be used for theproduction of one crop The land is deepploughed and allowed to lie fallow for severalyears Deep ploughing provides a reservoir forthe rain that falls and various techniques are usedto reduce losses by evapotranspiration Perhapsonly a quarter of the total precipitation is madeavailable to the plants in this way but in Kansasand Nebraska grain yields may double after athree- to four-year fallow (More 1969)Obviously this is only possible if rain falls in thefirst place It might be necessary to counter thelack of precipitation by introducing water fromelsewhere in the hydrologic cycle and making itavailable through direct irrigation Most of themajor rivers on the plains have been dammedand the underlying aquifers tapped to providethe moisture required Grandiose schemes suchas the North American Water and Power Alliance(NAWAPA) which would bring water to theplains from the Hudson Bay and Arcticwatersheds in northern Canada have also beensuggested (Schindler and Bayley 1990) While thismay be ideal for agriculture it is not without itsenvironmental problems The larger projects havebeen roundly criticized for their ability to causecontinental scale environmental disruption buteven local or regional schemes can be harmfulThe rivers downstream from dams experiencereduced flow which produces physical changesin the stream channel and disrupts the balancein the aquatic environment Return flow fromirrigated fields contains fertilizer and pesticideresidues which alter the chemical compositionof the water The classic example of such changesis the Colorado River which at its mouth is amere trickle of highly salinated water flowing in

a channel much larger than the present volumerequires (Turk 1980)

Agriculture has resorted to the use ofgroundwater in those areas with no fluvial watersupply available Groundwater has severaladvantages over surface water such as reliabilityof supply and uniform quality but many of theaquifers beneath the plains have been soextensively used that the recharge rate has fallenfar behind demand and they are becomingdepleted This in turn leads to the need fordeeper wells and increased pumping time witha consequent rise in costs

Whatever the source of the additionalmoisture irrigation has one major problemassociated with it worldwide that is the growthof salinization or the build up of salts in the upperlayers of the soil It is a problem as old asirrigation itself (see eg Jacobsen and Adams1971) In areas of high temperature evaporationrates are generally high and this causes thesalinity of surface water in these areas to be highGroundwater also tends to have a high salt levelparticularly if it has been in the system for a longtime When the water is applied to the crops theevaporation loss is high and the salts remainbehind in the soil after the water has evaporatedThe salt build-up may be so great over time thatit interferes with the growth processes in theplants and they die or provide only much reducedyields In some cases the salt may be flushed outof the soil but the process is costly and notalways completely successful As a result landwhich has been affected by salinization may haveto be abandoned

There are also economic factors which mustreceive consideration along with these physicalproblems Although modern irrigation techniquesare remarkably successful they are also costly amodern central pivot system for example willrequire an investment well in excess of $1000per hectare The capital costs of providingequipment to combat drought in areas ofcontingent drought such as the Great Plainswhere major drought may occur perhaps onlyevery 15 to 20 years have to be weighed againstthe losses that would occur if no action is taken

Losses of as much as $16 billion have beenreported for the 1980 drought in the US GreatPlains (Karl and Quayle 1981) and a figure of$25 billion has been estimated for the 1984drought on the Canadian prairies (Sweeney1985) Amounts such as these would seem tosupport investment in irrigation equipment butsuch direct economic considerations may notalways satisfy environmental concerns If theinvestment is made there may be a tendency to

introduce irrigation into areas not particularlysuited to arable agriculture rather than haveequipment lying idle This may create noproblems in the short term but during the longerperiods of drought common on the plains theseareas would be the first to suffer If no investmentis made in times of drought there may be anattempt to offset the lower yields by bringingmore land under cultivation Whatever thedecision the net result is that additional land

Figure 312 The causes and development of desertification

becomes susceptible to environmentaldegradation

DESERTIFICATION

Although often severe the problems which arisein most areas experiencing seasonal or contingentdrought are seen as transitory disappearing whenthe rains return If the rains do not return theland becomes progressively more arid untileventually desert conditions prevail This is theprocess of desertification in its simplest formWhen considered in this way desertification is anatural process which has existed for thousandsof years is reversible and has caused the worldrsquosdeserts to expand and contract in the past Thisis however only one approach todesertificationmdashone which sees the process as thenatural expansion of desert or desert-likeconditions into an area where they had notpreviously existed This process occurring alongthe tropical desert margins was referred tooriginally as lsquodesertizationrsquo (Le Houerou 1977)but the more common term now islsquodesertificationrsquo and the concept has beenexpanded to include a human element

There is no widely accepted definition ofdesertification Most modern approacheshowever recognize the combined impact ofadverse climatic conditions and the stress createdby human activity (Verstraete 1986) Both havebeen accepted by the United Nations as theelements that must be considered in any workingdefinition of the process (Glantz 1977) TheUnited Nations Environment Program (UNEP)has tended to emphasize the importance of thehuman impact over drought but the relativeimportance of each of these elements remainsvery controversial Some see drought as theprimary element with human interventionaggravating the situation to such an extent thatthe overall expansion of the desert is increasedand any recoverymdashfollowing a change in climaticconditions for examplemdashis lengthier thannormal Others see direct human activities asinstigating the process In reality there must bemany causes (see Figure 312) which together

bring desert-like conditions to perhaps as muchas 60000 sq km of the earthrsquos surface every yearand threaten up to 30 million sq km more Theareas directly threatened are those adjacent tothe deserts on all continents Africa is currentlyreceiving much of the attention but large sectionsof the Middle East the central Asian republicsof the former Soviet Union China adjacent tothe Gobi Desert northwest India and Pakistanalong with parts of Australia South America andthe United States are also susceptible todesertification Even areas not normallyconsidered as threatened such as southernEurope from Spain to Greece are not immune(see Figure 32b) At least 50 million people aredirectly at risk of losing life or livelihood in theseregions In a more graphic illustration ofdesertification the United States Agency forInternational Development (USAID) at theheight of the Sahelian drought in 1972 claimedthat the Sahara was advancing southwards at arate of as much as 30 miles (48 km) (Pearce1992f) Although these specific numbers are notuniversally acceptedmdashNelson (1990) forexample has suggested that all such data betreated with a healthy scepticismmdashthey do givean indication of the magnitude of the problemand the reason that there has been increasingcause for concern in recent years (van Yperseleand Verstraete 1986)

Desertification initiated by drought

Human nature being what it is attitudes todrought include a strong element of denial andwhen drought strikes there is a natural tendencyto hope that it will be short and of limitedintensity According to Heathcote (1987) forexample the traditional Australian approach todrought has been to denigrate it as a hazard andto react to its onset with surprise Theinhabitants of drought-prone areas thereforemay not react immediately to the increasedaridity particularly if they have progressedbeyond the huntinggathering socio-economiclevel They may continue to cultivate the samecrops perhaps even increasing the area under

cultivation to compensate for reduced yields orthey may try to retain flocks and herds whichhave expanded during the times of plenty If thedrought is prolonged in the arable areas thecrops die and the bare earth is exposed to theravages of soil erosion The Dustbowl in theGreat Plains developed in this way Once theavailable moisture had been evaporated andthe plants had died the wind removed thetopsoilmdashthe most fertile part of the soilprofilemdashleaving a barren landscape which eventhe most drought-resistant desert plants founddifficult to colonize (Borchert 1950) In theabsence of topsoil there was nothing to retainthe rain which did fall It rapidly ran off thesurface causing further erosion or percolatedinto the ground-water system where it wasbeyond the reach of most plants

Prolonged drought in pastoral areas isequally damaging It reduces the forage supplyand if no attempt is made to reduce the animalpopulation the land may fall victim toovergrazing The retention of larger herdsduring the early years of the Sahelian droughtfor example allowed the vegetation to beovergrazed to such an extent that even the plantroots died In their desperation for food theanimals also grazed on shrubs and even treesand effectively removed vegetation which hadhelped to protect the land The flocks and herdshad been depleted by starvation and death bythe time this stage was reached but the damagehad been done The wind its speed unhamperedby shrubs and trees lifted the exposed loose soilparticles and carried them away taking withthem the ability of the land to support plant andanimal life In combination these human andphysical activities seemed to be pushing theboundaries of the Sahara Desert inexorablysouthwards to lay claim to territory which onlyrecently supported a population living ascomfortably as it could within the constraints ofthe environment Out of this grew the image ofthe lsquoshifting sandsrsquo which came to representdesertification in the popular imagination Asan image it was evocative but the reality ofsuch a representation has been increasingly

questioned in the 1990s (Nelson 1990 Pearce1992f)

Desertification caused by human activity

Climatic variability clearly made a majorcontribution to desertification in both the Saheland the Great Plainsmdashperhaps even initiatingthe processmdashand in concert with humanactivities created serious environmentalproblems An alternative view sees humanactivity in itself capable of initiatingdesertification in the absence of increasedaridity (Verstraete 1986) For example humaninterference in areas where the environmentalbalance is a delicate one might be sufficient toset in motion a train of events leading eventuallyto desertification The introduction of arableagriculture into areas more suited to grazing orthe removal of forest cover to open upagricultural land or to provide fuelwood maydisturb the ecological balance to such an extentthat the quality of the environment begins todecline The soil takes a physical beating duringcultivation its crumb structure is broken downand its individual constituents are separatedfrom each other In addition cultivationdestroys the natural humus in the soil and thegrowing crops remove the nutrients both ofwhich normally help to bind the soil particlestogether into aggregates If nothing is done toreplace the organic material or the nutrients thesoil then becomes highly susceptible to erosionModern agricultural techniques which allowthe soil to lie exposed and unprotected byvegetation for a large part of the growingseason also contribute to the problem Whenwind and water erode the topsoil it becomesimpossible to cultivate the land and evennatural vegetation has difficulty re-establishingitself in the shifting mineral soil that remains

The removal of trees and shrubs to be used asfuel has had similar effects in many Third Worldnations where the main source of energy is woodIn Sudan for example the growing demand forfuelwood was a major factor in the reduction ofthe total wood resource by 36 per cent annually

in the 1970s and early 1980s (Callaghan et al1985) Le Houerou (1977) has estimated thatin the areas along the desert margins in Africaand the Middle East a family of five willconsume every year all of the fuel available onone hectare of woody steppe With a populationclose to 100 million dependent upon this formof fuel in the area concerned as much as 20million hectares per year are being destroyed andall of that area is potentially open todesertification

No change in the land-use is required toinitiate such a progression in some regions Theintroduction of too many animals into an areamay lead to overgrazing and cause suchenvironmental deterioration that after only a fewyears the land may no longer be able to supportthe new activity Forage species are graduallyreplaced by weeds of little use to the animalsand the soil becomes barren and unable to recovereven when grazing ceases In all of these casesthe land has been laid waste with little activecontribution from climate Human activities havedisturbed the environmental balance to such anextent that they have effectively created a desert

Although human activities have been widelyaccepted as causing desertification and theprocesses involved have been observed there isincreasing concern that the human contributionhas been overestimated Current academic andpopular attitudes to desertification owe a lot tothe findings of a United Nations Conference onDesertification (UNCOD) held in NairobiKenya in 1977 At that conference the role ofhuman activities in land degradation wasconsidered to be firmly established and thecontribution of drought was seen as secondaryat best Since human action had caused theproblem it seemed to follow that human actioncould solve it In keeping with this philosophyUNEP was given the responsibility for takingglobal initiatives to introduce preventivemeasures which would alleviate the problem ofdesertification (Grove 1986) Fifteen years and$6 billion later few effective counter measureshave been taken and the plan of action is widelyseen as a failure (Pearce 1992a)

The data upon which the UNEP responseswere based are now considered by manyresearchers to be unrepresentative of the realsituation Nelson (1990) for example hassuggested that the extent of irreversibledesertification has been over-estimatedalthough he does not deny that it remains aserious concern in many parts of the world Themain problems with the data arose from thetiming and method of collection and wereaggravated by the UNEP premise that humanactivity was the main cause of the landdegradation that produced desertification Thebasic data apparently indicating the rapidcreation of desert-like conditions were collectedin the early 1970s at a time of severe drought insub-Saharan Africa They therefore failed togive sufficient weight to the marked rainfallvariability characteristic of the area and inconsequence over-represented the effects of thedrought A great deal of the information wasobtained using remote sensing The changinglocation of vegetation boundaries wereidentified from satellite photography forexample This seemed to confirm the steadyencroachment of the desert in areas such as theSudan and the results were incorporated in theUNCOD report of 1977 Since then howeverthey have been widely disputed and subsequentstudies have found no evidence that the largescale desertification described in the 1970scontinued into the 1980s (For summaries ofcurrent thinking on desertification see Nelson(1990) Pearce (1992f) and Hulme and Kelly(1993))

Failure to appreciate the the extent of annualfluctuations in vegetation boundariesmdashdifferences of as much as 200 km were reportedon the SudanChad border between 1984 and1985mdashcombined with inadequate groundcontrol may have contributed to the problem(Nelson 1990) A general consensus seems to beforming among those investigating the issue thatthe approach to defining desertification in termsof vegetation needs to be re-examined In partsof East Africa for example drought and possiblyovergrazing have combined to allow the normal

grass cover to be replaced by thorn scrub Onsatellite photography this appears as animprovement in vegetative cover yet from ahuman and ecological viewpoint it is a retrogradestep (Warren and Agnew 1988) A more accurateapproach to land degradation might be to studythe soil Vegetation responds rapidly to short-term changes in moisture but damage to soiltakes much longer to reverse Thus themeasurement of soil conditionsmdashnutrient levelsfor examplemdashmight give a more accurateindication of the extent of land degradation(Pearce 1992f)

UNEPrsquos insistence on explaining mostdesertification as the result of human activitiesmay also have contributed to themisrepresentation of the extent of the problemNatural causes such as short-term drought andlonger-term climatic change were ignored orgiven less attention than they deserved yet bothcan produce desert-like conditions withoutinput from society With short-term droughtthe vegetation recovers once the drought is overwith changes induced by lengthier fluctuationslittle improvement is likely even if the humanuse of the land is altered The inclusion of areassuffering from short-term drought may wellhave inflated the final results in the UNEPaccounting of land degradation Failure toappreciate the various potential causes ofdesertification would also limit the response tothe problem Different causes would normallyelicit different responses and UNEPrsquosapplication of the societal response to all areaswithout distinguishing the cause may in partexplain the lack of success in dealing with theproblem (Pearce 1992f)

The prevention and reversal of desertification

The debunking of some of the myths associatedwith desertification and the realization that evenafter more than 15 years of study its nature andextent are inadequately understood does notmean that desertification should be ignoredThere are undoubtedly major problems of landdegradation in many of the earthrsquos arid lands

Perhaps sensing an increased vulnerability as aresult of the current controversy and certainlyfearful of being left behind in the rush to dealwith the problems of the developed world thenations occupying the land affected appeared atthe Rio Earth Summit in 1992 and proposed aDesertification Convention to address theirproblems Despite its lack of success followingUNDOC the provisions of the Convention arelikely to be managed by UNEP which has

Table 33 Action required for the prevention andreversal of desertification

proposed a budget of $450 billion over 20 years(Pearce 1992f)

Although obscured by the current controversyand lost in the complexity of the attempts todefine the issue of desertification more accuratelytwo questions remain of supreme importance tothe areas suffering land degradation Candesertification be prevented Can thedesertification which has already happened bereversed In the past the answer to both hasalways been a qualified yes (see Table 33) andseems likely to remain so although someresearchers take a more pessimistic view (egNelson 1990) In theory society could work withthe environment by developing a goodunderstanding of environmental relationships inthe threatened areas or by assessing the capabilityof the land to support certain activities and byworking within the constraints that these wouldprovide In practice non-environmentalelementsmdashsuch as politics and economicsmdashmayprevent the most ecologically appropriate use ofthe land A typical response to the variableprecipitation in areas prone to desertificationis to consider the good years as normal and toextend production into marginal areas at thattime (Riefler 1978) The stage is then set forprogressive desertification when the bad yearsreturn Experience in the United States has shownthat this can be prevented by good land-useplanning which includes not only considerationof the best use of the land but also the carryingcapacity of that land under a particular use(Sanders 1986) To be effective in an area suchas Africa this would involve restrictions ongrazing and cultivation in many regions but notonly that Estimates of the carrying capacity ofthe land would have to be based on conditionsin the worst years rather than in the good or evennormal years (Kellogg and Schneider 1977Mackenzie 1987b) Such actions wouldundoubtedly bring some improvement to thesituation but the transition between the old andnew systems might well be highly traumatic forthe inhabitants of the area Stewart and Tiessen(1990) for example point out that in the Sahelcattle are a form of crop insurance When the

crops fail farmers plan to survive by selling someof their cattle Any attempt to limit herds andprevent overgrazing must therefore beaccompanied by a suitable replacement for thistraditional safety net Without it the pastoralistswould lose the advantages of larger flocks andherds in the good years Some of the cultivatorsmight have to allow arable land to revert topasturemdashor reduce cash-cropping and return tosubsistence agriculturemdashand members of bothgroups might have to give up their rural life-styleand become urbanized

As with the other elements associated withland degradation the widely acceptedrelationship that linked growing numbers oflivestock to overgrazing and the eventual onsetof desertification has been questioned (Nelson1990) This in turn has led to a reassessment ofthe methods by which desertification in pastoralareas can be prevented Traditional herdingtechniques involving nomadism and theacceptance of fluctuating herd sizes seemparticularly suited to the unpredictableenvironment of an area such as the Sahel Thenomadic herdsmen of that region may in fact bebetter managers of the land than the farmers inthe wetter areas to the south (Warren and Agnew1988 Pearce 1992f) They may also know moreabout dealing with pastoral land-use problemsthan they are given credit for by scientists fromthe developed nations Pearce (1992f) hasconcluded that there is no evidence that nomadicherdsmen in places like the Sahel actually destroytheir pastures They represent the best way touse land in an area where there is no naturalecological equilibrium only constant flux Thatsituation may apply only as long as the fluxremains within certain established limits Toomuch change in one direction as occurs duringmajor drought episodes without concomitantchange in herding methods could still encouragedesertification Although the links between herdsize overgrazing and land degradation areconsidered less firm than was once thought theyhave not yet been disproved and it does notfollow that they are entirely absent In attemptsto deal with desertification in pastoral areas such

links cannot be ignored They clearly needadditional investigation

The problem of the destruction of woodlandwill also have to be addressed if desertificationis to be prevented Trees and shrubs protect theland against erosion yet they are being clearedat an alarming rate One hundred years ago inEthiopia 40 per cent of the land could beclassified as wooded today only 3 per cent canbe designated in that way (Mackenzie 1987b)Good land-use planning would recognize thatcertain areas are best left as woodland andwould prevent the clearing of that land for theexpansion of cultivation or the provision of fuelwood The latter problem is particularly seriousin most of sub-Saharan Africa where wood isthe only source of energy for most of theinhabitants It also has ramifications whichreach beyond fuel supply Experience hasshown that where wood is not available animaldung is burned as a fuel and although that maysupply the energy required it also represents aloss of nutrients which would normally havebeen returned to the soil Any planninginvolving the conservation of fuelwood mustconsider these factors and make provision foran alternative supply of energy or anothersource of fertilizer

Many of the techniques which could beemployed to prevent desertification are alsoconsidered capable of reversing the processCertainly there are areas where the destructionof the land is probably irreversible but there havealso been some successes In parts of NorthAmerica land apparently destroyed in the 1930shas been successfully rehabilitated through land-use planning and direct soil conservationtechniques such as contour ploughing stripcropping and the provision of windbreaksIrrigation has also become common and methodsof weather modification mainly rain makinghave been attempted although with inconclusiveresults (Rosenberg 1978) Many of these methodscould be applied with little modification in areassuch as Africa where desertification is rampantDry-farming techniques have been introducedinto the Sudan (CIDA 1985) in Ethiopia new

forms of cultivation similar to contour ploughinghave been developed to conserve water andprevent erosion (Cross 1985b) in Mali and otherparts of West Africa reforestation is beingattempted to try to stem the southward creep ofthe desert (CIDA 1985) Most observers considerthe success rate of such ventures to be limited(Pearce 1992f) Problems arise from theintroduction of inappropriate technology fromthe unwillingness of farmers or pastoralists toadopt the new methods and from a variety ofeconomic factors including for examplefertilizer costs and the availability of labour(Nelson 1990) There is clearly no universalpanacea for desertification Solutions will haveto continue to be specific to the issue and thelocation but even then there can be no guaranteethat solutions which appear ideal in the short-term will not ultimately exacerbate the problemThe lack of moisture has been tackled directly inmany areas for example by the drilling ofboreholes to provide access to groundwaterLogical as this may seem without strict controlit may not be the best approach Extra waterencourages larger flocks and herds whichovergraze the area around the borehole LeHouerou (1977) has pointed out that aroundsome of the boreholes drilled at the time of theSahelian drought the pasture was completelydestroyed within a radius of 15ndash30 km aroundthe bore Subsequent investigation however hassuggested that this was primarily a local problemat only a few wells and its impact was thereforemuch less than originally estimated (Pearce1992f)

All of these developments deal directly withthe physical symptoms of desertification but ithas been argued that many studies have over-looked economic and social constraints (Hekstraand Liveman 1986) Ware (1977) has suggestedfor example that insufficient development ofmarkets transportation and welfare systemsmade a major contribution to the problems inthe Sahel and future planning must give thesefactors due consideration The profit motive isalso an important factor in some areas In Kenyaexperience in agroforestry schemes indicates that

the best results are achieved when the farmerscan see clear and immediate rewards in additionto the less obvious longer-term environmentalbenefits (Peacutegorieacute 1990) On the human sidepopulation growth rates and densities must beexamined with a view to assessing humanpressure on the land Over-population hastraditionally been regarded as an integral part ofthe droughtfamine desertification relationshipbut that too has been re-examined Mortimore(1989) for example has suggested that highpopulation densities may not be out of place inareas where proposed soil and water conservationschemes are labour intensive Ironically someareas suffer from rural depopulation In the early1980s urban populations across Africa increasedat about 6 per cent per year due in large part tothe exodus from rural areas (Grove 1986) Whererelief from population pressure is needed it maycome in the form of family planning or throughrelocation Despite potentially serious social andpolitical concerns these may be the only waysto tackle the population problem (Mackenzie1987b)

The fight against desertification has beenmarked by a distinct lack of success Recentreassessments of the problem beginning in thelate 1980s suggest that this may be the result ofthe misinterpretation of the evidence and apoor understanding of the mechanisms thatcause and sustain the degradation of the landThe additional research required to resolve thatsituation will further slow direct action againstdesertification but it may be the price that hasto be paid to ensure future success

DROUGHT PREDICTION

Even if all of these methods of dealing withdrought famine and desertification were to beinitiated immediately the results would be along time coming In Africa this means that theexisting and recurring problems of drought andfamine must receive continuing and immediateattention To be effective such aid requires anearly warning of the problem fast response andtimely delivery of relief The last two are

essentially socio-economic elements but thefirst is physical and it has led to numerousattempts by climatologists in recent years todevise a method by which drought may bepredicted

Drought is not the sole cause of famine ordesertification but it is certainly a major causeoften initiating the problem only to have itintensified by other factors Prevention ofdrought is not feasible at present nor would itnecessarily bring about an end to famine anddesertification if it was If drought could bepredicted however responses could beplanned and the consequences therefore muchreduced The simplest approach is the actuarialforecast which estimates the probability offuture drought based on past occurrences Tobe successful actuarial forecasting requires alengthy sequence of data for analysis(Schneider 1978) In many areas includingsub-Saharan Africa the record is simply tooshort to provide a reliable prediction Problemswith the homogeneity of the meteorologicalrecord may also reduce the significance of theresults

An extension of the actuarial approach is thelinking of meteorological variables with someother environmental variable which includes arecognized periodicity in its behaviour(Oguntoyinbo 1986) One of the mostcommonly cited links of this type is therelationship between sunspot activity andprecipitation (see Figure 313) In NorthAmerica drought on the plains has beencorrelated with the minimum of the 22-yeardouble sunspot cycle The drought years of themid-1970s for example coincided with aperiod of minimum sunspot activity Theprevious drought some 20 years earlier in themid-1950s also fitted into the cycle Close assuch a correlation may seem it applies less welloutside the western United States Furthermorethe relationship remains a statistical one andas Schneider (1978) has pointed out there is nophysical theory to explain the connectionbetween the two phenomena

In the search for improved techniques of

drought prediction much time and effort has goneinto the study of the physical causes of droughtThe immediate causes commonly involve changesin atmospheric circulation patterns Drought inthe western prairies of Canada and the UnitedStates for example is promoted by a strong zonalairflow which brings mild Pacific air across thewestern mountains As it flows down the easternslopes it warms up and its relative humiditydecreases causing the mild dry conditions inwinter and the hot dry conditions in summerwhich produce drought (Sweeney 1985)Seasonal drought in the Sahel was long linked tothe failure of the ITCZ to move as far north asnormal during the northern summer (see Figure314) This is no longer generally accepted assufficient to explain the lengthier dry spellshowever and the Sahelian drought is now beingexamined as part of the broader pattern ofcontinent-wide rainfall variability associatedwith large-scale variations in the atmosphericcirculation (Nicholson 1989)

This type of knowledge is in itself of limitedhelp in predicting or coping with drought sinceby the time the patterns are recognized thedrought has already arrived It is necessary tomove back a stage to try to find out what caused

the circulation change in the first place OverNorth America for example circulation patternsseem to be related to changing sea surfacetemperatures in the North Pacific which causethe course of the upper westerlies to be alteredcreating a zonal flow across the continent Thefailure of the ITCZ to move as far north asnormal into the Sahel has also been linked to astrengthening of the westerlies in the northernhemisphere This prevents the polewardmigration of the sub-tropical anticyclone lyingover the Sahara and it remains in place to blockthe rain-bearing winds which would normallyflow over the area from the Atlantic (Bryson1973) The drought in Africa has also been

Figure 313 Drought and sunspot cycles in western North America

Source After Schneider and Mtesirow (1976) Lockwood (1979)

Figure 314 Variations in the northward penetration ofthe monsoon rains in the Sahel 1950ndash72

Source After Bryson and Murray (1977)

blamed on the intensification of the tropicalHadley cell The descending arm of that cell isresponsible for the general aridity in the regionand any increase in its strength or extent isaccompanied by further suppression ofprecipitation particularly in areas peripheral tothe desert The synchronicity of wet and dryyears north and south of the Sahara supportsthis explanation (Nicholson 1981)

It has also been suggested that human activitieshave caused lsquodegradation induced droughtrsquo bycontributing to the process by which the Hadleycell is intensified Charney (1975) proposed thatovergrazing and wood cutting in the Sahelincreased the surface albedo and disrupted theregional radiation balance Surface heatingdeclined as more solar radiation was reflectedand this in turn caused some cooling of theatmosphere This cooling encouraged subsidenceor augmented existing subsidence and helped toreduce the likelihood of precipitation by retardingconvective activity With less precipitationvegetation cover decreased and the albedo of thesurface was further enhanced This process wasdescribed as a biogeophysical feedbackmechanism It received considerable attention inthe 1980s because it seemed to fit the observationthat drought in the Sahel was more persistentthan elsewhere in Africa While such persistencemay appear to be a function of the positivefeedback mechanism central to the theory itcannot be taken as confirmation of it (Hulme1989)

As with many of the efforts to explain droughtin Africa it was difficult to develop the theoryof degradation-induced drought further becauseof the lack of empirical evidence Following astudy of long-term changes in African rainfallNicholson (1989) concluded that the humancause of drought as espoused by Charney didnot seem feasible but allowed that surfacechangesmdashwhatever the causemdashmight feed backinto the system to reinforce the droughtEstablishing the nature and extent of thesesurface changes has not been easy The mostobvious approach would be the use of satelliteremote sensing to estimate both vegetation

destruction and changes in the surface albedoHowever the controversy over the interpretationof satellite data in assessing the extent ofdesertification has yet to be resolved andproblems still remain in the use of satellite deriveddata in this type of modelling (Thomas andHenderson-Sellers 1987) The existing datarecord is also short Hulme (1989) in hisassessment of the theory estimated that changesof the magnitude proposed by Charneymdasha 14 to35 per cent increase in albedomdashwould requirevegetation destruction on at least a sub-continental scale over a period of as much as 20years As yet there is no evidence of this but thatmay be in part a function of the inadequacy ofthe data and he concluded that the theory is atbest lsquonot provenrsquo Since then Balling (1991) hascalculated that desertification may actually havecaused surface air temperatures to increase(rather than decrease as Charneyrsquos theoryrequires) although Hulme and Kelly (1993) havesuggested that Ballingrsquos estimates of warming aretoo high

While such studies may ultimately provide abetter understanding of the problems of droughtand the mechanisms involved they areinsufficient to provide a direct forecastingmechanism Researchers have re-examinedcertain relationships in the earthatmospheresystem in search of something more suitableTheir approach is based on the observation thatthe various units in the system are interconnectedin such a way that changes in one unit willautomatically set in motion changes in othersthrough autovariation Since many of the changesare time-lagged it should be possible to predictsubsequent developments if the original changecan be recognized This forms the basis of theconcept of teleconnection or the linking ofenvironmental events in time and place

In recent years the search for a droughtforecasting mechanism involving teleconnectionhas centred on changing conditions in the worldrsquosoceans Sea-surface temperatures (SSTs) havebeen examined as possible precursors of thecirculation patterns which cause drought in theSahel and a correlation between global SSTs and

drought has been established (Folland et al 1986Owen and Ward 1989) When southernhemisphere ocean temperatures exceed those inthe northern hemisphere rainfall in the Sahel isbelow average The warmer southern oceans mayreduce the strenghth of the ITCZ leading to lessuplift and therefore less precipitation When theworld began warming in the 1980s the southernoceans warmed first and fastest and the yearswith record global temperaturesmdash1983 1987and 1990mdashwere also years in which the rainsfailed Only 1988 did not fit the pattern andPearce (1991b) has drawn attention to thepossibility that if global warming continues asexpected drought in the Sahel might only getworse

The links established between SSTs anddrought in sub-Saharan Africa have allowed theUK Meteorological Office through the HadleyCentre for Prediction and Research to issuerainfall forecasts for the area (Owen and Ward1989) These involve multi-stage predictions Tobe useful the forecasts have to be supplied byApril but it is the SSTs in June and July thatcorrelate most closely with the rainfall Thus theprecipitation forecast is based on SSTs predictedtwo months ahead and any changes betweenApril and June will reduce the quality of theforecast Apart from 1988 when a strong LaNintildea caused a major cooling of the tropicalPacific and ruined the forecast the predictionshave been remarkably accurate (Pearce 1991b)Although the lead time is short the approach isone of the most promising yet developed fordrought prediction

ENSO events in the south Pacific have alsoreceived considerable attention as potentialprecursors of drought It has long been knownthat following an El Nintildeo changes occur in windfields sea surface temperatures and oceancirculation patterns The large shifts of air andwater associated with these developments causemajor alterations to energy distribution patternsZonal energy flow replaces the meridional flowwhich is normal in tropical Hadley cells andbecause of the integrated nature of theatmospheric circulation the effects are eventually

felt beyond the tropics Reduced rainfall has beennoted in a number of semi-arid areas followingsuch episodes and teleconnections have beenestablished between ENSO events andprecipitation in areas as far apart as Brazil IndiaIndonesia and Australia Drought in north-eastern Brazil commonly occurs in conjunctionwith an El Nintildeo event and India receives lessmonsoon rainfall during El Nintildeo years Therelationship is well-marked in India wheremonsoon rainfall over most of the country wasbelow normal in all of the 22 El Nintildeo yearsbetween 1871 and 1978 (Mooley andParthasarathy 1983) Similarly in Australia 74per cent of the El Nintildeo events between 1885 and1984 were associated with drought in some partof the interior of the continent (Heathcote 1987)Such figures suggest the possibility of El Nintildeoepisodes being used for drought prediction insome parts of the world

There is no clear relationship between droughtin the Sahel and the occurrence of ENSO events(Lockwood 1986) Semazzi et al (1988) havesuggested that sub-Saharan rainfall is linked toENSO events through the influence of the latteron SSTs in the Atlantic However SST anomaliesdo occur in years when El Nintildeos are poorlydeveloped or absent Thus although an ENSOepisode may be a contributory factor in thedevelopment of drought in the Sahel it does notseem to be a prerequisite for that developmentAssociated with ENSO is La Nintildea which hasphysical characteristics completely opposite tothose of El Nintildeo It is a cold current rather thana warm one and flows west instead of east Theclimatological impact of La Nintildea also seems tobe opposite to that of El Nintildeo At the time of thelast major La Nintildeamdashin 1988mdashheavy rain causedflooding in Bangladesh Sudan and Nigeria Thedrought forecast for the Sahel that year did notcome to pass Instead the region experienced oneof its wettest years in recent decades (Pearce1991b) There is as yet too little data to establisha link between La Nintildea and rainfall variabilitybut it is a relationship that merits additionalinvestigation

Teleconnection links remain largely

theoretical although most of the relationshipscan be shown to be statistically significantCertainly in the study of drougmht it has notbeen possible to establish physical relationshipswhich would allow increasing aridity to bepredicted with any accuracy However it seemsprobable that future developments in droughtprediction will include consideration ofteleconnections particularly those involvingtime-lags established in conjunction with theimprovement of general circulation models(Oguntoyinbo 1986)

SUMMARY

In many parts of the world low precipitationlevels combine with high evapotranspirationrates to produce an environment characterizedby its aridity Under natural conditions theecological elements in such areas are in balancewith each other and with the low moisture levelsIf these change there is a wholesale readjustmentas the environment attempts to attain balanceagain The early inhabitants of these areas alsohad to respond to the changes and as long astheir numbers remained small and their way oflife nomadic they coped remarkably well Ashuman activities became more varied andtechnologically more sophisticated as the wayof life became more sedentary and populationsincreased the stage was set for problems witharidity Environmentally appropriate responsesto aridity were no longer possible and the effectsof drought were magnified The failure of cropsand the decimation of flocks and herds causedstarvation and death In some areas the

combination of drought and unsuitableagricultural practices created desert-likeconditions

Many of the problems associated withdrought famine and desertification stem fromhumankindrsquos inability to live within theconstraints of an arid environment and onesolution would be to restrict activities in drought-prone regions Given existing political culturaland socio-economic realities such an approachis not feasible in most of the affected areasUnfortunately many of the alternative solutionsare short-term in their impactmdashof necessity inmany casesmdashand in some areas at least may besetting up even greater difficulties in the years tocome In short solutions to the problems ofdrought famine and desertification are unlikelyto be widely available in the foreseeable futureand the images generated throughout sub-Saharan Africa in the last two decades are likelyto recur with sickening frequency

Mortimer M (1989) Adapting to DroughtFarmers Famines and Desertification in WestAfrica Cambridge Cambridge UniversityPress

Royal Meteorological Society (1989) lsquoSpecialAfrica Issuersquo Weather 44(2) London RoyalMeteorological Society

Steinbeck J (1958) The Grapes of Wrath NewYork Viking Press

UNCOD (1977) Desertification Its Causes andConsequences Oxford Pergamon Press

The unrelenting pollution of the atmosphere bymodern society is at the root of several globalenvironmental issues The emission of pollutantsinto the atmosphere is one of the oldest humanactivities and the present problems are only themost recent in a lengthy continuum Issues suchas acid rain increased atmospheric turbidity andthe depletion of the ozone layer includeessentially the same processes which led to theurban air pollution episodes of a decade or twoago The main difference is one of scale Theprevious problems had a local or at most aregional impact The current issues are global inscope and therefore potentially morethreatening

THE NATURE AND DEVELOPMENT OFACID RAIN

Acid rain is normally considered to be a by-product of modern atmospheric pollution Evenin a pure uncontaminated world however it islikely that the rainfall would be acidic Theabsorption of carbon dioxide by atmosphericwater produces weak carbonic acid and nitricacid may be created during thunderstormswhich provide sufficient energy for the synthesisof oxides of nitrogen (NOX) from atmosphericoxygen and nitrogen During volcanic eruptionsor forest fires sulphur dioxide (SO2) is releasedinto the atmosphere to provide the essentialcomponent for the creation of sulphuric acidPhytoplankton in the oceans also emit sulphurduring their seasonal bloom period The sulphurtakes the form of dimethyl sulphide (DMS)

which is oxidized into SO2 and methanesulphonic acid (MSA) The MSA is ultimatelyconverted into sulphate (Cocks and Kallend1988) Acids formed in this way fall out of theatmosphere in rain to become involved in avariety of physical and biological processes oncethey reach the earthrsquos surface The return ofnitrogen and sulphur to the soil in naturally acidrain helps to maintain nutrient levels forexample The peculiar landscapes of limestoneareasmdashcharacterized by highly weatheredbedrock rivers flowing in steep-sided gorges orthrough inter-connected systems of under-ground stream channels and cavesmdashprovideexcellent examples of what even moderatelyacid rain can do

In reality since lsquoacid rainrsquo includes snow hailand fog as well as rain it would be moreappropriate to describe it as lsquoacid precipitationrsquoThe term lsquoacid rainrsquo is most commonly used forall types of lsquowet depositionrsquo however A relatedprocess is lsquodry depositionrsquo which involves thefallout of the oxides of sulphur and nitrogenfrom the atmosphere either as dry gases oradsorbed on other aerosols such as soot or flyash (Park 1987) As much as two-thirds of theacid precipitation over Britain falls as drydeposition in the form of gases and smallparticles (Mason 1990) On contact withmoisture in the form of fog dew or surfacewater they produce the same effects as theconstituents of wet deposition At present bothwet and dry deposition are normally included inthe term lsquoacid rainrsquo and to maintain continuitythat convention will be followed here

Acid rain

ACID RAIN 71

Current concern over acid rain is not with thenaturally produced variety but rather with thatwhich results from modern industrial activityTechnological advancement in a society oftendepends upon the availability of metallic oreswhich can be smelted to produce the great volumeand variety of metals needed for industrial andsocio-economic development Considerableamounts of SO2 are released into the atmosphereas a by-product of the smelting processparticularly when non-ferrous ores are involvedThe burning of coal and oil to provide energyfor space heating or to fuel thermal electric powerstations also produces SO2 The continuinggrowth of transportation systems using theinternal combustion enginemdashanothercharacteristic of a modern technological societymdashcontributes to acid rain through the release ofNOX into the atmosphere

Initially the effects of these pollutants were

restricted to the local areas in which theyoriginated and where their impact was oftenobvious The detrimental effects of SO2 onvegetation around the smelters at Sudbury(Ontario) Trail (British Columbia) Anaconda(Montana) and Sheffield (England) have longbeen recognized for example (Garnett 1967Hepting 1971) As emissions increased and thegases were gradually incorporated into the largerscale atmospheric circulation the stage was setfor an intensification of the problem Sulphurcompounds of anthropogenic origin are nowblamed for as much as 65 per cent of the acidrain in eastern North America with nitrogencompounds accounting for the remainder(Ontario Ministry of the Environment 1980)In Europe emission totals for SO2 and NOX arecommonly considered to split closer to 75 percent and 25 per cent (Park 1987) Since the early1970s however declining SO2 emissions and agrowing output of NOX have combined to bring

Figure 41 The pH scaleshowing the pH level ofacid rain in comparisonto that of other commonsubstances

the relative proportions of these gases close tothe North American values (Mason 1990)

Acid precipitation produced by humanactivities differs from natural acid precipitationnot only in its origins but also in its qualityAnthropogenically produced acid rain tends tobe many times more acidic than the naturalvariety for example The acidity of a solution isindicated by its hydrogen ion concentration orpH (potential hydrogen) the lower the pH thehigher the acidity A chemically neutral solutionsuch as distilled water has a value of 7 withincreasingly alkaline solutions ranging from 7 to14 and increasingly acidic solutions ranging from7 down to 0 (see Figure 41) Since this pH scaleis logarithmic a change of one point representsa tenfold increase or decrease in the hydrogenion concentration while a two-point changerepresents a one hundredfold increase ordecrease A solution with a pH of 40 is ten times

more acidic than one of pH 50 a solution ofpH 30 is one hundred times more acidic thanone of pH 50

The difference between lsquonormalrsquo and lsquoacidrsquorain is commonly of the order of 10 to 15 pointsIn North America for example naturally acidrain has a pH of about 56 whereasmeasurements of rain falling in southern OntarioCanada frequently provide values in the rangeof 45 to 40 (Ontario Ministry of theEnvironment 1980) To put these values inperspective it should be noted that vinegar has apH of 27 and milk a pH of 66 (see Figure 41)Thus Ontario rain is about 100 times more acidicthan milk but 100 times less acidic than vinegarSimilar values for background levels of acidic rainare indicated by studies in Europe The CentralElectricity Generating Board (CEGB) in Britainhas argued for pH 50 as the normal level fornaturally acid rain (Park 1987) but the average

Figure 42 Schematic representation of the formation distribution and impact of acid rain

Source Compiled from information in Park (1987) Miller (1984) LaBastille (1981)

ACID RAIN 73

annual pH of rain over Britain between 1978and 1980 was between 45 and 42 (Mason1990) Remarkably high levels of acidity havebeen recorded on a number of occasions on bothsides of the Atlantic In April 1974 for examplerain falling at Pitlochry Scotland had a pHmeasured at 24 (Last and Nicholson 1982) anda value of 27 was reported from western Norwaya few weeks later (Sage 1980) At Dorset northof Toronto Ontario snow with a pH of 297fell in the winter of 1976ndash77 (Howard and Perley1991) and an extreme value of pH 15 some11000 times more acid than normal wasrecorded for rain falling in West Virginia in 1979(LaBastille 1981) Although these values areexceptional the pattern is not Acid depositiontends to be episodic with a large proportion ofthe acidity at a particular site arriving in only afew days of heavy precipitation (Last 1989)Annual averages also mask large seasonalvariations Summer precipitation for exampleis often more acid than that in the winteralthough emissions are generally less in thesummer

The quality of the rain is determined by aseries of chemical processes set in motion whenacidic materials are released into theatmosphere Some of the SO2 and NOX emittedwill return to the surface quite quickly andclose to their source as dry deposition Theremainder will be carried up into theatmosphere to be converted into sulphuric andnitric acid which will eventually return toearth as acid rain (see Figure 42) Theprocesses involved are fundamentally simpleOxidation converts the gases into acids ineither a gas or liquid phase reaction The latteris more effective The conversion of SO2 intosulphuric acid in the gas phase is 16 per centper hour in summer and 3 per cent per hour inwinter Equivalent conversion rates in theliquid phase are 100 per cent per hour insummer and 20 per cent per hour in winter(Mason 1990) Despite the relatively slowconversion to acid in the gas phase it is themain source of acid rain when clouds and rainare absent or when humidity is low

The rate at which the chemical reactions takeplace will also depend upon such variables as theconcentration of heavy metals in the airborneparticulate matter the presence of ammonia andthe intensity of sunlight Airborne particles ofmanganese and iron for example act as catalyststo speed up the conversion of SO2 to sulphuricacid and sulphates Natural ammonia may havesimilar effects (Ontario Ministry of theEnvironment 1980) Sunshine provides the energyfor the production of photo-oxidantsmdashsuch asozone (O3) hydrogen peroxide (H2O2) and thehydroxyl radical (OH)mdashfrom other pollutantsin the atmosphere and these oxygen-richcompounds facilitate the oxidation of SO2 andthe NOX to sulphuric and nitric acid respectively(Cocks and Kallend 1988) The role of thephotochemical component in the conversionprocess may account for the greater acidity ofsummer rainfall in many areas (Mason 1990)In the presence of water these acids and the otherchemicals in the atmosphere will dissociate intopositively or negatively charged particles calledions For example sulphuric acid in solution is amixture of positively charged hydrogen ions(cations) and negatively charged sulphate ions(anions) It is these solutions or lsquococktails ofionsrsquo as Park (1987) calls them that constituteacid rain

Whatever the complexities involved in theformation of acid rain the time scale is crucialThe longer the original emissions remain in theatmosphere the more likely it is that the reactionswill be completed and the sulphuric and nitricacids produced Long Range Transportation ofAtmospheric Pollution (LRTAP)mdashtransportationin excess of 500 kmmdashis one of the mechanismsby which this is accomplished

Air pollution remained mainly a local problemin the past The effects were greatest in theimmediate vicinity of the sources and much ofthe effort of environmental groups in the 1960sand 1970s was expended in attempts to changethat situation Unfortunately some of the changesinadvertently contributed to the problem of acidrain One such was the tall stacks policy In anattempt to achieve the reduction in ground level

pollution required by the Clean Air Acts theCEGB in Britain erected 200 m high smokestacksat its generating stations (Pearce 1982d)Industrial plants and power stations in the UnitedStates took a similar approach increasing theheights of their stacks until by 1977 more than160 were over 150 m high (Howard and Perley1991) and by 1981 at least 20 were more than300 m high (LaBastille 1981) The InternationalNickle Company (INCO) added a 400 msuperstack to its nickel smelter complex atSudbury Ontario in 1972 (Sage 1980) Theintroduction of these taller smokestacks onsmelters and thermal electric power stationsalong with the higher exit velocities of theemissions allowed the pollutants to be pushedhigher into the atmosphere This effectivelyreduced local pollution concentrations butcaused the pollutants to remain in the atmospherefor longer periods of time thus increasing theprobability that the acid conversion processeswould be completed The release of pollutantsat greater altitudes also placed them outside theboundary layer circulation and into the largerscale atmospheric circulation system with itspotential for much greater dispersal through themechanisms of LRTAP The net result was asignificant increase in the geographical extent ofthe problem of acid rain

THE GEOGRAPHY OF ACID RAIN

Total global emissions of the SO2 and NOX themain ingredients of acid rain are difficult toestimate Fossil fuel combustion alone producesabout 91 million tonnes annually (Hameed andDignon 1992) and other activities both naturaland anthropogenic add to that The main sourcesare to be found in the industrialized areas of thenorthern hemisphere Northeastern NorthAmerica Britain and western Europe havereceived most attention (see Figure 43) buteastern Europe and the republics of the formerUSSRmdashRussia Ukraine and Kazakhstan alongwith eastern and western Europe emit amdasharealso important sources These republicscombined total of some 54 million tonnes of SO2

Figure 43 The geography of acid rain in Europe andNorth America

Source Compiled from data in Park (1987) Miller(1984)LaBastille (1981) Ontario Ministry of theEnvironment (1980)

ACID RAIN 75

every year or almost double that produced inNorth America (Barrie 1986) In Asia Japaneseindustries emit large quantities of SO2 (Park1987) while the industrial areas of China arealso major contributors Annual emissions of SO2

in China average 15 million tonnes but have beenas high as 24 million tonnes (Glaeser 1990)

Sulphur dioxide emission levels in the westernindustrialized nations peaked in the mid-1970sor early 1980s and have declined significantlysince then (see Figure 44) In Britain forexample the output of SO2 decreased by 35 percent between 1974 and 1990 (Mason 1990)Similar declines in SO2 have been recorded forNorth America and western Europe and thattrend is likely to continue as new environmentalregulations are introduced and enforced Incontrast emissions of NOX continue to rise Thismay be due in part to increased automobiletraffic but it also reflects the limited attention

given to NOX reduction in most acid pollutioncontrol programmes The trends are less clearand more difficult to forecast outside westernEurope and North America As long as theeconomic and political disarray continues ineastern Europe and the former Soviet Union itis unlikely that pollution control programmes willbe given top priority and acid gas emissions arelikely to remain high or even increase Chineseeconomic development will continue to be fuelledby low-quality sulphur-rich coal leading toincreased local and regional levels of atmosphericacidity (Glaeser 1990) In contrast Japanmdashtheleading industrial nation in Asiamdashwas quick toinstall pollution control equipment to reduce SO2

and NOX levels in the 1980s (Ridley 1993) andthe decline in emissions there is likely to continueThe problem of acid rain has received lessattention in Asia than in Europe or NorthAmerica but a new study initiated in 1992 under

Figure 44 Sulphur dioxide emissions in selected countries 1970ndash89

Source Based on data in World Resources Institute (1992)

the auspices of the World Bank has been designedto change that The study will be based on acomputer model called RAINS similar to onedeveloped by the International Institute forApplied Systems Analysis for the EuropeanCommunity Results from the model will allowresearchers to alert Asian governments to theextent and intensity of the acid rain problem andto recommend ways of dealing with it (Hunt1992)

Acid emissions remain limited outside themajor industrial nations but concern has beenexpressed over growing levels of air pollutionwhich may already have provided a base for acidrain in some Third World countries (Park 1987)The future extent of the problem will dependupon the rate at which these countriesindustrialize and the nature of thatindustrialization Experience in the developedworld shows that other possibilities exist also

For example large conurbationsmdashsuch as LosAngelesmdashwith few sulphur producing industriesbut with large volumes of vehicular traffic havebeen identified as sources of acidic pollution (Elliset al 1984) Many developing nations arebecoming rapidly urbanized and as a result mayprovide increased quantities of the ingredientsof acid rain in the future (Pearce 1982c) Thusalthough at present the geographical distributionof acid rain is largely restricted to theindustrialized nations of the northern hemisphereit has the potential to expand to a near-globalscale in the future (see Figure 45)

All of the areas presently producing largeamounts of acidic pollution lie within the mid-latitude westerly wind belt Emissions fromindustrial activity are therefore normally carriedeastwards or perhaps northeastwards often forseveral hundred kilometres before beingredeposited The distance and rate of travel are

Figure 45 The geography of acid rain showing areas with pH below 50

Source After Park (1991)

ACID RAIN 77

Source From Barrie (1986)

Figure 46 Annual emissions of sulphur dioxide (106 tonnes) in regions of the northern hemisphere that influencethe Arctic

closely linked to the height of emission Pollutionintroduced into the upper westerlies or jet streamsis taken further and kept aloft longer than thatemitted into the boundary layer circulation Forexample a parcel of air tracked from Torontoat an altitude of 5000 m was found well outover the Atlantic some 950 km east of its sourcein less than 12 hours In the same time span aparcel of air close to the surface covered less thana quarter of that distance (Cho et al 1984) Inthis way pollutants originating in the US Midwestcause acid rain in Ontario Quebec and the NewEngland states emissions from the smelters andpower stations of the English Midlands and theRuhr contribute to the acidity of precipitation inScandinavia

The ultimate example of LRTAP is providedby acidic pollution in the Arctic which has beentraced to sources some 8000 km away in NorthAmerica and Eurasia (Park 1987) The winteratmospheric circulation in high latitudes causespolluted air to be drawn into the Arctic air massin sufficient quantity to reduce visibilitymdashthrough the creation of Arctic haze (see Chapter5)mdashand increase environmental acidity Sulphurdioxide and sulphate particles are the mainconstituents of the pollution which originates inthe industrial regions of Eurasia and NorthAmerica with the former contributing as muchas 80 per cent of the total (see Figure 46) Thenet result at locations in the Canadian Arctic isan atmospheric sulphate content during thewinter months which is at least twenty to thirtytimes that of the summer months (Barrie andHoff 1985) Slightly higher winter concentrationshave been recorded in the Norwegian andRussian Arctic (Barrie 1986) This seasonalvariation is reflected in the acidity of theprecipitation and the snowpack which variesfrom a pH of about 56 in the summer to 49ndash52 in the winter months (Barrie 1986)

The problem of acid rain obviously transcendsnational boundaries introducing politicalovertones to the problem and creating the needfor international co-operation if a solution is tobe found That co-operation was not easilyachieved Since the ingredients of acid pollution

are invisible and the distances they are carriedare so great it is not possible to establish a visiblelink between the sources of the rain and the areaswhich suffer its effects It was therefore easy forpolluters to deny fault in the past Theintroduction of airborne sampling systems usingballoons (LaBastille 1981) or aircraft (Pearce1982d) has helped to change that Tracerelements added to a polluted airstream or source-specific chemicals allow emissions from aparticular area to be followed until they aredeposited in acid rain (Fowler and Barr 1984)Mathematical models developed in Europe alsoallow the final destination of specific acidemissions to be identified (Cocks and Kallend1988) With such developments in monitoringtechniques denial of guilt becomes more andmore difficult

ACID RAIN AND GEOLOGY

The impact of acid rain on the environmentdepends not only on the level of acidity in therain but also on the nature of the environmentitself Areas underlain by granitic or quartziticbedrock for example are particularly susceptibleto damage since the soils and water are alreadyacidic and lack the ability to lsquobufferrsquo or neutralizeadditional acidity from the precipitation Acidlevels therefore rise the environmental balanceis disturbed and serious ecological damage is theinevitable result In contrast areas which aregeologically basicmdashunderlain by limestone orchalk for examplemdashare much less sensitive andmay even benefit from the additional acidity Thehighly alkaline soils and water of these areasensure that the acid added to the environmentby the rain is very effectively neutralized In areascovered by glacial drift or some otherunconsolidated deposit the susceptibility of theenvironment to damage by acid rain will bedetermined by the nature of the superficialmaterial rather than by the composition of thebedrock In theory it is important to establishbackground levels of acidity or alkalinity so thatthe vulnerability of the environment toacidification can be estimated in reality this is

ACID RAIN 79

seldom possible since in most casesenvironmental conditions had already beenaltered by acid rain by the time monitoring wasintroduced

The areas at greatest risk from acid rain inthe northern hemisphere are the pre-CambrianShield areas of Canada and Scandinavia wherethe acidity of the rocks is reflected in highly acidicsoils and water The folded mountain structuresof eastern Canada and the United StatesScotland Germany and Norway are alsovulnerable (see Figure 43) Most of these areashave already suffered but the potential forfurther damage is high Should the presentemission levels of SO2 and NOX be maintainedfor the next ten to twenty years it is likely thatsusceptible areas presently little affected by acidrainmdashin western North America and the Arcticfor examplemdashwould also suffer damage as thelevel of atmospheric acidity rises

ACID RAIN AND THE AQUATICENVIRONMENT

The earliest concerns over the impact of acid rainon the environment were expressed by RobertSmith in England as long ago as 1852 (Park1987) but modern interest in the problem datesonly from the 1960s Initial attentionconcentrated on the impact of acid rain on theaquatic environment which can be particularlysensitive to even moderate increases in acidityand it was in the lakes and streams on both sidesof the Atlantic that the effects were first apparentBoth LaBastille (1981) and Park (1987) creditSvente Oden a Swedish soil scientist withbringing the problem to the attention of thescientific community in a campaign which beganin 1963 Both also mention the work of EvilleGorham in the late 1950s in the English LakeDistrict and the studies of Gene Likens andHerbert Bormann in New Hampshire datingback to 1963 Gorham was also one of the firstto become involved in the study of acid lakes inCanada in the area polluted by smelter emissionsaround Sudbury Ontario (Gorham and Gordon1960) From these limited beginnings the

scientific investigation of the problem has grownrapidly By the mid-1980s a majority of scientistsaccepted that a link existed between the emissionsof SO2 and NOX and the acidification of lakesA minority remained unconvinced Along withpoliticians in the United States and Britain theycontinued to counter requests for emissionreductions with calls for more studies (Park1987) despite an estimate that research over aquarter of a century had resulted in more than3000 studies in North America alone (Israelson1987)

There is a tendency for all lakes to becomemore acidic with time as a result of naturalageing processes but studies of acid-sensitivelakes suggest that observed rates of change inpH values since the middle of the nineteenthcentury have exceeded the expected natural ratesThe analysis of acid sensitive diatom species inlake sediments has allowed pH-age profiles tobe constructed for some 800 lakes in NorthAmerica and Europe (Charles et al 1990) Mostof these profiles indicate that lake acidificationis a relatively recent phenomenon with littlechange in pH values indicated prior to 1850 Inalmost a century and a half since then pH valuesin the lakes studied have decreased by between05 and 15 units (see Figure 47) In contrastless sensitive lakesmdashthose with adequatebuffering for examplemdashshowed little change

Some of the increased acidity could beexplained by changing land use In theNetherlands for example agricultural practicessuch as sheep washing and the foddering of duckskept levels of acidity artificially low in the pastbut as these activities declined the acidity of thelakes began to rise again (Charles et al 1990)In south-west Scotland reforestation has beenaccompanied by increasing acidity because thetrees have a greater ability than the vegetationthey replaced to scavenge acid aerosols frompassing air streams (Mason 1990) Suchsituations seem to be the exception however andthe general consensus is that declining pH valuesare the result of increased acidic deposition since the Industrial Revolution Rising levels of copperzinc and lead in the lake deposits plus the

presence of soot from the burning of fossil fuelssupports the industrial origin of the increasedacidity (Last 1989) The corollary that decreasedacid emissions from industrial activities should

lead to a reduction in lake acidity is perhapsreflected in rising pH values in some lakes inBritain and Scandinavia but the data are as yettoo limited to provide conclusive results (Last1989 Mason 1990)

Figure 47 Decrease in lake pH asinferred from the diatom populationstructure The graphs represent changeswhich have occurred over the periodfrom pre-industrial times until about1980 to 1985 and indicate thepercentage of lakes in each region inwhich the pH has declined by specificamounts

Source Form Charles et al (1990)

ACID RAIN 81

Some uncertainties over the relationshipbetween industrial emissions and acid rain willalways remain because of the complexity of theenvironment and the variety of its possibleresponses to any input There can be no doubthowever that acid rain does fall and when itdoes its effects on the environment are oftendetrimental

In addition to reduced pH values acidic lakesare characterized by low levels of calcium andmagnesium and elevated sulphate levels Theyalso have above-normal concentrations ofpotentially toxic metals such as aluminium(Brakke et al 1988)(see Figure 48) The initialeffect of continued acid loading varies from laketo lake since waterbodies differ in their sensitivityto such inputs Harmful effects will begin to befelt by most waterbodies when their pH falls to53 (Henriksen and Brakke 1988) althoughdamage to aquatic ecosystems will occur in somelakes before that level is reached and someauthorities consider pH 60 as a more appropriatevalue (Park 1987) Whatever the value once thecritical pH level has been passed the net effect

will be the gradual destruction of the biologicalcommunities in the ecosystem

Most of the investigations into the impact ofacid rain on aquatic communities have involvedfish populations There is clear evidence fromareas as far apart as New York State NovaScotia Norway and Sweden that increasedsurface water acidity has adverse effects on fish(Baker and Schofield 1985) The processesinvolved are complex and their effectivenessvaries from species to species (see Figure 49)For example direct exposure to acid water maydamage some species Brook trout and rainbowtrout cannot tolerate pH levels much below 60(Ontario Ministry of the Environment 1980)and at 55 smallmouth bass succumb(LaBastille 1981) The salmonid group of fish ismuch less tolerant than coarser fish such as pikeand perch (Ontario Ministry of theEnvironment 1980) Thus as lakes becomeprogressively more acid the composition of thefish population changes

The stage of development of the organism isalso important (see Figure 410) Adult fish forexample may be able to survive relatively lowpH values but newly hatched fry or even thespawn itself may be much less tolerant (OntarioMinistry of the Environment 1980) As a resultthe fish population in acid lakes is usually wipedout by low reproductive rates even before thepH reaches levels which would kill mature fish(Jensen and Snekvik 1972)

Fish in acid lakes also succumb to toxicconcentrations of metals such as aluminiummercury manganese zinc and lead leached fromthe surrounding rocks by the acids Many acidlakes for example have elevated concentrationsof aluminium (Brakke et al 1988) which hasbeen recognized as a particularly potent toxin(Cronan and Schofield 1979) The toxic effectsof aluminium are complex but in lakes wherethe pH has fallen below 56 it is commonlypresent in sufficient quantity to kill fish (Stokeset al 1989) The fish respond to the presence ofthe aluminium by producing mucus which clogsthe gills inhibiting breathing and causing abreakdown of their salt regulation systems

Figure 48 Aluminium concentration in relation to pHfor 20 lakes near Sudbury Ontario

Source From Harvey (1989)

(Mason 1990) Aluminium also kills a variety ofinvertebrates but the progressive concentrationof the metal through food chains and food websensures that higher level organisms such as fishare particularly vulnerable and it is possible thatfish kills previously attributed to high aciditywere in fact the result of aluminium poisoning(Park 1987) The mobilization of heavy metalsby leaching may also help to reduce fishpopulations indirectly by killing the insects andmicroscopic aquatic organisms on which the fishfeed Birds such as herons ospreys andkingfishers which feed on the fish and those such

as ducks and other waterfowl which dependupon molluscs and aquatic insects for their foodalso feel the effects of this disruption of the foodchain (Howard and Perley 1991)

The fish are particularly vulnerable during theannual spring flush of highly acidic water intothe lakes and streams This is a well-documentedphenomenon which causes stress to all organismsin the aquatic environment (Park 1987) All ofthe areas presently affected by acid rain receivea proportion of their total precipitation in theform of snow The acids falling in the snow duringthe winter accumulate on land and on the frozen

Figure 49 The impact ofacid rain on aquaticorganisms

Source Compiled from datain Israelson (1987) Bakerand Schofield (1985)LaBastille (1981) OntarioMinistry of the Environment(1980)

ACID RAIN 83

waterways until the spring melt occurs At thattime they are flushed into the system inconcentrations many times higher than normalMeasurements in some Ontario lakes have showna reduction in pH values of more than two unitsin a matter of a few days although decreases ofthe order of one pH unit are more common(Ontario Ministry of the Environment 1980Jeffries 1990) This augmented level of aciditymay last for several weeks and unfortunatelyit often coincides with the beginnings of theannual hatch The recently hatched fry cannotsurvive the shock and fish populations in acidiclakes often have reduced or missing age groupswhich reflect this high mortality (Baker andSchofield 1985) The aquatic ecosystems of lakeswhich are normally well-buffered are not immuneto major damage if the melt is rapid and highlyacidic (Ontario Ministry of the Environment1980)

Fish populations in many rivers and lakes ineastern North America Britain and Scandinaviahave declined noticeably in the last two to threedecades as a consequence of the effects of acidrain Surveys of European and North American

lakes have established that 50 per cent of lakeswith a pH lt5 are likely to contain no fish (Mason1990) More than 300 lakes in Ontario Canadamainly in the vicinity of Sudbury are fishless andmany others have experienced a reduction in thevariety of species they contain (Ontario Ministryof the Environment 1980 Park 1991) severalrivers in Nova Scotia once famous for theirAtlantic salmon are now too acidic to supportthat fish (Israelson 1987) In the AdirondackMountains of New York State between 200 and400 lakes have lost some or all of their fish species(Harvey 1989) Damage to fish populations hasoccurred in an area of 33000 sq km in southernNorway with brown trout particularly hard hit(Baker and Schofield 1985) In nearby Sweden100 lakes sampled in the mid-1970s had alreadylost 43 per cent of their minnow 32 per cent oftheir roach 10 per cent of their arctic char and14 per cent of their brown trout populations asa result of acidification (Almer et al 1974) andby the end of the decade it was estimated thatsome 4000 lakes in Sweden were fishless(LaBastille 1981) In Britain no obviousproblems emerged until the 1970s when riversand lakes in southwest Scotland the English LakeDistrict and Wales showed the first signs ofdecline in the numbers of trout salmon and othergame fish (Park 1987) Since then a number ofstudies have shown that the problem isparticularly acute in those streams drainingforested catchment areas which are commonlymore acidic Many of these streams arecompletely devoid of fish or support only a fewspecies in their lower reaches where the acidityis usually less (Elwood and Mulholland 1989)

Waterbodies which have lost or are in theprocess of losing their fish populations are oftendescribed as lsquodeadrsquo or lsquodyingrsquo This is not strictlycorrect All aquatic flora and fauna will declinein number and variety during progressiveacidification but even at pH 35 water boatmenand whirligig beetles survive and multiply(LaBastille 1981) and species of protozoans arefound at pH levels as low as 20 (Hendrey 1985)Phytoplankton will disappear when pH fallsbelow 58 (Almer et al 1974) but acid-tolerant

Figure 410 The age-class composition of samples ofthe white sucker populations in lakes Skidway (pH50) and Bentshoe (pH 59)

Sphagnum mosses will colonize the lake bottoms(Pearce 1982d) Rapid Sphagnum growth hasbeen reported in Scandinavia but in easternNorth America large quantities of green algaeare more common The absence of insect larvaeand other organisms which normally graze onthe algae may in part explain its abundance(Stokes et al 1989) Leaves or twigs falling intothe water will be slow to decompose becausethe bacteria which would normally promotedecay have been killed by the acidic conditions(Park 1987) The absence of phytoplankton andthe general reduction in organic activity allowsgreater light penetration which makes acid lakesunnaturally clear and bluish in colour (LaBastille1981) This ethereal appearance may suggestdeath but even the most acid lakes have somelife in them

ACID RAIN AND THE TERRESTRIALENVIRONMENT

Terrestrial ecosystems take much longer to showthe effects of acid rain than aquatic ecosystemsAs a result the nature and magnitude of theimpact of acid precipitation on the terrestrialenvironment has been recognized only recentlyAs early as 1965 however air pollution wasknown to be killing oak and pine trees on LeoTolstoyrsquos historic estate at Yosnaya Polyana(Goldman 1971) There is growing evidence thatthose areas in which the waterbodies have alreadysuccumbed to acidification must also face theeffects of increasing acid stress on their forestsand soils The threat is not universally recognizedhowever and there remains a great deal ofcontroversy over the amount of damage directlyattributable to acid rain Reduction in forestgrowth in Sweden (LaBastille 1981) physicaldamage to trees in West Germany (Pearce 1982b)and the death of sugar maples in Quebec andVermont (Norton 1985) have all been blamedon the increased acidity of the precipitation inthese areas Some of the soils developed on thehill-peats of Scotland appear to have become soacid that they can longer support the acid-tolerantheather native to the area (Last 1989) Although

the relationship seems obvious in many casesthe growth development and decline of plantshas always reflected the integrated effects ofmany variables including site microclimatologyhydrology land-use change age and speciescompetition Acid rain has now been added tothat list Even those individuals or groups withgreatest concern for the problem admit that it isnext to impossible to isolate the impact of anyone element from such a combination of variables(Ontario Ministry of the Environment 1980)Thus it may not be possible to establish definitiveproof of the link between acid precipitation andvegetation damage The body of circumstantialevidence is large however and adverse effectshave been produced in laboratory experimentsTogether these support the view that theterrestrial environment is under some threat fromacid rain

Assessment of the threat is made difficult bythe complexity of the relationships in theterrestrial environment In areas experiencingacid rain dry and wet deposition over land isintercepted initially by the vegetation growingthere The effects of this precipitation on theplants may be direct brought about by thepresence of acid particles on the leaves forexample or indirect associated with changes inthe soil or the biological processes controllingplant growth (see Figure 411) Acid precipitationintercepted by trees may promote necrosis of leaftissue leaching of leaf nutrients and chlorophylldegradation (Shriner amp Johnston 1985) all ofwhich cause visible damage Vegetation growingat high altitudes and therefore enveloped in cloudfor long periods frequently displays suchsymptoms since cloud moisture is often moreacidic than rain (Hendrey 1985) Ultimately theacid particles will be washed off the vegetationand into the soil where they can begin to affectthe plants indirectly but no less seriously

By intercepting acid deposition plantsparticularly trees also act as concentrators Forexample dry deposition allowed to accumulateon the leaves or needles of the forest canopywill increase the acidity of the rainfall whichwashes it out to the forest floor (Park 1987)

ACID RAIN 85

Episodic increases in acidity of this nature maybe likened to the spring flush in the aquaticenvironment although their intensity andduration is usually less

Once the acid rain enters the soil its impactwill depend very much on the soil type and theunderlying bedrock Soils derived from granitefor example will already be acidic and thereforevulnerable to further increases In contrast soilsdeveloped over limestone or some other calcium-rich source will have the ability to neutralizelarge quantities of additional acid Naturalprocesses such as the decay of organic matter orthe weathering of minerals increase the acidityof many soils and it is often difficult to assessthe contribution of acid rain to the total Indeedit has been argued that the addition of

atmospheric acids may be relatively insignificantcompared with those from in-soil processes (Krugand Frink 1983) The situation is furthercomplicated when such soils are developed foragriculture To maintain productivity it isnecessary to make regular applications offertilizer and lime which mask acidification

Acidic water interferes with soil biology andsoil chemistry disturbing nutrient cycles andcausing physiological damage to plant rootsystems Increased acidity inhibits the bacterialactivity which is instrumental in releasingnutrients from dead or decaying animal andvegetable matter the ability of nitrifying bacteriato fix atmospheric nitrogen may also berestricted leading to reduced soil fertility(Ontario Ministry of the Environment 1980)

Figure 411 The impact of acid rain on the terrestrial environment

Source Compiled from data in Fernandez (1985) Shriner and Johnston (1985) Tomlinson (1985)

This reduction in bacterial activity and theassociated disruption of nutrient supply may besufficient to offset the benefits that some soilsreceive from acid rain in the form of extranitrogen compounds (LaBastille 1981)

Changes in soil chemistry initiated by acidrain also lead to nutrient depletion In anormally fertile soil nutrientsmdashsuch as calciumpotassium and magnesiummdashare present in theform of positively charged microscopic particles(cations) bonded by way of their electricalcharge to clay and humus particles or other soilcolloids They can be removed from there byplants as required and are normally replacedby additional cations released into the soil bymineral weathering (Steila 1976) As acidicsolutions pass through the soil hydrogen ionsreplace the basic nutrient cations which arethen leached out of the soil in solution withsulphate and nitrate anions (Fernandez 1985)Regular acid-induced leaching of this type leadsto reduced soil fertility and consequently affectsplant growth Needle yellowing in coniferoustrees for example has been attributed to theremoval of magnesium from acidified forestsoils (Ulrich 1989)

The mobilization of toxic metals such asaluminium cadmium zinc mercury lead copperand iron is another feature which accompaniessoil acidification (Fernandez 1985) Most of thesemetals are derived from bedrock or soil mineralsby natural weathering but there is evidence fromsome areas such as the northeastern UnitedStates that atmospheric loading is also important(Johnson and Siccama 1983) Acid rain liberatesthe metals in ionic form and they are carried insolution into groundwater lakes and streams orabsorbed by plants (Park 1987) The detrimentaleffects of these toxic metals such as aluminiumon the aquatic environment are well-documented Their impact on the terrestrialenvironment is less clear however Some metalssuch as iron are essential for growth and onlybecome toxic at higher concentrations othermetals may be present in amounts normallyconsidered toxic yet the vegetation isundamaged presumably because it has adapted

to the higher concentrations (Park 1987) Themain claims that metals have initiated vegetationdamage have come from western Germanywhere high levels of aluminium in soils have beenblamed in part for the forest decline in that area(Fernandez 1985) The aluminium restricts thedevelopment of the fine root systems of the treesto the upper part of the soil profile If the topsoildries out the surviving deep roots are unable tosupply enough water to meet the needs of thetrees Water stress occurs and growth ratesdecline (Ulrich 1989) Elsewhere there is no clearevidence that tree growth has been impaired bytoxic metal mobilization

The damage attributed to acid rain is bothvisible and invisible In some cases the impact isonly apparent after detailed observation andmeasurement For example a survey of annualrings in a mixed spruce fir and birch forestexposed to acid rain in Vermont revealed aprogressive reduction in growth rates between1965 and 1979 (Johnson and Siccama 1983)Between 1965 and 1983 in the same generalarea there was also a 25 per cent decline in theabove-ground biomass of natural sugar mapleforest (Norton 1985) Physical damage to thefine root systems is another element common totrees in areas subject to acid rain (Tomlinson1985)

Most of these symptoms can only berecognized following careful and systematicsurvey but there are other effects which are moredirectly obvious and which have received mostpublic attention They can be grouped togetherunder the general term lsquotree diebackrsquo whichdescribes the gradual wasting of the tree inwardsfrom the outermost tips of its branches

Dieback has been likened to the prematurearrival of autumn (Norton 1985) On deciduoustrees the leaves on the outermost branches beginto turn yellow or red in mid-summer they dryout and eventually fall well ahead of scheduleThese branches will fail to leaf-out in the springIn succeeding years the problem will spread fromthe crown until the entire tree is devoid of foliageand takes on a skeletal appearance even insummer (Norton 1985) Coniferous trees react

ACID RAIN 87

in much the same way Needles turn yellow dryup and fall off the branches new buds fail toopen or if they do produce stunted and distortedgrowth (Park 1987) The trees gradually weakenduring these changes and become more and morevulnerable to insect attack disease and theravages of weather all of which contribute totheir demise (Norton 1985)

The maple groves of Ontario Quebec andVermont have been suffering progressivedieback since 1980 (Norton 1985) and it is nowestimated that in Quebec alone more than 80per cent of the maple stands show signs ofdamage (Robitaille 1986) Mortality rates formaples growing in Quebec have increased from2 per cent per year to 16 per cent (presenting apotential $6 million loss for the provincersquosmaple sugar producers) and regeneration iswell below normal (Norton 1985) There is asyet no conclusive proof that acid rain is thecause of dieback nor that current acid rainlevels are sufficient to prevent or diminish thegermination and growth of maple seeds (Pitelkaand Raynal 1989) Maple seedlings are sensitiveto the presence of aluminium in the soil but inmost of the affected area aluminium levels arenot high enough to be toxic (Thornton et al1986) Alternative explanations for the diebacksuch as poor sugar bush management ordiseasemdashwith or without a contribution fromacid rainmdashhave been put forward However theproblem is common to both natural andmanaged groves while disease usually followsrather than precedes the onset of dieback(Norton 1985) Dieback is now becomingprevalent in beech and white ash stands but it isthe damage to the maple which is causinggreatest concern particularly in Quebeccurrently the source of 75 per cent of the worldrsquossupply of maple sugar (Norton 1985)

Deciduous trees are not the only victims ofdieback The coniferous forest of eastern NorthAmerica has also suffered considerable decline(Johnson and Siccama 1983) The red spruceforests in the Adirondack Green and WhiteMountains were particularly badly hit betweenthe early 1960s and mid-1980s (Johnson et al

1989) Since the greatest dieback occurred athigher elevations where the trees are oftenenveloped in cloud for days at a time it washypothesized that persistent exposure to the highlevels of acid common in these clouds was themain cause of the damage Subsequent studieshowever provided no proof of a direct linkbetween acidity and dieback in that setting(Johnson et al 1989) There is evidence thatwinterkill played an important part in initiatingthe spruce decline (Johnson et al 1989) and ithas been suggested that exposure to acid stresscaused the trees to be less capable of with-standing the extreme cold of winter (Haines andCarlson 1989)

Dieback is extensive in Europe also By themid-1980s the symptoms had been recognizedacross fifteen countries in forests covering some70000 sq km (Park 1991) Damage wasparticularly extensive in what was then WestGermany where Ulrich and his colleagues firstlinked it to acid rain (Ulrich et al 1980) It wasestimated that one third of the trees in thatcountry had suffered some degree of dieback 75per cent of the fir trees and 41 per cent of thespruce trees in the state of Baden-Wurttembergwhich includes the Black Forest were damaged(Anon 1983) 1500 hectares of forest died inBavaria in the late 1970s and early 1980s (Pearce1982d) For a nation which prides itself in goodforest management such figures werehorrendous The death of the forests orWaldsterben as it has became known wasprogressing at an alarming and apparentlyquickening pace in the mid-1980s (Park 1987)Some recovery took place in the German forestsafter 1985 (Blank et al 1988) but by the end ofthe decade perhaps as many as half the trees inthe country were showing signs of damage (Park1991) Other nations in Europe are affected fromSweden with 38 per cent of its trees damaged toGreece with 64 per cent As more informationbecomes available for eastern Europe it is clearthat forest decline is also a serious problem inthe Czech and Slovak republics eastern Germanyand Poland In contrast Norway does not fit thepattern despite the exposure of its forests to acid

rain Decline is not widespread and is presenteven in areas where acid precipitation is minimal(Abrahamsen et al 1989)

Damage to existing trees is only part of theproblem The future of the forests is at stakealso Natural regeneration is no longer takingplace in much of Central Europe and even theplanting of nursery-raised stock provides noguarantee of success (Tomlinson 1985)Developments such as these raise the spectre ofthe wholesale and irreversible loss of forestland Such is the importance of the forestindustry to many of the regions involved thatthis would lead to massive economic disruptionand it is therefore not surprising that calls foraction on acid rain from these areas becameincreasingly more stridentmdashperhaps evendesperatemdashthrough the 1980s (Pearce 1982bPiette 1986) Although acid gas emissions havedeclined in Europe and North America in thelast decade allowing some limited recovery in anumber of aquatic ecosystems (Last 1989)there is little evidence of similar improvementsin terrestrial ecosystems This may reflect thelonger response time associated with the greaterecological complexity of the latter but it mayalso support the claims of many scientists thatthere are no well-established physical linksbetween acid rain and forest decline (Hainesand Carlson 1989 Pitelka and Raynal 1989)The decline of European forests for example isnow seen as a multifaceted problem in whichacid precipitation may only be a minorcontributor along with tree harvesting practicesdrought and fungal attacks (Blank et al 1988)In contrast Ulrichmdashwho first formulated theprocesses by which acid rain was thought tocause forest damage (see for example Ulrich1983)mdashaccepts the contributions of otherfactors but continues to claim that they onlycome into play fully once soil acidification hasmade the forest vulnerable (Hauhs and Ulrich1989) Until these differences are resolved andthe role of acid rain is better understood there isno guarantee that the greater control of acidemissions will have the desired effect of savingthe forests

ACID RAIN AND THE BUILTENVIRONMENT

Present concern over acid rain is concentratedmainly on its effect on the natural environmentbut acid rain also contributes to deterioration inthe built environment Naturally acid rain hasalways been involved in the weathering of rocksat the earthrsquos surface It destroys the integrity ofthe rock by breaking down the mineralconstituents and carrying some of them off insolution All rocks are affected to some extentbut chalk limestone and marble are particularlysusceptible to this type of chemical weatheringInevitably when these rocks are used as buildingstone the weathering will continue In recentyears however it has accelerated in line withthe increasing acidity of the atmosphere

Limestone is a common building stonebecause of its abundance natural beauty and easeof working Its main constituents are calcium andmagnesium carbonates which react with thesulphuric acid in acid rain to form the appropriatesulphate These sulphates are soluble and arewashed out of the stone gradually destroyingthe fabric of the building in the process Furtherdamage occurs when the solutions evaporateCrystals of calcium and magnesium sulphatebegin to form on or beneath the surface of thestone As they grow they create sufficientpressure to cause cracking flaking and crumblingof the surface which exposes fresh material toattack by acid rain (Anon 1984) Limestone andmarble suffer most from such processesSandstone and granite may become discolouredbut are generally quite resistant to acidity as isbrick Acid damage to the lime-rich mortarbinding the bricks may weaken brick-builtstructures however (Park 1987) Structural steeland other metals used in modern buildings mayalso deteriorate under attack from acid rain(Ontario Ministry of the Environment 1980)

By attacking the fabric of buildings acid raincauses physical and economic damage but it doesmore than that it also threatens the worldrsquoscultural heritage Buildings which have survivedthousands of years of political and economic

ACID RAIN 89

change or the predation of warfare and civilstrife are now crumbling under the attack of acidrain The treasures of ancient Greece and Romehave probably suffered more damage in the last50 years than they did in the previous 2000ndash3000 years (Park 1987) On the great cathedralsof Europemdashsuch as those in Cologne Canterburyand Chartresmdashthe craftsmanship of medievalstone masons and carvers may now be damagedbeyond repair (Pearce 1982a Park 1987) Fewbuildings in the industrialized regions of theworld are immune and damage to the Taj Mahalin India from sulphur pollution may be only thefirst indication that the problem is spreading tothe developing world also (Park 1987) Thefaceless statues and crumbling cornices of theworldrsquos famous buildings receive most publicitybut less spectacular structures may provideimportant information on the rate at whichdamage is occurring and its relationship to acidemissions In the United States for exampleresearchers investigating the effects of acid rainon building materials have used the standardizedmarble gravestones in national militarycemeteries as controls in their field studies (Anon1984)

The chemical processes involved in acid rainattacks on the built environment are essentiallythe same as those involved in the naturalenvironment but there are some differences inthe nature and provenance of the acidity Damagein urban areas is more often associated with drydeposition than with wet for example (Park1987) Acidic particles falling out of theatmosphere close to their source land onbuildings and once moisture is added corrosionbegins Damage is usually attributed todeposition from local sources such as the smeltersor power stations commonly found in urbanareas with little of the long range transportationassociated with acidification in the naturalenvironment However in cities with littleindustrial activity such as Ottawa Canada mostof the damage will be caused by wet depositionoriginating some distance upwind (LaBastille1981) and the same probably applies to acidcorrosion of isolated rural structures (Park 1987)

Since the various Clean Air Acts introduced inEurope and North America in the 1960s and1970s had their greatest impact on urbanpollution levels it might be expected that theeffects of acid rain on the built environmentwould be decreasing There is some indicationthat this is so but there appears to be a time laginvolved and it may be some time before thereduction in emissions is reflected in a reductionof acid damage to buildings and other structures(Park 1987)

ACID RAIN AND HUMAN HEALTH

The infamous London Smog of 1952 developedas a result of meteorological conditions whichallowed the build-up of pollutants within theurban atmosphere Smoke produced by domesticfires power stations and coal-burning industrieswas the most obvious pollutant but the mostdangerous was sulphuric acid floating free inaerosol form or attached to the smoke particles(Williamson 1973) Drawn deep into the lungsthe sulphuric acid caused or aggravated breathingproblems and many of the 4000 deathsattributed to the smog were brought about bythe effect of sulphuric acid on the humanrespiratory system (Bach 1972) Although theClean Air Acts of the 1960s and 1970s alongwith such developments as the tall stacks policyreduced the amount of sulphur compounds inurban air recent studies in Ontario andPennsylvania have indicated that elevatedatmospheric acidity continues to cause chronicrespiratory problems in these areas (Lippmann1986)

The acid rain which causes respiratoryproblems is in a dry gaseous or aerosol formand mainly of local origin It is therefore quitedifferent from the far-travelled wet depositionthat has caused major problems in the naturalenvironment There is as yet no evidence thatwet deposition is directly damaging to humanhealth but because of its ability to mobilizemetals it may have important indirect effects(Park 1987) For example in Norway the intakeof aluminium in acidified water has been linked

to chronic renal failure (Abrahamsen et al1989) Heavy metals such as copper cadmiumzinc and mercury liberated from soil andbedrock by acid rain may eventually reach thehuman body via plants and animals in the foodchain or through drinking water supplies Thecorrosion of storage tanks and distributionpipes by acidified water can also add metals todrinking water the liberation of lead from leadpiping or from solder on copper piping is aparticular concern Although quality control inwater treatment plants can deal with suchproblems (Ontario Ministry of theEnvironment 1980) many areas subject to acidrain depend upon wells springs and lakeswhich provide an untreated water supply Thismay expose users to elevated levels of suchmetals as lead and copper and althoughindividual doses in all of these situations wouldbe small regular consumption might allow themetals to accumulate to toxic levels

SOLUTIONS TO THE PROBLEM OF ACIDRAIN

Although the cause and effect relationshipbetween emissions of SO2 and NOX and acidrain damage is not universally accepted most ofthe solutions proposed for the problem involvethe disruption of that relationship The basicapproach is deceptively simple In theory areduction in the emission rate of acid forminggases is all that is required to slow down andeventually stop the damage being caused by theacidification of the environment Translatingthat concept into reality has proved difficulthowever

The reduction in emissions of SO2 and NOX

is a long-term solution based on prevention Itis also a solution in which the environment itselfhas a major role As emissions decline it mustadjust until some new level of equilibriumreflecting the decreased acidity is attained Thereis concern that in some areas the damage hasgone too far to be reversed completely and thereis currently some support for this point of viewFor example SO2 emissions in Britain have

declined by nearly 40 per cent since 1970(Caulfield and Pearce 1984) and in Canadaemissions decreased by 45 per cent between 1970and 1985 (Environment Canada 1991) yet thereduction in aquatic or terrestrial aciditydownwind from these areas remains less thanexpected The discrepancy may be explained inpart by the continuing rise in NOX emissionsoffsetting the decline in SO2 but it is also possiblethat the link between reduced acidity andecological recovery is not linear In that case aspecific reduction in acid emissions might notbring about an equivalent reduction inenvironmental damage (Park 1991) In LakeOxsjon in Sweden for example the pH fell from68 to 45 between 1968 and 1977 Despite themarked reduction in SO2 emissions from Britainand Sweden in the decade or so since then thepH has only recovered to 49 (Mason 1990)Reductions of a similar magnitude have beenidentified in some Scottish lochs (Last 1989)Surveys in lakes in Ontario indicate that pHvalues may rise by small but significant amountssoon after acid input is reduced but it may be10 to 15 years before aquatic biota respond tothe reduced acidity (Havas 1990) This indicatesthat natural recovery does seem to be possiblebut that it is a slow process taking much longerthan the original acidification It may alsoindicate that some direct human input will benecessary either to initiate the recovery processor to speed it up

One possible input is the addition of limewhich would produce an immediate reductionin acidity and allow the recovery mechanismsto work more effectively Lime has been used asa means of sweetening acid soils for many yearsand may be the reason that in areas of acid soilsagricultural land is less affected by acid rainthan the natural environment In areas wherenatural regeneration is no longer possible therestoration of the original chemical balance ofthe soil by liming and appropriate fertilizerapplication might allow reforestation to besuccessful

The same situation might apply in the aquaticenvironment Simply re-stocking an acidified

ACID RAIN 91

lake with fish cannot be successful unless somebuffering agent is added In 1973 several lakesin the Sudbury area were treated with calciumcarbonate and calcium hydroxide in an attemptto reduce acid levels (Scheider et al 1975) Acidityreturned to normal and there was an increase innutrient levels but in the lakes closest toSudbury copper and nickel remained atconcentrations toxic to fish (Ontario Ministry

of the Environment 1980) Similar experimentsin Sweden since the mid-1970s have involved theliming of some 3000 lakes and 100 streams andhave provided encouraging results (Porcella etal 1990) Reduced acidity is often followed byrecovery among the aquatic biota Lowerorganismsmdashsuch as phytoplanktonmdashwhichreproduce rapidly recover first followed

Figure 412 Reduction of sulphur dioxide through emission controls

Source Various see text

eventually by amphibians and fish (Havas 1990)Artificial buffering of lakes in this way is only atemporary measure which may be likened to theuse of antacid to reduce acid indigestion Theneutralizing effects of the lime may last longerthan those of the antacid but they do wear offin 3 to 5 years and re-liming is necessary as longas acid loading continues (Ontario Ministry ofthe Environment 1980) The treatment of theenvironment with lime to combat acidity is onlya temporary measure at best It can be used toinitiate recovery or to control the problem untilabatement procedures take effect but since itdeals only with the consequences of acid rainrather than the causes it can never provide asolution

Most of the current proposals for dealing withacid rain tackle the problem at its source Theyattempt to prevent or at least reduce emissionsof acid gases into the atmosphere The only wayto stop acid emissions completely is to stop thesmelting of metallic ores and the burning of fossilfuels Modern society could not function withoutmetals but advocates of alternative energysourcesmdashsuch as the sun wind falling water andthe seamdashhave long supported a reduction in theuse of fossil fuels These alternative sources canbe important locally but it is unlikely that theywill ever have the capacity to replaceconventional systems Nuclear power has alsobeen touted as a replacement since it can be usedto produce electricity without adding gases tothe atmosphere It has problems of its ownhowever Difficulties associated with the disposalof radioactive wastes remain to be resolved andevents such as the Chernobyl disaster of 1986do little to inspire public confidence in nuclearpower Thus although the replacement offossilfuel-based energy systems with non-polluting alternatives has the potential to reduceacid rain it is unlikely to have much effect in thenear future

Since SO2 makes the greatest contribution toacid rain in North America and Europe it hasreceived most attention in the development ofabatement procedures whereas emissions ofNOXmdashwhich are both lower in volume and more

difficult to deal withmdashhave been largelyneglected Similarly the development of controltechnology has tended to concentrate on systemssuitable for conventional power stations sincethey are the main sources of acid gases (Kyte1986a) Sulphur dioxide is formed when coal andoil are burned to release energy and thetechnology to control it may be applied beforeduring or after combustion The exact timing willdepend upon such factors as the amount of acidreduction required the type and age of the systemand the cost-effectiveness of the particularprocess (see Figure 412)

One of the simplest approaches to theproblem is fuel switching which involves thereplacement of high sulphur fuels with lowsulphur alternatives This may mean the use ofoil or natural gas rather than coal In Britainfor example the recently privatized powerindustry is actively exploring the increased useof North Sea gas as a means of reducing SO2

output despite concern that this approach is awaste of a high premium fuel with a relativelyshort lifespan (Stevenson 1993) Howeversince most power stations use coal and are noteasily converted to handle other fuels fuelswitching usually involves the replacement ofone type of coal with another or even theblending of low and high sulphur coal Muchdepends on the availability of the low sulphurproduct In Britain for example the supply islimited (Park 1987) but in western Canada andthe western United States abundant supplies oflow sulphur coal are available with a sulphurcontent only one-fifth of that which is normalin eastern coal (Cortese 1986) Such adifference suggests that fuel switching has aconsiderable potential for reducing SO2

production yet wholesale substitution isuncommon The problem is a geographical oneThe main reserves of low-sulphur coal are inthe west far removed from the large consumersin the east Transport costs are therefore highand complete switching becomes economicallyless attractive than other methods of reducingSO2 output Compromise is possible Ratherthan switching entirely Ontario Hydro the

ACID RAIN 93

major public electricity producer in theprovince has a well-established practice ofblending low-sulphur western Canadian coalwith the high-sulphur product from the easternUnited States As a result of this plus the use ofwashed coal the utilityrsquos SO2 output per unit ofelectricity has been declining with someregularity since the early 1970s (OntarioMinistry of the Environment 1980)

The amount of SO2 released duringcombustion can be reduced if the coal or oil istreated beforehand to remove some of theincluded sulphur in a process called fueldesulphurization The methods can be quitesimple and quite cost effective Crushing andwashing the coal for example can reducesubsequent SO2 emissions by 8 to 15 per cent(Park 1987) which represents a reduction of 15to 2 million tonnes of SO2 per year in the easternUnited States alone (Cortese 1986) Morecomplex chemical cleaning methods involvingthe gasification or liquefaction of the coal arealso possible but at considerable cost (Ramage1983)

If it is not possible to reduce sulphur levelssignificantly prior to combustion there aretechniques which allow reduction during thecombustion process itself Basically they involvethe burning of coal in the presence of limeAlthough the technology has been studied sincethe 1950s it has yet to be adopted on a largescale (Ramage 1983) There are two promisingdevelopments however which are expected tobe available in the early 1990s These are limeinjection multi-stage burning (LIMB) andfluidized bed combustion (FBC) LIMB involvesthe injection of fine lime into the combustionchamber where it fixes the sulphur released fromthe burning coal to produce a sulphate-rich limeash This process can reduce SO2 emissions by35ndash50 per cent (Burdett et al 1985) In the FBCsystem air under pressure is injected into amixture of coal limestone and sand until thewhole mass begins to act like a boiling fluid(Ramage 1983) The continual mixing of thematerials under such conditions ensures thatcombustion is very efficient and that up to 90

per cent of the sulphur in the fuel is removed(Kyte 1986b) It has the added advantage thatsince furnace temperatures are relatively low itreduces NOX emissions also In the United Statesfour thermal electric generating stations utilizingFBC technology came on stream in the late 1980sTogether they produce only 400 MW but it hasbeen estimated that a further 150 stationsmdashproducing 20000 MWmdashcould be retrofitted(Ellis et al 1990)

Flue gas desulphurization (FGD) is the namegiven to a group of processes which remove SO

2from the gases given off during combustion Thedevices involved are called scrubbers and maybe either dry or wet operations The simplest dryscrubbers act much like filters removing the gason contact by chemical or physical meansSulphur dioxide passing through a dry pulverizedlimestone filter for example will react chemicallywith the calcium carbonate to leave the sulphurbehind in calcium sulphate (Williamson 1973)Other filtersmdashsuch as activated charcoalmdashworkby adsorbing the gas on to the filter (Turk andTurk 1988)

Wet scrubbers are more common than the dryvariety The flue gases may be bubbled throughan alkaline liquid reagent which neutralizes theSO2 and produces calcium sulphate in the process(Kyte 1981) A variation on this approachinvolves the use of a lime slurry through whichthe flue gases are passed and in more modernsystems the combustion gases are bombarded byjets of lime (LaBastille 1981) or pass throughspray systems (Ramage 1983) The systemdescribed by LaBastille removes 92 per cent ofthe SO2 from the exhaust gases and manyscrubbers achieve a reduction of between 80 and95 per cent (Cortese 1986) By 1978 Japaneseindustry had installed scrubbers on more than500 plants (Howard and Perley 1991) In theUnited States some 70000 MW of electricity iscurrently being produced from plants employingFGD technology and in Canada a FGD retrofitprogramme is in place with the goal of reducingSO2 emissions by about 50 per cent by 1994 (Elliset al 1990) The European Community directiveof 1988mdashrequiring all EC nations to reduce SO2

emissions by stages up to 70 per cent by 2003mdashwill be met in large part by the installation ofFGD equipment (Park 1991)

Flue gas desulphurization is thus one of themost common methods of SO2 removal in partbecause of the high efficiencies possible but forother reasons also Scrubbers are technicallyquite simple and can be added to existingpower plants relatively easily Retrofittingexisting plants in this way is less expensive thanbuilding entirely new ones and in some systemsthe recovery of sulphuric acid for sale can helpto offset the cost

None of the FGD systems described workswell to reduce emissions of NOX from powerplants The best results for NOX controlmdashup to80 per cent reductionmdashhave been obtainedusing a selective catalytic reduction process(SCR) which breaks the NOX down into the

original N and O but the price is high Toretrofit an existing thermal power station wouldcost an estimated $10000 for every 1000 kg ofNOX removed and maintenance costs would besubstantial because the life of the catalyst isshort (Ellis et al 1990) Difficulties also existwith emission reductions from automobileexhausts The development of technology thatcan produce a cooler burning internalcombustion engine or perhaps replace itcompletely may be required before emissions ofNOX from that source are reduced significantly(Park 1987)

It is now technically possible to reduce SO2

emissions to very low levels However as iscommon with many environmental problemstechnology is only one of the elements involvedIn many cases economics and politics haveretarded the implementation of technical solutions

Figure 413 Technologiesand trade-offs inpower stationemission control

Source From Ridley(1993)

ACID RAIN 95

to environmental problems and that is certainlythe case with acid rain

THE ECONOMICS AND POLITICS OFACID RAIN

Government participation in pollutionabatement is not a new phenomenon but inrecent years particularly following the CleanAir legislation of the 1960s and 1970s the roleof government has intensified and pollutionproblems have become increasingly a focus forpolitical intervention Thus when acid rainemerged as a major environmental problem itwas inevitable that any solution would involveconsiderable governmental and politicalactivity Given the magnitude of the problem italso became clear that it could be solved only atconsiderable cost and economic and politicalfactors are now inexorably linked in anyconsideration of acid rain reduction Additionalcomplexity is provided by the internationalnature of the problem

The costs of acid rain reduction will varydepending upon such factors as the type ofabatement equipment required the reduction ofemission levels considered desirable and theamount of direct rehabilitation of theenvironment considered necessary (see Figure413) For example one report prepared by theOffice of Technology Assessment of the USCongress (Anon 1984) estimated that a 35 percent reduction in SO

2 levels in the eastern United

States by 1995 would cost between $3 and $6billion In a series of five bills presented to theUS Congress in 1986 and 1987 costs rangedfrom $25 to $226 billion (Ellis et al 1990) A50 per cent reduction in Canada has beenestimated to cost $300 million (Israelson 1987)In Britain a 60 per cent reduction of SO

2 from

the 1980 levels had an estimated price-tag ofbetween pound1ndash2 billion (c $2ndash4 billion) (Park 1991Ridley 1993) while a 90 per cent reduction waspriced at pound5 billion (c $10 billion) (Pearce 1992e)Such costs are not insignificant and in alllikelihood would be imposed directly orindirectly on the consumer

Increased costs of this type have to be setagainst the costs of continuing environmentaldamage if no abatement is attempted but thelatter often involve less tangible elements whichare difficult to evaluate The death of a lakefrom increased acidity for example mayinvolve a measurable economic loss through thedecline of the sports or commercial fishery(Forster 1985) but there are other items such asthe aesthetic value of the lake which cannotalways be assessed in real monetary termsThus the traditional costbenefit analysisapproach is not always feasible when dealingwith the environmental impact and abatementof acid rain

The first major international initiative to dealwith acid rain took place in 1979 when the UNEconomic Commission for Europe (UNECE)drafted a Convention on the Long RangeTransportation of Air Pollutants TheConvention was aimed at encouraging acoordinated effort to reduce SO

2 emissions in

Europe but the thirty-five signatories alsoincluded Canada and the United States (Park

Table 41 Commitment of selected nations to thereduction of sulphur dioxide emissions

Source Various sources including Park (1991)

1987) Although not a legally bindingdocument it did impose a certain moralobligation on the signatories to reduce acidpollution That was generally insufficienthowever and after additional conferences inStockholm (1982) Ottawa (1984) and Munich(1984) the UNECE was forced in 1985 toprepare an additional protocol on sulphuremissions This was a legally binding documentwhich required its signatories to reducetransboundary emissions of SO2 by 30 per cent(of the 1980 levels) before 1993 Manysubsequently improved on that figure (see Table41) but fourteen of the original thirty-fiveparticipants in the 1979 ECELRTAPConvention refused to sign among them Britainand the United States (Park 1987)Subsequently both have become embroiled withneighbouring states which signed the protocoland the resulting confrontation providesexcellent examples of the economic and politicalproblems accompanying attempts to reduceacid rain at the international level

Canada and the United States

Both Canada and the United States are majorproducers of acid gases (see Figure 414) Canadaranks fifth overall in the global SO2 emissionstable (Park 1987) but produces less than onefifth of the US total Canadian emissions of NOX

are only one tenth of those in the US When percapita emissions are considered however theCanadian output of SO2 is about double that ofthe US and NOX per capita output is about thesame north and south of the border (Ellis et al1990) Both countries are also exporters of acidgases with the United States sending three timesas much SO2 to Canada as Canada sends to theUnited States (Cortese 1986) It is thisdiscrepancy and the damage that it causes whichis at the root of the North American acid raincontroversy

As a member of the so-called lsquo30 per cent clubrsquoCanada is obligated to reduce SO2 emissions by30 per cent of the 1980 base level before 1993and has already implemented abatement

programmes which will make a 50 per centreduction possible by 1994 (Israelson 1987)Over 80 per cent of the 1994 objective had beenmet by 1991 (Environment Canada 1991) Suchcommitments as have been made by the UnitedStates have been in the form of finance forincreased research Sulphur dioxide emissionshave been reduced in New England and in theMid-Atlantic states but emissions continue toincrease in the mid-west and southeastern states(Cortese 1986) It is possible that by the year2000 SO2 emissions will have risen by 8 to 13per cent (Ellis et al 1990) although legislationhas been put in place to reduce emissions by some9 million tonnes (Howard and Perley 1991)

The pollution most affecting Canada comesfrom the mid-west particularly the Ohio valleywhere six states emit more than a million tonnesof SO2 per year (Israelson 1987) This area toois the most resistant to abatement proceduresbecause of their potentially negative impact on aregional economy based on high-sulphurbituminous coal and its representatives havelobbied strongly and successfully against theinstitution of emission controls

Such opposition to acid rain control souredrelationships between Canada and the UnitedStates Discussions between the two countrieswent on for over a decade before any substantiveagreement on the problem of transboundarytransportation of acid rain was reached Thiscame about finally as part of a comprehensiveUS Clean Air Act passed into law in 1990(Howard and Perley 1991) The full effects willnot be felt until the end of the century but thelegislation ensures a brighter future for the acidsensitive environment of eastern Canada and theUnited States

Britain and Scandinavia

Britain was one of the few nations in Europenot to join the lsquo30 per cent clubrsquo when it wasformed although it subsequently revealedplans to reduce SO2 emissions by 60 per centSituated to the north-west of the continent itwas relatively free from external emissions of

ACID RAIN 97

acid pollution but added more than 45million tonnes annually to the westerly airstream (Park 1987) In the early 1980s anestimated 25 per cent of the SO2 leaving Britainwas deposited in the North Sea 4 per centlanded in Norway and Sweden and 6 per centcontinued on to fall in the USSR (Pearce1982d) Between 1980 and 1985 SO2

emissions from British industry fell by about25 per cent but levelled off or rose slightlyafter that (Mason 1990) At that time Britainwas the largest producer of acid pollution inEurope after the USSR Norway and Swedenwere among the lowest and made a limitedcontribution to their own acid rain problem(Park 1987) (see Figure 415) In the acid rainfalling on Norway for example thecontribution from British sources was twicethat from local Norwegian sources (Pearce

1982d) A considerable proportion of the acidrain falling in Scandinavia originated in EastGermany but it was to Britain that theScandinavians turned to seek help in reducingthe environmental damage being caused byacid rain

Much of the blame for the acid rain damagein Scandinavia came to rest on the shoulders ofthe Central Electricity Generating Board (CEGB)the body which controlled electric powerproduction in England Emissions from CEGBpower stations were responsible for more thanhalf the SO2 produced in Britain (Pearce 1984)Thus any planned reduction in emissionsrequired the cooperation of the CEGB TheBoard in common with most authorities inBritain argued that the problem required furtherstudy but that position eventually becameuntenable In 1986 for the first time Britain

Figure 414 Emissions of SO2 and NO

2 in Canada and the United States 1970ndash89

agreed with Norway that acid rain originatingin Britain was causing problems in the Norwegianenvironment Subsequently the Britishgovernment announced its intentions to cut SO2

emissions by 14 per centmdashcommendableperhaps but still well below that required forentry into the lsquo30 per cent clubrsquo (Park 1987)

An agreement reached by the EnvironmentMinisters of the EC in June 1988 required a 70

per cent reduction in emissions by the year 2003Britain agreed only to a 60 per cent reductionand the CEGB initiated a series of measuresaimed at meeting that requirement (Kyte 1988)By the following year planning was in place tofit FGD equipment to six coal-fired powerstations at a total cost of some pound2 billion (c $4billion) but the whole process was thrown intodisarray by the privatization of the electricity

Figure 415 The balance of trade in sulphur dioxide in 1980

Source From Park (1987)

ACID RAIN 99

generating industry in Britain which saw theCEGB replaced by two private com-panics in1990 After some consideration of fuel-switchingas a cheaper alternative both companies revertedto FGD technology for emission control but onlyat three stations rather than the original six (Park1991)

Although this procrastination means that theproposed emission reductions will take longer toachieve it may allow Britain to take advantageof new technology currently under developmentExperiments with systems such as LIMB whichinvolve the furnace injection of acid reducingchemicals promise emission reductions that aremore cost-effective than those of existingscrubber technology (Ridley 1993) Theinstallation of these new cheaper systems maydeal with some of the economic issues which havecontributed to the slow adoption of emissioncontrol strategies in the past Ironically theenvironmental effects of reduced acid emissionsmay never be fully known Cooperative studiesby British and Scandinavian scientists have allbut ended and many of the research fundsavailable for the study of acid rain in the 1970sand 1980s have been lost to apparently morepressing issues including global warming andozone depletion (Pearce 1990)

SUMMARY

Acid rain is a natural product of atmosphericchemical reactions Inadvertent humaninterference in the composition of theatmosphere through the addition of SO2 and

NOX has caused it to develop into a majorenvironmental problem It is mainly confined tothe industrialized areas of the northernhemisphere at present but has the potential tobecome a problem of global proportions Acidrain damage is extensive in the aquaticenvironment and is increasingly recognized inthe terrestrial environment Damage to buildingsis common in some areas but the effect of acidrain on human health is less obvious

Progress towards the large scale abatement ofacid emissions has been slow and methods forcontrolling NOX lag behind those for dealing withSO2 Emissions of sulphur dioxide are beginningto decline in many areas Although lakes andforests damaged by acid rain will take some timeto recover action is being taken to improve thesituation and that in itself is psychologicallyimportant Finally much has been written aboutthe necessity to lsquoclean-uprsquo acid rain Ironicallythe acidity is really the end-product of a series ofnatural cleansing processes by which theatmosphere attempts to maintain some degreeof internal chemical balance

Adriano DC and Haas M (1989) AcidicPrecipitation vol 1 Case Studies New YorkSpringer-Verlag

Howard R and Perley M (1991) PoisonedSkies Toronto Stoddart

Park CC (1987) Acid Rain Rhetoric andReality London Methuen

One of the more obvious indications ofatmospheric pollution is the presence of solid orliquid particles called aerosols dispersed in theair These aerosols are responsible forphenomena as diverse as the urban smogs thatbedevil the worldrsquos major cities and thespectacular sunsets which often follow majorvolcanic eruptions The concentration anddistribution of particulate matter in theatmosphere is closely linked to climaticconditions Some local or regional climatesencourage high aerosol concentrations as inLos Angeles for example with its combinationof high atmospheric pressure light winds andabundant solar radiation On a global scale themid-latitude westerlies and their associatedweather systems already implicated in thedistribution of acid rain are responsible for thetransportation of aerosols over long distancesin the troposphere The jet streams in the upperatmosphere are also involved in the distributionof aerosols carrying particles around the worldseveral times before releasing them Knowledgeof such relationships has important practicalimplications At the local level the success ofpollution abatement programmes oftendepends upon an understanding of the impactof climate on aerosol distribution At acontinental or even hemispheric scale therelationship between atmospheric circulationpatterns and the spread of particulate mattercan be used to provide an early warning ofpotential problems following catastrophicevents such as volcanic eruptions or nuclearaccidents In such situations the atmosphericaerosols are responding to existing climatic

conditions There has been growing concern inrecent years that they may do more than thatthey may also be capable of initiating climaticchange

AEROSOL TYPES PRODUCTION ANDDISTRIBUTION

The total global aerosol production is presentlyestimated to be between 2-3times109 tonnes perannum and on any given day perhaps as manyas 1times107 tonnes of solid particulate matter issuspended in the atmosphere (Bach 1979Cunningham and Saigo 1992 Ahrens 1993)Under normal circumstances almost all of thetotal weight of particulate matter isconcentrated in the lower 2 km of theatmosphere in a latitudinal zone between 30degNand 60degN (Fennelly 1981) The mean residencetime for aerosols in the lower troposphere isbetween 5 and 9 days which is sufficientlyshort that the air can be cleansed in a few daysonce emissions have stopped (Williamson1973) The equivalent time in the uppertroposphere is about one month and in thestratosphere the residence time increases to twoto three years (Williamson 1973) As a resultanything added to the upper troposphere orstratosphere will remain in circulation for alonger time and its potential environmentalimpact will increase

Aerosols can be classified in a number of waysbut most classifications include such elements asorigin size and development sometimesindividually sometimes in combination (seeFigure 51) Most of the atmospherersquos aerosol

Atmospheric turbidity

ATMOSPHERIC TURBIDITY 101

contentmdashperhaps as much as 90 per cent (Bach1979)mdashis natural in origin althoughanthropogenic sources may be dominant locallyas they are in urban areas for example (see Figure52) Dust particles created during volcanicactivity or carried into the troposphere duringdust storms are examples of common naturalaerosols Dust blown eastwards from the GobiDesert and adjacent arid lands of northern Chinais a significant constituent of the atmosphere inBeijing in April and May Less frequent stormscan also have local significance A major duststorm in Australia in the summer of 1983 carried

dust as high as 36 km into the atmosphere anddeposited 106 kg of dust per hectare over thecity of Melbourne (Lourensz and Abe 1983)That dust was picked up from the desiccatedwheat fields and overgrazed pastures of Victoriaand New South Wales and farming practicesalong with quarrying and a variety of industrialprocesses make an important contribution to dustof anthropogenic origin (Williamson 1973)Natural particles of organic originmdashsuch as theterpenes considered responsible for the hazecommon over such areas as the Blue Mountainsof Virginiamdashjoin hydrocarbon emissions fromhuman activities to provide an important groupof organic aerosols Total organic emissions inthe world have been estimated at as much as44times108 tonnes per annum (Fennelly 1981)

Bolle et al (1986) have grouped aerosols intoseveral mixtures which include combinations ofthe different particle types (see Figure 51) Threeof these are geographically based In thecontinental mixture dust-like particles of naturalorigin predominate (70 per cent of the total) andmost of these are confined to the troposphereThey may be carried over considerable distancehowever dust from the Sahara and Sahel regionsof Africa has been carried on the tropicaleasterlies to the Caribbean (Morales 1986)Logically volcanically produced aerosols couldbe included in the continental mixture butperhaps because of the major contribution ofvolcanic eruptions to aerosol loading volcanicash has been included as a separate item in theclassification In the urbanindustrial mix watersoluble aerosols make up more than 60 per centof the total They are of anthropogenic originand include a high proportion of sulphates Sootand water vapour are other important productsof urban and industrial activities butmechanically produced dust particles are ofrelatively minor importance The effects ofindividual urbanindustrial aerosols may beenhanced by combination with other constituentsof the atmosphere Chemical particlesmdashsoot andfine dust for examplemdashmay act as condensationnuclei for the water vapour and the net result isto increase cloudiness in urbanindustrial areas

Figure 51 A sample of the different approaches usedin the classification of atmospheric aerosols

Aerosols of urban and industrial origin arenormally considered to be restricted in theirdistribution since they are mainly released intothe lower troposphere However products ofurban combustion found in the Arctic and theincreasing levels of sulphates in the stratospheresuggest that their effects now extend beyond thelocal area (Shaw 1980) The third geographically

based aerosol mixture recognized by Bolle et al(1986) is maritime in origin It includes waterand sea-spray particles for the most part withonly a small proportion of water-soluble particles(c 5 per cent)

Aerosols can range in size from a fewmolecules to a visible grain of dust but thedistribution across that range is not even The

Figure 52 Diagrammatic representation of the sources and types of atmospheric aerosols

Figure 53 A comparison of the size-range of common aerosols with radiation wavelength

main mass of particulates is concentrated in twopeaks one between 001 micrometre (microm) and 1microm centred at 01 microm and the other between 1microm and 100 microm (see Figure 53) centred at 10microm (Shaw 1987) The smaller particles in the firstgroup are called secondary aerosols since theyresult from chemical and physical processeswhich take place in the atmosphere They includeaggregates of gaseous molecules water dropletsand chemical products such as sulphateshydrocarbons and nitrates As much as 64 percent of total global aerosols are secondaryparticulates 8 per cent of them anthropogenicin origin from combustion systems vehicleemissions and industrial processes The other 56per cent are from natural sources such asvolcanoes the oceans and a wide range of organicprocesses (Fennelly 1981) Some estimatessuggest that sulphate particles are now the largestgroup of atmospheric aerosols accounting foras much as 50 per cent of all secondary particles(Toon and Pollack 1981) Most of thetropospheric sulphates are of anthropogenicorigin and contribute to the problems of acidrain (see Chapter 4) whereas those found in thestratosphere are most likely to be the productsof volcanic eruptions The larger particles withdiameters between 1 and 100 microm are calledprimary aerosols and include soil dust and solidindustrial emissions usually formed by thephysical breakup of material at the earthrsquos surface(Fennelly 1981) There is some evidence that thesegroupings are a direct result of the processes bywhich the aerosols are formed Mechanicalprocesses are unable to break substances intopieces smaller than 1 microm in diameter whereasthe growth of secondary particles appears tocease as diameters approach 1 microm (Shaw 1987)

AEROSOLS AND RADIATION

Atmospheric aerosols comprise a veryheterogeneous group of particles and the mixwithin the group changes with time and placeFollowing volcanic activity for example theproportion of dust particles in the atmospheremay be particularly high in urban areas such as

Los Angeles photochemical action on vehicleemissions causes major increases in secondaryparticulate matter over the oceans 95 per centof the aerosols may consist of coarse sea-saltparticles Such variability makes it difficult toestablish the nature of the relationship betweenatmospheric aerosols and climate It is clearhowever that the aerosols exert their influenceon climate by disrupting the flow of radiationwithin the earthatmosphere system and thereare certain elements which are central to therelationship The overall concentration ofparticulate matter in the atmosphere controls theamount of radiation intercepted while the opticalproperties associated with the size shape andtransparency of the aerosols determines whetherthe radiation is scattered transmitted or absorbed(Toon and Pollack 1981) The attenuation ofsolar radiation caused by the presence of aerosolsprovides a measure of atmospheric turbidity aproperty which for most purposes can beconsidered as an indication of the dustiness ordirtiness of the atmosphere

Several things may happen when radiationstrikes an aerosol in the atmosphere If theparticle is optically transparent the radiantenergy passes through unaltered and nochange takes place in the atmospheric energybalance More commonly the radiation isreflected scattered or absorbedmdashinproportions which depend upon the size colourand concentration of particles in theatmosphere and upon the nature of theradiation itself (see Figure 53) Aerosols whichscatter or reflect radiation increase the albedoof the atmosphere and reduce the amount ofinsolation arriving at the earthrsquos surfaceAbsorbent aerosols will have the oppositeeffect Each process through its ability tochange the path of the radiation through theatmosphere has the potential to alter theearthrsquos energy budget The water droplets inclouds for example are very effective inreflecting solar energy back into space before itcan become involved in earthatmosphereprocesses Some of the energy scattered byaerosols will also be lost to the system but a

proportion will be scattered forward towardsits original destination Most aerosolsparticularly sulphates and fine rock particlesscatter solar radiation very effectively

The most obvious effects of scattering arefound in the visible light sector of the radiationspectrum Particles in the 01 to 10 microm size rangescatter light in the wavelengths at the blue endof the spectrum while the red wavelengthscontinue through As a result when the aerosolcontent of the atmosphere is high the skybecomes red (Fennelly 1981) This is common inpolluted urban areas towards sunset when thepath taken by the light through the atmosphereis lengthened and interception by aerosols isincreased Natural aerosols released duringvolcanic eruptions produce similar results Theoptical effects which followed the eruption ofKrakatoa in 1883 for example included not onlymagnificent red and yellow sunsets but also asalmon pink afterglow and a green colourationwhen the sun was about 10deg above the horizon(Lamb 1970) As well as being aestheticallypleasing the sequence and development of thesecolours allowed observers to calculate the sizeof particles responsible for such opticalphenomena (Austin 1983)

Among the atmospheric aerosols desert dustand soot particles readily absorb the shorter solarwave lengths (Toon and Pollack 1981 Lacis etal 1992) with soot a particularly strong absorberacross the entire solar spectrum (Turco et al1990) The degree to which a substance is capableof absorbing radiation is indicated by its specificabsorption coefficient For soot this value is 8ndash10 m2g-1 which means that 1 g of soot can blockout about two-thirds of the light falling on anarea of 8ndash10 m2 (Appleby and Harrison 1989)Individual soot particles in the atmosphere areapproximately 01 microm in diameter and tend tolink together in branching chains or looseaggregates With time these clusters become morespherical and their absorption coefficientdeclines but even when the aggregate diametersexceed 04 microm the specific absorptivity mayremain as high as 6 m2g-1 (Turco et al 1990)Thus the injection of large amounts of soot into

the atmosphere has major implications for theearthrsquos energy budget

In addition to disrupting the flow of incomingsolar radiation the presence of aerosols also hasan effect on terrestrial radiation Being at a lowerenergy level the earthrsquos surface radiates energyat the infrared end of the spectrum Aerosolsmdashsuch as soot soil and dust particlesmdashreleased intothe boundary layer absorb infrared energy quitereadily particularly if they are larger than 10microm in diameter (Toon and Pollack 1981) and asa result will tend to raise the temperature of thetroposphere However the absorption efficiencyof specific particles varies with the wavelengthof the radiation being intercepted The absorptioncoefficient of soot at infrared wavelengths forexample is only about one-tenth of its value atthe shorter wavelengths of solar radiation Inaddition since they are almost as warm as theearthrsquos surface tropospheric aerosols are lessefficient at blocking the escape of infraredradiation than colder particles such as those inthe stratosphere (Bolle et al 1986) Thus longer-wave terrestrial radiation can be absorbed byparticles in the stratosphere and re-radiated backtowards the lower atmosphere where it has awarming effect Much depends upon the size ofthe stratospheric aerosols If they are smaller indiameter than the wavelengths of the outgoingterrestrial radiation as is often the case they tendto encourage scattering and allow less absorption(Lamb 1970) The net radiative effects ofparticulate matter in the atmosphere are difficultto measure or even estimate They include acomplexity which depends upon the size shapeand optical properties of the aerosols involvedand upon their distribution in the stratosphereand troposphere

Any disruption of energy fluxes in the earthatmosphere system will be reflected ultimatelyin changing values of such climatologicalparameters as cloudiness temperature or hoursof bright sunshine Although atmosphericaerosols produce changes in the earthrsquos energybudget it is no easy task to assess theirclimatological significance Many attempts atthat type of assessment have concentrated on

volcanic dust which for a number of reasons isparticularly suitable for such studies Forexample the source of the aerosols can beeasily pin-pointed and the volume of materialinjected into the atmosphere can often becalculated the dust includes particles from abroad size-range and it is found in both thetroposphere and the stratosphere Recentstudies however have suggested that thesulphate particles produced during volcaniceruptions have a greater impact on the energybudget than volcanic dust and ash

VOLCANIC ERUPTIONS ANDATMOSPHERIC TURBIDITY

The large volumes of particulate matter throwninto the atmosphere during periods of volcanicactivity are gradually carried away from theirsources to be redistributed by the wind andpressure patterns of the atmosphericcirculation Dust ejected during the explosiveeruption of Krakatoa in 1883 encircled theearth in about two weeks following the originaleruption (Austin 1983) and within 8 to 12weeks had spread sufficiently to increaseatmospheric turbidity between 35degN and 35degS(Lamb 1970) The diffusion of dust from theMount Agung eruption in 1963 followed asimilar pattern (Mossop 1964) and in bothcases the debris eventually spread polewardsuntil it formed a complete veil over the entireearth The cloud of sulphate particles ejectedfrom El Chichoacuten in 1982 was carried aroundthe earth by the tropical easterlies in less than20 days and within a year had blanketed theglobe (Rampino and Self 1984) Similarly thevolcanic debris injected into the atmosphereduring the eruption of Mount Pinatubo in June1991 circled the earth at the equator in about23 days (Gobbi et al 1992) It reached Japantwo weeks after the eruption began (Hayashidaand Sasano 1993) Within 20 days the edge ofthe debris cloud had reached southern Europe(Gobbi et al 1992) and less than 2 monthslater was recognized in the stratosphere abovesouthern Australia (Barton et al 1992) By

October of 1991 the cloud was present aboveResolute at 74degN in the Canadian Arctic andby early 1992 had spread worldwide (Rosen etal 1992) Sulphate aerosols and fine ashparticles from Mount Hudson which erupted inChile in August 1991 were carried by strongzonal westerlies between 45degS and 46degS to passover southeastern Australia within 5 days ofthe eruption and around the earth in just over aweek (Barton et al 1992)

The build up of the dust veil and its eventualdispersal will depend upon the amount ofmaterial ejected during the eruption and theheight to which the dust is projected into theatmosphere The eruption of Krakatoa releasedat least 6 cubic km (and perhaps as much as 18cubic km) of volcanic debris into the atmosphere(Lamb 1970) In comparison Mt St Helensproduced only about 27 cubic km (Burroughs1981) Neither of these can match the volume ofdebris from Tambora an Indonesian volcanowhich erupted in 1815 producing an estimated80 cubic km of ejecta (Findley 1981) Moreimportant than the total particulate productionhowever is its distribution in the atmosphereThat depends very much on the altitude to whichthe debris is carried and whether or not itpenetrates beyond the tropopause The maximumheight reached by dust ejected from Krakatoahas been estimated at 50 km and a similar altitudewas reached by the dust column from MountAgung in 1963 (Lamb 1970) A particularlyviolent eruption at Bezymianny in Kamchatkain 1956 threw ash and other debris to a heightof 45 km (Cronin 1971) but Mt St Helensdespite the explosive nature of its eruption failedto push dust higher than 20 km perhaps becausethe main force of the explosion was directedhorizontally rather than vertically (Findley 1981)As a result it has been estimated that Mt StHelens injected only 5 million tonnes of debrisinto the stratosphere compared with 10 milliontonnes for Mount Agung and as much as 50million tonnes for Krakatoa (Burroughs 1981)The eruptions of El Chichoacuten in 1982 and MountPinatubo in 1991 injected an estimated 20 and30 million tonnes of material respectively into

the stratosphere mostly in the form of sulphurdioxide (SO2) which ultimately producedsulphuric acid aerosols (Brasseur and Granier1992) In the case of Mount Pinatubo theamount of SO2 injected was probably as muchas that from Krakatoa (Groisman 1992)although the total debris production from thelatter was higher

Since the altitude of the tropopause decreaseswith latitude (see Chapter 2) even relativelyminor eruptions may contribute dust to thestratosphere in high latitudes The dust plumefrom the Surtsey eruption off Iceland in 1963for example penetrated the tropopause at 105km (Cronin 1971) whereas the products of acomparable eruption in equatorial regionswould have remained entirely within thetroposphere When Nyamuragira in Zaireerupted in 1981 for example it producedalmost as much SO2 as El Chichoacuten but little ofit reached the stratosphere In contrast the forceof the eruption of Mount Pinatubo penetratedthe tropopause at about 14 km and carrieddebris up for another 10 to 15 km (Gobbi et al1992) Particulate matter which is injected intothe stratosphere in high latitudes graduallyspreads out from its source but its distributionremains restricted Most high latitude volcanoesin the northern hemisphere are located in a beltclose to the Arctic Circle and there is noevidence of dust from an eruption in this beltreaching the southern hemisphere (Cronin1971) In contrast products of eruptions inequatorial areas commonly spread to form aworld-wide dust veil (Lamb 1970) As a result ofthis it might be expected that when volcanoesare active in both regions turbidity in thenorthern hemisphere would be greater than inthe southern Atmospheric turbidity patterns inthe period between 1963 and 1970 when fourvolcanic plumes in the Arctic Circle belt andthree in equatorial regions penetrated thetropopause tend to confirm the greaterturbidity of the northern stratosphere undersuch conditions (Cronin 1971) There are fewervolcanoes in high latitudes in the southernhemisphere than in the north but the same

restrictions on the distribution of volcanicdebris seem to apply For example the volcanicash and sulphuric acid from the eruption ofMount Hudson in southern Chile in 1991penetrated the tropopause at about 9 km andwas carried around the world quite rapidly onthe upper westerlies However simulationscarried out to estimate the subsequent spread ofthe debris cloud suggest that it remainedrestricted to the area between 70degS and 30degS(Barton et al 1992)

The dust veil index

Individual volcanic eruptions differ from eachother in such properties as the amount of dustejected the geographical extent of its diffusionand the length of time it remains in theatmosphere Comparison is possible using theseelements but to simplify the process and tomake it easier to compare the effects of differenteruptions on weather and climate Lamb (1970)developed a rating system which he called a dustveil index (DVI) It was derived using formulaewhich took into account such parameters asradiation depletion the estimated lowering ofaverage temperatures the volume of dustejected and the extent and duration of the veilThe final scale of values was adjusted so that theDVI for the 1883 eruption of Krakatoa had avalue of 1000 Other eruptions were thencompared to that base The 1963 eruption ofMount Agung was rated at 800 for examplewhereas the DVI for Tambora in 1815 was3000 (Lamb 1972)

Individual dust veils may combine to producea cumulative effect when eruptions are frequent(see Figure 54) The 1815 eruption of Tamborafor example was only the worst of severalbetween 1811 and 1818 The net DVI for thatperiod was therefore 4400 Similarly Lamb(1972) estimated that between 1694 and 1698the world DVI was 3000 to 3500 At times whenvolcanic activity is infrequent the DVI is low asit was between 1956 and 1963 when no eruptionsinjected debris into the stratosphere (Lockwood1979)

The DVI provides an indication of thepotential disruption of weather and climate byvolcanic activity Dust in the atmosphere reducesthe amount of solar radiation reaching the earthrsquossurface and at high index levels that reductioncan be considerable This is particularly so inhigher latitudes where the sunrsquos rays follow alonger path through the atmosphere and aretherefore more likely to be scattered Majoreruptions producing a DVI in excess of 1000have caused reductions in direct beam solarradiation of between 20 and 30 per cent forseveral months (Lamb 1972) The effect isdiminished to some extent by an increase indiffuse radiation but the impact on net radiationis negative Observations in Australia followingthe eruption of Mount Agung in 1963 showed amaximum reduction of 24 per cent in direct beamsolar radiation yet because of the increase indiffuse radiation net radiation fell by only 6 per

cent (Dyer and Hicks 1965) Similar values wererecorded at the Mauna Loa Observatory inHawaii at that time (Ellis and Pueschel 1971) ElChichoacuten reduced net radiation by 2ndash3 per centat ground level (Pollack and Ackerman 1983)while the reduction caused by Pinatubo averaged27 per cent over a 10-month period followingthe eruption with individual monthly valuesreaching as high as 5 per cent (Dutton and Christy1992)

A number of problems with the DVI have beenidentified since it was first developed and thesehave been summarized by Chester (1988) TheDVI was based solely on dust and did not includethe measurement of sulphates or sulphuric acidaerosols which are now recognized as being veryeffective at scattering solar radiation As a resultthe impact of sulphur-rich eruptions such as ElChichoacuten or Mount Pinatubo would beunderestimated At the same time there is no way

Figure 54 Cumulative DVI for the northern hemisphere dust from an individual eruption is apportioned over4 yearsndash40 to year 1 30 to year 2 20 to year 3 and 10 to year 4

Source From Bradley and Jones (1992)

of preventing non-volcanic sources of dust frombeing included in the index The use of climaticparameters in the calculation of certain indexvalues may introduce the possibility of circularreasoning For example falling temperatures aretaken as an indication of an increasing DVI yeta high DVI may also be used to postulate orconfirm falling temperatures

In an attempt to deal with some of theseproblems a number of researchers reassessedLambrsquos index but introduced only limitedrefinement and modification (Mitchell 1970Robock 1981) Other indices have also beenproposed although none has been as widely usedas the original DVI One example is the volcanicexplosivity index (VEI) developed as a result ofresearch sponsored by the Smithsonian Instituteinto historic eruptions (Chester 1988) It is basedonly on volcanological criteria such as theintensity dispersive power and destructivepotential of the eruption as well as the volumeof material ejected It also includes a means ofdifferentiating between instantaneous andsustained eruptions (Newhall and Self 1982)Being derived entirely from volcanologicalcriteria it eliminates some of the problems ofthe DVImdashsuch as circular reasoningmdashforexample but it does not differentiate betweensulphates and dust nor does it include correctionsfor the latitude or altitude of the volcanoes(Chester 1988) Thus although the VEI isconsidered by many to be the best index ofexplosive volcanism it is not without its problemswhen used as a tool in the study of climaticchange

A glaciological volcanic index (GVI) has alsobeen proposed (Legrand and Delmas 1987)Based on ice cores it would reveal acidity levelsin glacial ice and therefore give an indication ofthe SO2 levels associated with past volcaniceruptions Since neither the DVI nor the VEI dealadequately with volcanic SO2 emissions the GVIhas the potential to improve knowledge of thecomposition of volcanic debris However currentice core availability is limited and the GVI addslittle to the information available fromestablished indices (Bradley and Jones 1992)

Volcanic activity weather and climate

Many of the major volcanic eruptions inhistorical times have been followed by short-termvariations in climate which lasted only as longas the dust veil associated with the eruptionpersisted The most celebrated event of this typewas the cooling which followed the eruption ofTambora in 1815 It produced in 1816 lsquothe yearwithout a summerrsquo remembered in Europe andNorth America for its summer snowstorms andunseasonable frosts Its net effect on worldtemperature was a reduction of the mean annualvalue by 07degC but the impact in mid-latitudesin the northern hemisphere was greater with areduction of 1degC in mean annual temperatureand average summer temperatures in parts ofEngland some 2ndash3degC below normal (Lamb1970) The eruption of Krakatoa in 1883 wasalso followed by lower temperatures which made1884 the coolest year between 1880 and thepresent (Hansen and Lebedeff 1988)

Increased volcanic activity may have been acontributing factor in the development of theLittle Ice Agemdashwhich persisted with varyingintensity from the mid-fifteenth to themidnineteenth century The eruption of Tamborafalls within that time span and other eruptionshave at least a circumstantial relationship withclimatic change A volcanic dust veil may havebeen responsible for the cool damp summers andthe long cold winters of the late 1690s in thenorthern hemisphere which ruined harvests andled to famine disease and an elevated death ratein Iceland Scotland and Scandinavia (Parry1978) Lamb (1970) has suggested that dust veilswere important during the Little Ice Age becausetheir cumulative effects promoted an increase inthe amount of ice on the polar seas which inturn disturbed the general atmosphericcirculation He also points out however thatcontemporaneity between increased volcanicactivity and climatic deterioration was notcomplete Some of the most severe winters inEuropemdashsuch as those in 1607ndash08 and 1739ndash40mdashoccurred when the DVI was low and theperiod of lowest average winter temperatures did

not coincide with the greatest cumulative DVIThus although increased volcanic activity andthe associated dust veils can be linked todeterioration between 1430 and 1850 it is likelythat volcanic dust was only one of a number offactors which contributed to the development ofthe Little Ice Age at that time

Major volcanic episodes in modern times haveusually been accompanied by prognosticationson their impact on weather and climate althoughit is not always possible to establish the existenceof any cause and effect relationship The Agungeruption produced the second largest DVI thiscentury but its impact on temperatures was lessthan expected perhaps because the dust fell outof the atmosphere quite rapidly (Lamb 1970) Itis estimated that it depressed the meantemperature of the northern hemisphere by a fewtenths of a degree Celcius for a year or two(Burroughs 1981) but such a value is well withinthe normal range of annual temperaturevariation The spectacular eruption of Mt StHelens in 1980mdashenhanced in the popularimagination by intense media coveragemdashpromoted the expectation that it would have asignificant effect on climate and it was blamedfor the poor summer of 1980 in Britain Incomparison to other major eruptions in the pasthowever Mt St Helens was relatively insignificantin climatological terms It may have produced a

cooling of a few hundredths of a degree Celciusin the northern hemisphere where its effectswould have been greatest (Burroughs 1981) Theeruption of El Chichoacuten in Mexico in 1982produced the densest aerosol cloud sinceKrakatoa nearly a century earlier Within a yearit had caused global temperatures to decline byat least 02degC and perhaps as much as 05degC(Rampino and Self 1984) However the coolingproduced by El Chichoacuten may have been offsetby as much as 02degC as the result of an El Nintildeoevent which closely followed the eruption andeffectively prevented cooling in the southernhemisphere (Dutton and Christy 1992) (seeFigure 55) Past experience suggested that theeruption of Mount Pinatubo would also lead tolower temperatures It was blamed for the coolsummer of 1992 in eastern North America andby September of that year it was linked withreductions in global and northern hemispheretemperatures of 05degC and 07degC respectively(Dutton and Christy 1992) Estimates bymodellers studying world climatic changeindicate that such cooling would be sufficient toreversemdashat least temporarilymdashthe globalwarming trends characteristic of the 1980s(Hansen et al 1992)

Although volcanic activity is most commonlyassociated with cooling there is some evidencethat it may also cause short-term local warming

Figure 55 Monthly mean global temperature anomalies obtained using microwave sounding units (MSU)The events marked are AmdashLa Nintildea OmdashEl Nintildeo EmdashEl Chichoacuten PmdashPinatubo

Source After Dutton and Christy (1992)

Groisman (1992) has compared temperaturerecords in Europe and the northeastern UnitedStates with major volcanic eruptions between1815 and 1963 The results show that in westernEurope and as far east as central Russia andUkraine statistically significant positivetemperature anomalies occur in the wintermonths some 2 to 3 years after an eruption thesize of Krakatoa (see Figure 56) The analysis ofdata following the eruption of El Chichoacuten

produced a similar pattern and Groismansuggests that the unusually mild winter of 1991ndash92 in central Russia was an indication of apositive anomaly associated with MountPinatubo The warming is seen as a result of thegreater frequency of westerly winds over Europewhich owe their development to an increase inzonal temperature gradients following the generalglobal cooling associated with major volcaniceruptions

Volcanoes have the ability to contribute tochanges in weather and climate at a variety oftemporal and spatial scales At a time when thehuman input into global environmental changeis being emphasized it is important not to ignorethe contribution of physical processes such asvolcanic activity which have the ability toaugment or diminish the effects of theanthropogenic disruption of the earthatmosphere system

THE HUMAN CONTRIBUTION TOATMOSPHERIC TURBIDITY

The volume of particulate matter produced byhuman activities cannot match the quantitiesemitted naturally (see Figure 57) Estimates ofthe human contribution to total global particulateproduction vary from as low as 10 per cent (Bach1979) to more than 15 per cent (Lockwood 1979)with values tending to vary according to the size-fractions included in the estimate Humanactivities may provide as much as 22 per cent ofthe particulate matter finer than 5 microm forexample (Peterson and Junge 1971) It might beexpected that the human contribution toatmospheric turbidity would come mainly fromindustrial activity in the developed nations of thenorthern hemisphere and that was undoubtedlythe case in the past but data from someindustrialized nations indicate that emissions ofparticulate matter declined in the 1980s (seeFigure 58) There is also some evidence that theburning of tropical grasslands is an importantsource of aerosols (Bach 1976) and althoughspecific volumes are difficult to estimate smokeand soot released during the burning of tropical

Figure 56 Mean anomalies of surface air temperature(degC) for the period 2 to 3 years after volcaniceruptions with volcanic explosivity powerapproximately equalto the Krakatoa eruption (a) Winter (b) SummerPoints show the stations used Dashed regions depict negative anomalies

Source From Groisman (1992)

rainforests must make a significant contributionto turbidity Some authorities would also includesoil erosionmdashcreated by inefficient or destructiveagricultural practicesmdashamong anthropogenicsources of aerosols (Lockwood 1979)Anthropogenically generated particulate mattertends to remain closer to the earthrsquos surface thanthe natural variety as is readily apparent in mostlarge urban areas One of the few exceptions isthe direct injection of soot particles into thestratosphere by high-flying commercial aircraftThe annual production of elemental carbon fromthat source is estimated at 16000 tonnes ofwhich 10 per cent is added directly to thestratosphere With the introduction of a newgeneration of supersonic aircraftmdashwhich wouldspend a greater proportion of their flight time inthe stratospheremdashstratospheric soot loadingwould probably double (Pueschel et al 1992)

Various attempts have been made to estimatethe impact of human activities on backgroundlevels of atmospheric turbidity One approach tothis is to examine turbidity levels in locations suchas the high Alps and the Caucasus (renownedfor their clean air) or mid-ocean and polarlocations (far removed from the common sourcesof aerosols) These are seen as reference pointsfrom which trends can be established As itstands the evidence is remarkably contradictory

There are data which support the view thathuman activities are enhancing global aerosollevels A set of observations from Davos inSwitzerland suggests an 88 per cent increase inturbidity in the thirty years up to the mid-1960s(McCormick and Ludwig 1967) and dustfall inthe Caucasus Mountains showed a rapid increaseduring a similar time span (Bryson 1968) AtMount St Katherine over 2500 m up into the

Figure 57 A comparison of natural and anthropogenic sources of particulate matter

Note The volcanic contributions appear small Their major impact is produced by their ability to make arapid and intense contribution to the aerosol content of the atmosphere

atmosphere in the Sinai measurements indicate a10 per cent reduction in direct beam solar energyup to the mid-1970s in an area far removed frommajor industrial sources (Lockwood 1979) Thefine particle content of the air above the NorthAtlantic doubled between 1907 and 1969 and atMauna Loa in Hawaii one estimate suggests anincrease in turbidity of 30 per cent in the ten yearsbetween 1957 and 1967 (Bryson 1968) Thatperiod included the Mount Agung eruption butthe increase was still apparent after the effects ofthe volcanic emissions had been removed fromthe calculations (Bryson and Peterson 1968) Onthe assumption that natural background levels ofatmospheric turbidity should remain constant orfluctuate within a very narrow range these risingaerosol levels were ascribed to human activities

The development of the Arctic Haze in recentyears is generally considered to be an indication

of continuing anthropogenic aerosol loading ofthe atmosphere The haze is not of local originIt is created in mid-latitudes as atmosphericpollution which is subsequently carriedpolewards to settle over the Arctic It is mostpronounced between December and March forseveral reasons including the increased emissionof pollutants at that season the more rapid andefficient poleward transport in winter and thelonger residence time of haze particles in thehighly stable Arctic air at that time of year (Shaw1980) Vertical profiles through the air mass showthat the bulk of the aerosols are concentrated inthe lowest 2 km of the atmosphere (see Figure59) Arctic air pollution has increased since themid-1950s in parallel with increased aerosolemissions in Europe and the net result has beena measurable reduction in visibility andperturbation of the regional radiation budget(Barrie 1986)

Figure 58 Emissions of particulate matter from selected nations 1980ndash89

Arctic Haze includes a variety ofcomponentsmdashfrom dust and soot particles topesticidesmdashbut the most common constituent issulphate particles Their presence at increasinglyhigh concentrations not only in the Arctic butin other remote areas such as the republic ofGeorgia in the former Soviet Union far removedfrom major pollution sources is also causingconcern (Shaw 1987) because of their ability todisrupt the flow of energy in the atmosphere andbecause of the contribution they make to acidprecipitation (see Chapter 4)

Providing a direct contrast to thesedevelopments are studies which claim that it isnot possible to detect any human imprint inchanging atmospheric turbidity Observations inthe Antarctic in the mid-1960s revealed nosignificant change in aerosol levels in that areabetween 1950 and 1966 (Fischer 1967) Datafrom Mauna Loa where Bryson and Peterson

(1968) claimed to find evidence of rising levelsof turbidity induced by human activities werere-interpreted to show that there was no evidencethat human activity affected atmosphericturbidity on a global scale (Ellis and Pueschel1971) The short term fluctuations revealed inthe data were associated with naturally producedaerosols The contradictory results from MaunaLoa reflect differences in the scientificinterpretation of the data from one observatoryDifferences between stations are the result of acombination of physical and human factors andin reality are only to be expected Emission sitesare unevenly distributed Most are located in mid-latitudes in the northern hemisphere and thereare great gapsmdashover the oceans for examplemdashwhere little human activity takes place Theatmospheric circulation helps to spread thepollutants but since the bulk of the emissionsfrom anthropogenic sources are confined to the

Figure 59 An estimate of the mean vertical profile of the concentration of anthropogenic aerosol mass in thehigh Arctic during March and April

Source After Barrie (1986)Note C(H)C(0) is the concentration at a specific altitude divided by the concentration at the surface

lower troposphere their residence time in theatmosphere is relatively short Their eventualdistribution is therefore less widespread thanvolcanic emissionsmdashwhich are concentrated inthe upper troposphere and stratosphere Aerosolsproduced in the industrial areas of thenortheastern United States have almost entirelydropped out of the westerly air-stream before itreaches Europe but aerosols from Europeansources spread over most of North Africa inJanuary and may drift out over the Atlantic alsodepending upon the strength of the continentalhigh pressure system (Lockwood 1979) Suchpatterns may help to explain the rising turbiditylevels in the Alps and the Caucasus and even inthe Sinai In the southern hemisphere theparticulate contribution from industrial activityis negligible and there is only limited cross-equatorial flow from the north Aerosolsproduced during the burning of tropical forestsand grasslands will offset the reduction inindustrial aerosols (Bach 1976) but theproportion of aerosols of direct anthropogenicorigin is usually considered to be much less thanin the northern hemisphere The absence ofanthropogenic aerosol sources may explain thelack of change in turbidity over the Antarcticbut the presence of high pressure at the surfaceand the strong westerlies of the circumpolarvortex aloft may combine to prevent thetransport of aerosols into the area This iscertainly the case with natural aerosolsParticulate matter from Pinatubo and MountHudson was unable to penetrate the Antarcticcircumpolar vortex during the first winterfollowing its emission (Deshler et al 1992) Theexact contribution of anthropogenic sources ofaerosols to atmospheric turbidity is difficult todetermine and is likely to remain so until thenumber of measuring sites is increased and theircurrent uneven distribution is improved

THE KUWAIT OIL FIRES

In the final stages of the Gulf War in February1991 between 500 and 600 oil wells were setalight by the retreating Iraqi army These wells

continued to burn for several months kept alightby oil and gas brought to the surface underpressure from the underlying oil fields Duringthat time they added massive amounts of smokesulphur dioxide carbon dioxide unburnedhydrocarbons and oxides of nitrogen to theatmosphere Most of these products wereconfined to the lower half of the tropospherewith the top of the plume never exceeding 5 kmAt the height of the fires it was estimated thatsulphur dioxide was being added to theatmosphere at an equivalent rate of 61 milliontonnes per year and soot at 64 million tonnesper year (Johnson et al 1991) Although most ofthe original emissions were retained in the regionas the result of stable air masses those aerosolsthat had reached higher in the atmosphere werecarried northeastwards bringing acid rain to Iranand black snow to the mountains of PakistanSeveral months later unexpectedly high levels ofcarbon soot in the upper troposphere above Japanwere also identified as products of the oil fires(Okada et al 1992)

With particulate mass densities of 500ndash1000microgm-3 the impact of the pollution cloud onincoming solar radiation was spectacularBeneath the centre of the plume the shortwaveradiation flux was measured at zero (Johnson etal 1991) This led to daytime temperaturereductions beneath the cloud of as much as 55degC(Seager 1991) and although some infraredradiation was returned from the cloud it did littleto ameliorate the cooling Mean monthlytemperatures between March and Septemberwere reduced by 08 to 24degC and record lowmean monthly temperatures were established inJuly and August (Shaw 1992) With largeamounts of energy being intercepted by the plumeand absorbed by the soot particles the plumeitself warmed up In earlier assessments of theimpact of the oil fires it was suggested that suchinternal heating would be sufficient to causelofting of aerosols into the stratosphere whichwould in turn increase the climatic consequencesof the fires (Pearce 1991a) This did not happenhowever perhaps because of the low altitude ofthe initial plume and the rapidity with which

the fires were extinguished As a result althoughthe fires had a regional climatic impact the globalprognostications which included the failure ofthe Asian summer monsoon did not come to pass(Pearce 1991a Johnson et al 1991)

NUCLEAR WINTER

The Kuwait oil field fires provided theatmosphere with probably the greatest amountof anthropogenically generated aerosols everproduced by a single event and to a number ofobservers they shared similarities with the majorfires expected to follow a nuclear war (Pearce1991 a) Urban fires for example with readyaccess to smoke producing wood paper plasticsand fossil fuels would produce similarcombustion products The fires likely to beignited by nuclear explosions were estimated tobe much larger in scale however capable ofadding more than 200 million tonnes of smoketo the atmosphere over several weeks (Turco etal 1983) Together with the estimated 960

million tonnes of dust expected to be injectedinto the atmosphere by the initial nuclearexplosions this would increase the turbidity ofthe atmosphere to such an extent that majorglobal cooling would take place (see Figure 510)

The concept of nuclear winter grew out of astudy of the climatic consequences of nuclear warpublished by Turco et al in 1983 The studypostulated that a major nuclear conflict wouldbe followed by a very rapid cooling of the earthsufficient to cause temperatures to fall belowfreezing in some areas even in mid-summerBecause such an event would reduce temperaturesto winter levels even after a midsummer war itwas given the name nuclear winter and the workbecame known as the TTAPS study from the firstletters of the names of the investigators

The TTAPS hypothesis was based on theassumption that smoke and dust thrown into theatmosphere during a nuclear war would increaseatmospheric turbidity to such an extent that ahigh proportion of incoming solar radiationwould be prevented from reaching the earthrsquos

Figure 510 The development of nuclear winter (a) the conflict (b) post-conflict fires (c) nuclear winter (d)the after-effects

surface The net effect would be to drivetemperatures down Since none of this could bemeasured directly estimates were based on theresults of a series of mathematical simulationmodels

According to the TTAPS baseline scenario a5000 megatonne nuclear war would inject 960million tonnes of dust into the atmosphere (1megatonne is equivalent to 1 million tonnes ofTNT) Eighty per cent would reach thestratosphere and remain there for more than ayear This dust veil would be augmented by asmuch as 225 million tonnes of smoke producedover a period of several weeks by the massive firesignited by the initial explosions Incorporatedinto the atmospheric circulation the dust andsmoke would first spread over the northernhemispheremdashwhere it was assumed most of thenuclear explosions would take placemdashbeforebeing carried into the southern hemisphereeventually to blanket the entire globe

The net result of this massive increase inatmospheric turbidity would be the reduction ofincoming solar radiation by as much as 95 percent causing an average land surface cooling of10ndash20degC and maximum cooling of up to 35degCin the the interior of the continents in a matterof weeks The temperature reductions over theoceans and in coastal areas would be perhapsonly 1ndash3degC because the cooling would be offsetby the great heat capacity of the oceans Thejuxtaposition of these relatively mild conditionswith the cold over the land masses would createstrong horizontal temperature and pressuregradients and lead to severe coastal stormsChanges in the vertical temperature structure ofthe atmosphere induced by the absorption oflarge amounts of solar energy in the upperatmosphere would also contribute to a muchmodified global circulation It was estimated thatrecovery from all of these changes would takemore than a year The potential environmentalimpact of gases such as CO2 and NOX releasedinto the atmosphere along with the particulatematter was recognized and the biologicalconsequences of nuclear winter were noted butneither was elaborated

The TTAPS hypothesis elicited a strongresponse from the scientific community Theassumptions underlying the estimated enduranceand intensity of the smoke and dust clouds werestrongly criticized for example and themagnitude of the postulated temperature declinewas regarded as questionable (see eg Maddox1984 Singer 1984) Much of this initial criticismmerely detailed perceived flaws in the study itselfand did little to develop the concept or toinvalidate it (Teller 1984)

Other investigators examined the mechanicsof the study and replaced its simplistic one-dimensional atmospheric model with two- andthree-dimensional versions The state-of-the-art3-D general circulation models (GCMs)ultimately used included a greater number ofvariables and finer geographic resolution thanthe TTAPS model and incorporated earthatmosphere feedback loops In general theyindicated that planetary cooling would be muchless than postulated in the TTAPS scenario andit was suggested that the cooling following anuclear war would be more appropriatelyreferred to as lsquonuclear autumnrsquo (or lsquonuclear fallrsquo)than nuclear winter (Thompson and Schneider1986)

The environmental consequences of nuclearwar (ENUWAR) received little attention in theTTAPS scenario but a contemporary study by agroup of life scientists indicated that the net effectof a large scale nuclear war would be the globaldisruption of the biosphere and the disruptionof the biological support systems of civilization(Ehrlich et al 1983) (see Figure 511) Theseverity of the impact has been mitigated in linewith the successive modifications of the nuclearwinter hypothesis but the situation is stillconsidered serious The report of the ScientificCommittee on Problems of the Environment(SCOPE) concerning the ecological andagricultural impact of nuclear war (Harwell andHutchinson 1985) sponsored by the InternationalCouncil of Scientific Unions (ICSU) identifiedindirect biological effects as a major threat tosociety and subsequent studies by the SCOPE-ENUWAR researchers have confirmed this

(SCOPE-ENUWAR 1987 Appleby and mentsHarrison 1989)

The original TTAPS team has re-examinednuclear winter in light of the new informationavailable from laboratory studies field experiand

numerical modelling (Turco et al 1990) Whileareas of uncertainty remain the new researchreaffirms the basic findings of the original workincluding the cooling estimates upon whichnuclear winter was based

Figure 511 The impact of niclearwinter on the natural environment

COOLING OR WARMING

In the mid-1970s increasing atmosphericturbidity associated with human activity wasconsidered to be one of the mechanisms capableof inducing global cooling (Calder 1974 Ponte1976) The processes involved seemed plausibleand logical at least in qualitative terms Theintroduction of pollutants into the atmosphereat a rate greater than they could be removed bynatural processes would allow the progressivebuild up of aerosols until sufficient quantities hadaccumulated to cause a rise in global turbiditylevels The net result would be a reduction ininsolation values at the earthrsquos surface as moreof the direct beam solar radiation was scatteredor reflected by the particulate matter in theatmosphere The ability of some of the aerosolsto act as condensation nuclei would also tend toincrease cloudiness and further reduce the receiptof insolation It has been estimated that naturalaerosols in the troposphere probably reduceglobal surface temperatures by about 15degC(Toon and Pollack 1981) and results obtainedby atmospheric modelling techniques suggest thata doubling of the atmospheric aerosol contentwould reduce surface temperature by up to 5degC(Sellers 1973) Global cooling following majorvolcanic eruptions would tend to support suchestimates and other natural events such as forestfires have been linked with regional coolingWildfires in Alberta Canada in 1982 reducedaverage daytime temperatures in the north-central United States by 15ndash40degC and fires inChina in 1987 were followed by reductions of20ndash60degC in daytime temperatures in Alaska(Appleby and Harrison 1989) The local coolingin the Middle East at the height of the Kuwaitoil fires also fits this pattern

No realistic value for the impact ofanthropogenic aerosols on global temperaturesis available It has been estimated that a 3 to 4per cent increase in global turbidity levels wouldbe sufficient to reduce the mean temperature ofthe earth by 04degC and according to proponentsof anthropogenically produced dust as a factorin climatic changemdashsuch as Reid Bryson

(1968)mdashthis would be enough to account for theglobal cooling which took place between 1940and 1960 Results such as these were based onthe observation that global cooling oftenfollowed major volcanic eruptions Particulatematter produced by human activity wasconsidered equivalent to volcanic dust andtherefore capable of contributing directly toglobal cooling In addition by elevatingbackground turbidity levels it allowed smallervolcanic eruptions to be more effective inproducing climatic change (Bryson 1968) Thiscomparison of aerosols of human and volcanicorigin has been questioned however (Kellogg1980 Toon and Pollack 1981) Most particulatematter injected into the atmosphere duringhuman activities does not rise beyond thetropopause As a result its residence time islimited and its impact is confined to an areacommonly between 1000 and 2000 kmdownwind from its source (Kellogg 1980) Mostsources of anthropogenic aerosols are on landand there the addition of particles to theboundary layer tends to reduce the combinedalbedo of the surface and the lower atmosphereThe reduced reflection of incoming radiation thenpromotes warming The opposite effect isexperienced over the oceans where the combinedalbedo is increased producing greater reflectivityand therefore cooling (Bolle et al 1986)Differential changes such as these might in timealter local circulation patterns through theirinfluence on atmospheric stability

Little anthropogenically producedparticulate matter enters the stratosphere atpresent but should that change the effectswould be greater and more prolonged thanthose produced by the tropospheric aerosols(Bolle et al 1986) Stratospheric aerosols alterthe energy budget in two ways They scatterreflect and to a lesser extent absorb incomingsolar radiation which reduces the amount ofenergy reaching the earthrsquos surface andcontributes to cooling They also absorboutgoing terrestrial infrared radiation The netresult is a warming of the stratosphereInformation on the warming ability of

anthropogenic aerosols is not readily availablebut the aerosols ejected by Mount Pinatuboraised stratospheric temperatures by between35 and 7degC (McCormick 1992 Gobbi et al1992) Some of the energy trapped in this waywill be reradiated back towards the earthrsquossurface but most will be lost into spaceparticularly if the aerosols have been injected tohigh levels in the stratosphere (Lacis et al1992) As the particles begin to sink howeverthe zone of warming will be brought closer tothe surface and an increased proportion of theenergy radiated from it will reach thetroposphere helping to offset the cooling Thewarming of the stratosphere will also intensifythe stratospheric temperature inversioncreating greater stability and reducing thevigour of the atmospheric circulationExperiments with GCMs suggest that anincrease in particulate matter in the stratospherewould dampen the Hadley circulation (seeChapter 2) slowing down the easterlies in thetropics and the westerlies in the sub-tropics(Bolle et al 1986) However from their studiesof the Mount Pinatubo aerosols Brasseur andGranier (1992) have estimated that thestratospheric warming following the eruptionstrengthened the mean meridional circulationby approximately 10 per cent

Records from which anthropogenicallygenerated aerosol trends can be determined aresparse and estimates of their impact have beendeveloped mainly through theoretical studyrather than by direct observation in theatmosphere In reality it is not yet possible toprove that human activities have or have notinduced climatic change through the release ofaerosols nor is it possible to make realistic futureprojections The SMIC Report (1971) recognizedthe ability of particulate matter in the atmosphereto cause warming but suggested that it wasinsufficient to compensate for the cooling causedby the attenuation of solar radiation In contrastsome estimates suggest that it is possible that thenet effect of elevated atmospheric aerosol levelscould be a slight warming rather than a cooling(Bach 1979)

Atmospheric turbidity has received lessattention from academics and the media in recentyears Perhaps the success of local air pollutioncontrol measures has helped to reduce the generallevel of anxiety In the early 1970s it seemedpossible that anthropogenic aerosols wouldincrease turbidity sufficiently to cause globalcooling and possibly contribute to thedevelopment of a new Ice Age (Calder 1974)Concern for cooling has been replaced by concernover global warming mainly as a consequenceof the intensification of the greenhouse effect (seeChapter 7) It has been argued that the presenceof particulate matter in the atmosphere hastempered the impact of the greenhouse effect(Bryson and Dittberner 1976) but it now appearspossible that under some conditions aerosolsactually add to the warming (Bach 1979) Kellogg(1980) has suggested that on a regional scalethe warming effect of aerosols is more importantthan the effect of increased carbon dioxidealthough on a global scale the situation isreversed He also points out that efforts to controlair pollution in industrial areas will ensure thataerosol effects will decline while the impact ofthe greenhouse effect will continue to growSimilar issues are considered by Charlson et al(1992) in their examination of climate forcingby anthropogenic aerosols in the troposphereThey claim that current levels of anthropogenicsulphate particles are sufficiently high to offsetsubstantially the global warming produced bygreenhouse gas forcing but acknowledge themany uncertainties which remain to be resolved

The lack of solid data means that manyquestions involving the impact of atmosphericturbidity on climate remain inadequatelyanswered The extent of the human contributionto atmospheric turbidity is still a matter ofspeculation Air pollution monitoring at the localand regional level provides data on changingconcentrations of particulate matter over citiesand industrial areas but there is as yet insufficientinformation to project these results to the globalscale In studying the impact of turbidity onclimate most of the work has dealt withtemperature change but it is also possible that

aerosols influence precipitation processesbecause of their ability to act as condensationnuclei The extent and direction of that influenceis largely unknown A general consensusappeared to emerge in the mid-1980s thatchanges in climate brought on by increasingaerosol concentrations have been relativelyminor taking the form of a slight warming ratherthan the cooling postulated a decade earlier It isentirely possible however that in the event of asignificant global temperature reduction at sometime in the future atmospheric turbidity will beresurrected as a possible cause The developmentof new improved GCMs will help to provideinformation on the climatic effects of atmosphericaerosols but it is recognized by the WorldMeteorological Organization and a number ofother international scientific and environmentalgroups that direct atmospheric observation andmonitoring is essential if the necessary aerosolclimatology is to be established (Kellogg 1980Bolle et al 1986)

SUMMARY

Particulate matter has undoubtedly been aconstituent of the atmosphere from the verybeginning and natural processes which existedthen continue to make the major contribution toatmospheric turbidity Volcanic activity duststorms and a variety of physical and organicprocesses provide aerosols which areincorporated into the gaseous atmosphereHuman industrial and agricultural activities alsohelp to increase turbidity levels The aerosols varyin size shape and composition from fine chemicalcrystals to relatively large inert soil particles

Once into the atmosphere they are redistributedby way of the wind and pressure patternsremaining in suspension for periods ranging fromseveral hours to several years depending uponparticle size and altitude attained The presenceof aerosols disrupts the inward and outward flowof energy through the atmosphere Studies ofperiods of intense volcanic activity suggest thatthe net effect of increased atmospheric turbidityis cooling and some of the coldest years of theLittle Ice Agemdashbetween 1430 and 1850mdashhavebeen correlated with major volcanic eruptionsAerosols produced by human activities cannotmatch the volume of material produced naturallybut in the 1960s and early 1970s some studiessuggested that the cumulative effects of relativelysmall amounts of anthropogenic aerosols couldalso cause cooling Present opinion seesatmospheric turbidity actually producing a slightwarming The greatest problem in the study ofthe impact of atmospheric turbidity on climateis the scarcity of appropriate data and thatsituation can only be changed by the introductionof systematic observation and monitoring tocomplement the theoretical analysismdashbased onatmospheric modelling techniquesmdashwhich hasbeen developed in recent years

Royal Meteorological Society (1992) lsquoGulf WarMeteorology Special Issuersquo Weather 47(6)London Royal Meteorological Society

Sagan C and Turco R (1990) A Path WhereNo Man Thought Nuclear Winter and theEnd of the Arms Race New York RandomHouse

One of the most important functions of theatmosphere is to provide the surface of the earthwith protection from solar radiation This mayseem contradictory at first sight since solarradiation provides the energy which allows theentire earthatmosphere system to function Aswith most essentials however there are optimumlevels beyond which a normally beneficial inputbecomes harmful This is particularly so with theradiation at the ultraviolet end of the spectrum(see Table 61) At normal levels for example itis an important germicide and is essential forthe synthesis of Vitamin D in humans At elevatedlevels it can cause skin cancer and producechanges in the genetic make-up of organisms Inaddition since ultraviolet radiation is an integralpart of the earthrsquos energy budget changes inultraviolet levels have the potential to contributeto climatic change

Under normal circumstances a layer ofozone gas in the upper atmosphere keeps theultraviolet rays within manageable limitsClose to the equator ozone allows only 30 percent of the ultraviolet-B (UV-B) radiation to

reach the surface A comparable value forhigher latitudes is about 10 per cent althoughduring the summer months radiation receiptsmay approach equatorial levels (Gadd 1992)Specific amounts vary in the short-term withchanges in such factors as cloudiness airpollution and natural fluctuations in ozoneconcentrations Ozone is a relatively minorconstituent of the atmosphere It is diffusedthrough the stratosphere between 10 and 50km above the surface reaching its maximumconcentration at an altitude of 20 to 25 km Ifbrought to normal pressure at sea-level all ofthe existing atmospheric ozone would form aband no more than 3 mm thick (Dotto andSchiff 1978) This small amount of a minor gaswith an ability to filter out a very highproportion of the incoming ultravioletradiation is essential for the survival of life onearth It removes most of the extremelyhazardous UV-C wavelengths and between 70and 90 per cent of the UV-B rays (Gadd 1992)The amount of ozone in the upper atmosphereis not fixed it may fluctuate by as much as 30per cent from day to day and by 10 per centover several years (Hammond and Maugh1974) Such fluctuations are to be expected in adynamic system and are kept under control bybuilt-in checks and balances By the early1970s however there were indications that thechecks and balances were failing to prevent agradual decline of ozone levels Inadvertenthuman interference in the chemistry of theozone layer was identified as the cause of thedecline and there was growing concern overthe potentially disastrous consequences of

The threat to the ozone layer

Table 61 Different forms of ultraviolet radiation

1 nm (nanometre)=1times10-9m=1times10-3microm

elevated levels of ultraviolet radiation at theearthrsquos surface The depletion of the ozonelayer became a major environmentalcontroversy by the middle of the decade Itstechnological complexity caused dissension inscientific and political arenas andmdashwith morethan a hint of science fiction in its make-upmdashitgarnered lots of popular attention In commonwith many environmental concerns of that erahowever interest waned in the late 1970s andearly 1980s only to be revived again with thediscovery in 1985 of what has come to be calledthe Antarctic ozone hole

THE PHYSICAL CHEMISTRY OF THEOZONE LAYER

Ozone owes its existence to the impact ofultraviolet radiation on oxygen molecules in thestratosphere with the main production takingplace in tropical regions where radiation levelsare high (Rodriguez 1993) Oxygen moleculesnormally consist of two atoms and in the loweratmosphere they retain that configuration At thehigh energy levels associated with ultravioletradiation in the upper atmosphere however thesemolecules split apart to produce atomic oxygen(see Figure 61) Before long these free atomscombine with the available molecular oxygen tocreate triatomic oxygen or ozone That reactionis reversible The ozone molecule may breakdown again into its original componentsmolecular oxygen and atomic oxygen as a resultof further absorption of ultraviolet radiation orit may combine with atomic oxygen to bereconverted to the molecular form (Crutzen1974) The total amount of ozone in thestratosphere at any given time represents abalance between the rate at which the gas is beingproduced and the rate at which it is beingdestroyed These rates are directly linked anyfluctuation in the rate of production will bematched by changes in the rate of decay untilsome degree of equilibrium is attained (Dotto andSchiff 1978) Thus the ozone layer is in aconstant state of flux as the molecular structureof its constituents changes

The role of ultraviolet radiation andmolecular oxygen in the formation of the ozonelayer was first explained by Chapman in 1930Later measurement indicated that the basictheory was valid but observed levels of ozonewere much less than expected given the rate ofdecay possible through natural processes Sincenone of the other normal constituents of theatmosphere such as molecular oxygennitrogen water vapour or carbon dioxide wasconsidered capable of destroying the ozoneattention was eventually attracted to traceelements in the stratosphere Initially it seemedthat these were present in insufficient quantityto have the necessary effect but the problemwas solved with the discovery of catalytic chainreactions in the atmosphere (Dotto and Schiff1978) A catalyst is a substance whichfacilitates a chemical reaction yet remains itselfunchanged when the reaction is over Beingunchanged it can go on to promote the samereaction again and again as long as the reagentsare available or until the catalyst itself isremoved In this form of chain reaction acatalyst in the stratosphere may destroythousands of ozone molecules before it is finallyremoved The ozone layer is capable of dealingwith the relatively small amounts of naturallyoccurring catalysts Recent concern over thethinning of the ozone layer has focused onanthropogenically produced catalysts (seeFigure 62) which were recognized in thestratosphere in the early 1970s and which havenow accumulated in quantities well beyond thesystemrsquos ability to cope

NATURALLY OCCURRING OZONEDESTROYING CATALYSTS

Natural catalysts have probably always been partof the atmospheric system and manymdashsuch ashydrogen nitrogen and chlorine oxidesmdasharesimilar to those now being added to theatmosphere by human activities The maindifference is in production and accumulation Thenatural catalysts tend to be produced in smallerquantities and remain in the atmosphere for a

THE THREAT TO THE OZONE LAYER 123

shorter time than their anthropogeniccounterparts

Hydrogen oxides

Hydrogen oxides (HOX) include atomic

hydrogen the hydroxyl radical (OH) and theperhydroxyl radical (HO2)mdashall derived fromwater vapour (H2O) methane (CH4) andmolecular hydrogen (H2) which occur naturallyin the stratosphere They are referred tocollectively as odd hydrogen particles (Crutzen

Figure 61 Schematic representation of the formation of stratospheric ozone

1972) and although relatively low in totalvolume they affect ozone strongly particularlyabove 40km Hammond and Maugh (1974) haveestimated that the HOX group through itscatalytic properties is responsible for about 11per cent of the natural destruction of ozone inthe stratosphere (see Figure 63) The oddhydrogens lose their catalytic capabilities whenthey are converted to water vapour

Nitrogen oxides

Nitrogen oxides (NOx) are very effectivedestroyers of ozone (see Figure 64) Nitric oxide(NO) is most important being responsible for 50ndash70 per cent of the natural destruction ofstratospheric ozone (Hammond and Maugh1974) It is produced in the stratosphere by theoxidation of nitrous oxide (N2O) which has beenformed at the earthrsquos surface by the action ofdenitrifying bacteria on nitrites and nitrates It

may also be produced in smaller quantities by theaction of cosmic rays on atmospheric gases(Hammond and Maugh 1974) Major cosmic rayactivity in the past associated with supernovaspossibly produced sufficient NOx to cause a 90 percent reduction in the ozone concentration forperiods of as much as a century (Ruderman 1974)The catalytic chain reaction created by NO is along one Nitric oxide diffuses only slowly into thelower stratosphere where it is converted into nitricacid and eventually falls out of the atmosphere inrain In contrast to the other oxides of nitrogenthe presence of nitrogen dioxide (NO2) in thelower stratosphere can be beneficial to the ozonelayer It readily combines with chlorine monoxide(ClO) one of the most efficient ozone destroyersto produce chlorine nitrate (ClONO2) a much lessreactive compound thus providing someprotection for lower stratospheric ozone(Brasseur and Granier 1992)

Figure 62 Diagrammatic representation of the sources of natural and anthropogenic ozone-destroyers

Figure 63 The destruction of ozone by HOX

Source Based on formulae in Crutzen (1972)

Figure 64 The destruction of ozone by NOx

Chlorine oxides

The extent to which naturally produced chlorinemonoxide (CIO) contributes to the destructionof the ozone layer is not clear The most abundantnatural chlorine compound is hydrochloric acid(HC1) Although it is present in large quantitiesin the lower atmosphere HCl is highly reactiveand soluble in water and Crutzen (1974)considered that it was unlikely to diffuse into thestratosphere in sufficient quantity to have a majoreffect on the ozone layer The addition of largeamounts of chlorine compounds to thestratosphere during volcanic eruptions was alsoproposed as a mechanism for the naturaldestruction of the ozone layer (Stolarski andCicerone 1974) but observations of the impactof large volcanic eruptionsmdashsuch as that of MtAgung (see Chapter 5)mdashon the ozone layer donot support that proposition (Crutzen 1974) Theimpact of naturally occurring CIO on the ozonelayer was therefore considered relativelyinsignificant compared to that of NOx and HOxBy 1977 however measurements showed thatthe contribution of chlorine to ozone destructionwas growing (Dotto and Schiff 1978) but largelyfrom human rather than natural sourcesAnthropogenically produced chlorine now posesa major threat to the ozone layer

THE HUMAN IMPACT ON THE OZONELAYER

It had become clear by the mid-1970s that humanactivities had the potential to bring aboutsufficient degradation of the ozone layer that itmight never recover The threat was seen to comefrom four main sources associated with moderntechnological developments in warfare aviationlife-style and agriculture (see Figure 62) andinvolving a variety of complex chemicalcompounds both old and new

Nuclear war and the ozone layer

When a modern thermonuclear device isexploded in the atmosphere so much energy is

released so rapidly that the normally inertatmospheric nitrogen combines with oxygen toproduce quantities of oxides of nitrogen (NOx)The rapid heating of the air also sets up strongconvection currents which carry the gases andother debris into the stratosphere and it is therethat most of the NOx is deposited Since naturalNOx is known to destroy ozone it is only to beexpected that the anthropogenically producedvariety would have the same effect and one ofthe many results of nuclear war might be the largescale destruction of the ozone layer Most of thestudies which originally investigated the effectsof nuclear explosions on the atmosphere useddata generated during the nuclear bomb tests ofthe 1950s and 1960s After 1963 when amoratorium on atmospheric tests of nucleardevices was declared information from thesesources was no longer available and recentinvestigations have been based on statisticalmodels

The results of the studies of the effects ofnuclear-weapon tests on the ozone layer were notconclusive The analysis of data collected duringa period of intense testing in 1961 and 1962produced no proof that the tests had had anyeffect on the ozone (Foley and Ruderman 1973)although it was estimated that the explosionsshould have been sufficient to cause a reductionof 3 per cent in stratospheric ozone levels(Crutzen 1974) Techniques for measuring ozonelevels were not particularly sophisticated in the1960s and in addition a reduction of 3 per centis well within the normal annual fluctuation inlevels of atmospheric ozone Thus it was notpossible to confirm or refute the predicted effectsof nuclear explosions on the ozone layer byanalysis of the test data Circumstantial evidencedid indicate a possible link however Since ozonelevels are known to fluctuate in phase withsunspot cycles it was expected that peakconcentrations of ozone in 1941 and 1952 wouldbe followed by a similar peak in 1963 inaccordance with the eleven-year cycle That didnot happen Instead the minimum level reachedin 1962 increased only gradually through theremainder of the decade (Crutzen 1974) The

missing sunspot cycle peak was considered to bethe result of the nuclear tests and the gradualincrease in ozone in the years following wasinterpreted as representing the recovery from theeffects of the tests as well as the return to thenormal cyclical patterns (Hammond and Maugh1974)

Although it was not possible to establishconclusive links between nuclear explosions andozone depletion on the basis of these individualtests a number of theoretical studies attemptedto predict the impact of a full-scale nuclear waron stratospheric ozone Hampson (1974)estimated that even a relatively minor nuclearconflict involving the detonation of 50megatonnesmdashequivalent to 50 million tonnes ofconventional TNT explosivemdashwould lead to areduction in global ozone levels of about 20 percent with a recovery period of several years Hepointed out the importance of thinking beyondthe direct military casualties of a nuclear conflictto those who would suffer the consequences of amajor thinning of the ozone layer Since thedestruction of the ozone layer would not remainlocalized the effects would be felt worldwidenot just among the combatant nationsSubsequent studies by US military authorities atthe Pentagon supported Hampsonrsquos predictionsThey indicated that following a major nuclearconflict 50ndash70 per cent of the ozone layer mightbe destroyedmdashwith the greater depletion takingplace in the northern hemisphere where most ofthe explosions would occur (Dotto and Schiff1978)

Similar results were obtained by Crutzen(1974) using a photochemical-diffusion modelHe calculated that the amount of nitric oxideinjected into the stratosphere by a 500-megatonne conflict would be more than ten timesthe annual volume provided by natural processesThis was considered sufficient to reduce ozonelevels in the northern hemisphere by 50 per centDramatic as these values may appear they remainapproximationsmdashbased on data and analysescontaining many inadequacies Interest in theimpact of nuclear war on the ozone layer peakedin the mid-1970s and declined thereafter It

emerged again a decade later as part of a largerpackage dealing with nuclear war andclimatology which emphasized nuclear winter(see Chapter 5)

Supersonic transports and the ozone layer

The planning and development of a newgeneration of transport aircraft was well underway in North America Europe and the USSR bythe early 1970s These were the supersonictransports (or SSTs) designed to fly higher andfaster than conventional subsonic civil airlinersand undoubtedly a major technologicalachievement It became clear however that theycould lead to serious environmental problems ifever produced in large numbers Initial concernsincluded elevated noise levels at airports and theeffects of the sonic boom produced when theaircraft passed through the sound barrier butmany scientists and environmentalists saw theimpact of these high-flying jets on the structureof the ozone layer as many times more seriousand more universal in its effects

Supersonic transports received a great deal ofattention between 1971 and 1974 as a result ofCongressional hearings in the United States intothe funding of the Boeing SST and a subsequentClimatic Impact Assessment Program (CIAP)commissioned by the US Department ofTransportation to study the effects of SSTs onthe ozone layer The findings in both cases wereextremely controversial and gave rise to a debatewhich continued for several years at times highlyemotional and acrimonious It was fuelled furtherby a series of legal and legislative battles whichended only in 1977 when the US Supreme Courtgranted permission for the Anglo-FrenchConcorde to land at New York The proceedingsand findings of the Congressional hearings andthe CIAP plus the debate that followed havebeen summarized and evaluated by Schneider andMesirow (1976) and Dotto and Schiff (1978)The arguments for and against the SSTs were asmuch political and economic as they werescientific or environmental They did revealhowever a society with the advanced technology

necessary to build an SST yet possessing aremarkably incomplete understanding of theenvironment into which the aircraft was to beintroduced

Like all aircraft SSTs produce exhaust gaseswhich include water vapour carbon dioxidecarbon monoxide oxides of nitrogen and someunburned hydrocarbons These are injecteddirectly into the ozone layer since SSTscommonly cruise at about 20 km above thesurfacemdashjust below the zone of maximumstratospheric ozone concentration Much of theinitial concern over the effect of SSTs on ozonecentred on the impact of water vapour whichwas considered capable of reducing ozone levelsthrough the creation of the hydroxyl radical aknown ozone-destroying catalyst Laterobservations which indicated that a 35 per centincrease in water vapour had been accompaniedby a 10 per cent increase in ozone rather thanthe expected decrease (Crutzen 1972) causedthe role of water vapour to be re-evaluated Itwas suggested that water vapour helped topreserve the ozone layer through its interactionwith other potential catalysts It converted NOx

to nitric acid for example and thereforenullified its ozone-destroying properties(Crutzen 1972 Johnson 1972) By the time thishad been confirmed in 1977 NOx had alreadyreplaced HOx as the villain in SST operations(Dotto and Schiff 1978)

In 1970 Crutzen drew attention to the roleof NOx in the destruction of ozone throughcatalytic chain reactions and in the followingyear just as the SST debate was beginning to takeoff Johnston (1971) warned that NOx emittedin the exhaust gases of 500 SSTs could reduceozone levels by as much as 22ndash50 per cent Laterpredictions by (Crutzen 1972) suggested a 3ndash22per cent reduction while Hammond and Maughreported in 1974 that the net effect of the NOx

emissions from a fleet of 500 SSTs would be a16 per cent reduction in ozone in the northernhemisphere and an 8 per cent reduction in thesouthern hemisphere

When all of this was under consideration inthe early 1970s it was estimated that the worldrsquos

fleet of SSTs would grow to several hundredaircraft by the end of the century and perhaps asmany as 5000 by the year 2025 (Dotto and Schiff1978) The NOx emissions from such a fleet wereconsidered capable of thinning the ozone layersufficiently to produce an additional 20000 to60000 cases of skin cancer in the United Statesalone (Hammond and Maugh 1974) Otherpredicted environmental impacts includeddamage to vegetation and changes in the natureand growth of some species as a result ofmutation The extent to which such threatshelped to kill SST development is difficult toestimate At the time the environmentalarguments seemed strong but the economicconditions were not really right for developmentand that as much as anything else led to thescrapping of the projected Boeing SSTDevelopment of the Soviet Tupolev-144 and theAnglo-French Concorde went ahead with thelatter being the more successful of the two interms of production numbers and commercialroute development Less than 10 SSTs arecurrently in operation and the effects on theozone layer are generally considered to benegligible

Chlorofluorocarbons halons and the ozonelayer

If there was some doubt about the impact of SSTexhaust emissions on the ozone layer the effectsof some other chemicals seemed less uncertainAmong these the chlorofluorocarbon (CFC)group and related bromofluorocarbons or halonshave been identified as potentially the mostdangerous (see Table 62) The CFCs sometimesreferred to by their trade name Freon came toprominence as a result of lifestyle changes whichhave occurred since the 1930s They are used inrefrigeration and air conditioning units but untilrecently their major use was as propellants inaerosol spray cans containing deodorants hairspray paint insect repellant and a host of othersubstances When the energy crisis broke theywere much in demand as foaming agents in theproduction of polyurethane and polystyrene

foam used to improve home insulation Polymerfoams are also included in furniture and car seatsand with the growth of the convenience foodindustry they were used increasingly in themanufacture of fast food containers and coffeecups The gases are released into the atmospherefrom leaking refrigeration or air conditioningsystems or sprayed directly from aerosol cansThey also escape during the manufacture of thepolymer foams and are gradually released as thefoams age Halons are widely used in fire

extinguishers and fire protection systems forcomputer centres industrial control rooms andaircraft Although they are less abundant in theatmosphere than CFCs their ability to causeozone depletion may be 3ndash10 times greater(Environment Canada 1989)

Advantages of CFCs and halons for suchpurposes include their stability and low toxicityunder normal conditions of temperature andpressure they are inertmdashthat is they do notcombine readily with other chemicals nor are they

Table 62 Some common anthropogenically produced ozone destroying chemicals

Source Compiled from data in Environment Canada (1989) MacKenzie (1992) Tickell (1992) ODP is a measure of the capacity of a chemical to destroy ozone with the ODP of CFC-1 1 and CFC-1 2as a base of 1 0

easily soluble in water In consequence theyremain in the environment relatively unchangedOver the years they have gradually accumulatedand diffused into the upper atmosphere Oncethey reach the upper atmosphere however theyencounter conditions under which they are nolonger inertmdashconditions which cause them tobreak down and release by-products which are

immensely destructive to the ozone layer (seeFigure 65)

In 1974 two scientists working in the UnitedStates on the photochemistry of the stratospherecame to the conclusion that CFCs however inertthey may be at the earthrsquos surface are highlysusceptible to break down by the ultravioletradiation present in the upper atmosphere

Figure 65 The break-down of a chlorofluorocarbon molecule (CFCl3) and its effect on ozone

Molina and Rowland (1974) recognized that thephotochemical degradation of CFCs releaseschlorine which through catalytic action has aremarkable ability to destroy ozone Theimportance of the chlorine catalytic chain lies inits efficiency it is six times more efficientcatalytically than the NO cycle The chain is onlybroken when the Cl or ClO gains a hydrogenatom from the odd hydrogen group or from ahydrocarbon such as methane (CH4) and isconverted into HCl which diffuses into the loweratmosphere eventually to be washed out by rain(Hammond and Maugh 1974) Similarconclusions were reached independently at aboutthe same time by other researchers (Crutzen1974 Cicerone et al 1974 Wofsy et al 1975)and with the knowledge that the use of CFCshad been growing since the late 1950s the stageseemed set for an increasingly rapid thinning ofthe earthrsquos ozone shield followed by a rise in thelevel of ultraviolet radiation reaching the earthrsquossurface

The world production of CFCs reached700000 tonnes in 1973 (Crutzen 1974) aftergrowing at an average rate of about 9 per centin the 1960s (Molina and Rowland 1974) Thetotal production of CFCs and halons amountedto 1260000 tonnes in 1986 before falling to870000 tonnes in 1990 (Environment Canada1992) The effects of such increases inproduction were exacerbated by the stability ofthe products which allowed them to remain inthe atmosphere for periods of 40 to 150 yearsand measurements in the troposphere in theearly 1970s indicated that almost all of theCFCs produced in the previous two decadeswere still there (Molina and Rowland 1974)This persistence means that even after acomplete ban on the production of CFCs theeffects on the ozone layer might continue to befelt for a further 20 to 30 years and undercertain circumstances for as long as 200 yearsafter production ceased (Crutzen 1974 Wofsyet al 1975)

The predicted effects of all of this on theozone layer varied Molina and Rowland(1974) estimated that the destruction of ozone

by chlorine was already equivalent to thatproduced by naturally occurring catalystsCrutzen (1974) predicted that a doubling ofCFC production would cause a corresponding10 per cent reduction in ozone levels whereasWofsy et al (1975) estimated that with agrowth rate of 10 per cent per year CFCs couldbring about a 20 per cent reduction by the endof the century All indicated the preliminarynature of their estimates and the inadequacy ofthe existing knowledge of the photochemistryof the stratosphere but such cautions wereignored as the topic took on a momentum of itsown and the dire predictions made followingthe SST studies were repeated The spectre ofthousands of cases of skin cancer linked to aseemingly innocuous product like hairspray ordeodorant was sufficiently different that itexcited the media and through them thegeneral public Although CFCs were beingemployed as refrigerants and used in theproduction of insulation the problem wasusually presented as one in which theconvenience of aerosol spray products wasbeing bought at the expense of the globalenvironment In 1975 there was somejustification for this since at that time 72 percent of CFCs were used as propellants inaerosol spray cans (Webster 1988) and thecampaign against that product grew rapidly

The multi-million dollar aerosol industry ledby major CFC producers such as DuPont reactedstrongly Through advertising and participationin US government hearings they emphasized thespeculative nature of the Molina-Rowlandhypothesis and the lack of hard scientific factsto support it The level of concern was highhowever and the anti-aerosol forces met withconsiderable success Eventually manufacturerswere forced to replace CFCs with less hazardouspropellants (Dotto and Schiff 1978) A partialban on CFCs covering their use in hair anddeodorant sprays was introduced in the UnitedStates in 1978 and in Canada in 1980 CFCaerosol spray use remained high in Europe wherethere was no ban In 1989 however theEuropean Community agreed to eliminate the

production and use of CFCs by the end of thecentury

The CFC controversy had ceased to makeheadlines by the late 1970s and the level of publicconcern had fallen away Monitoring of the ozonelayer showed little change Ozone levels were notincreasing despite the ban on aerosol sprayswhich was only to be expected given the slowrate of decay of the existing CFCs but thesituation did not seem to be worsening Quiteunexpectedly in 1985 scientists working in theAntarctic announced that they had discovered alsquoholersquo in the ozone layer and all of the fearssuddenly returned

The Antarctic ozone hole

Total ozone levels have been measured at theHalley Bay base of the British Antarctic Surveyfor more than thirty years beginning in the late1950s Seasonal fluctuations were observed for

most of that time and included a thinning of theozone above the Antarctic during the southernspring which was considered part of the normalvariability of the atmosphere (Schoeberl andKrueger 1986) This regular minimum in the totalozone level began to intensify in the early 1980showever (see Figure 66) Farman et al (1985)reported that it commonly became evident in lateAugust and got progressively worse until bymid-October as much as 40 per cent of the ozonelayer above the Antarctic had been destroyedUsually the hole would fill by November butduring the 1980s it began to persist intoDecember The intensity of the thinning and itsgeographical extent were originally establishedby ground based measurements and laterconfirmed by remote sensing from the Nimbus-7 polar orbiting satellite (Stolarski et al 1986)

As was only to be expected after theaerosolspray-can experience in the 1970s theimmediate response was to implicate CFCs

Figure 66 Changing ozone levels at the South Pole (1964ndash85) for the months of October November andDecember

Source After Komhyr et al 1986Note Dobson units (DU) are used to represent the thickness of the ozone layer at standard (sea-level)temperature and pressure (1 DU is equivalent to 001 mm)

Measurements of CFCs at the South Poleindicating a continuous increase of 5 per centper year tended to support this although theoriginal fluctuation had been present before CFCswere released into the atmosphere in any quantity(Schoeberl and Krueger 1986) Other researcherssuggested that gases such as NOx (Callis andNatarajan 1986) and the oxides of chlorine(ClOx) and bromine (BrOx) (Crutzen and Arnold1986) were the culprits As the investigation ofthe Antarctic ozone layer intensified it becameclear that the chemistry of the polar stratosphereis particularly complex Although thestratosphere is very dry it becomes saturated atthe very low temperatures reached during thewinter months and clouds form Nitric andhydrochloric acid particles in these polarstratospheric clouds enter into a complex seriesof reactionsmdashinvolving for exampledenitrification and dehydrationmdashwhichultimately lead to the release of chlorine In theform of ClO it then attacks the ozone (Shine1988) The evaporation of the polar stratosphericclouds in the spring as temperatures rise bringsan end to the reactions and allows the recoveryof the ozone layer

The chemical reactions which lead to thedestruction of the ozone following the formationof polar stratospheric clouds take place initiallyon the surface of ice particles in the cloudsSimilar heterogeneous chemical reactions on thesurface of sulphate aerosol particles have alsobeen identified as contributing to major thinningof the Antarctic ozone layer (Brasseur andGranier 1992 Deshler et al 1992 Keys et al1993) The injection of large volumes of sulphateinto the lower stratosphere during the eruptionsof Mount Pinatubo and Mount Hudson in 1991was followed by rapid destruction of the ozonelayer at altitudes of 9ndash13 km During September1991 alone almost 50 per cent of the ozone inthe lower stratosphere was destroyed (Deshleret al 1992) The role of the sulphate aerosol wasconfirmed by modelling chemical radiative anddynamical processes in the stratosphere (Braseurand Granier 1992) by direct measurement of thelevels of volcanic aerosol and ozone depletion

(Deshler et al 1992) and by the presence ofchemicals normally associated withheterogeneous chemical reactions at a time whenstratospheric temperatures remained too high forpolar stratospheric clouds to form Thebackground sulphate aerosols in the stratosphereprovided an alternative surface upon which thereactions took place (Keys et al 1993)

Following the initial identification of thesulphateozone relationship in the Antarctic inlate 1991 additional evidence was obtained fromThule in Greenland Measurements taken in thefirst three months of 1992 showed a negativecorrelation between aerosol counts and ozonelevels in the middle stratosphere withfluctuations of as much as 50 per cent in ozonecontent (di Sarra et al 1992) One year later overCanada record low ozone values were measuredat altitudes at which aerosols from the MountPinatubo eruption had been observed (Kerr etal 1993)

When the aerosols from Pinatubo and Hudsoninitially spread into the Antarctic in 1991 theywere unable to penetrate beyond an altitude of14ndash15 km because of the presence of thecircumpolar vortex As a result the thinning ofthe ozone layer in the upper stratosphereremained close to normal levels By late 1992however the aerosols had spread evenly over thepolar region Record low levels of ozone werereported over the pole and over southernArgentina and Chile while the thinning of theozone layer also reached record levels over thenorthern hemisphere (Gribbin 1992) and globalozone levels were more than 4 per cent belownormal (Kiernan 1993)

In contrast to these approaches whichemphasize the chemistry of the stratosphere thereare the so-called dynamic hypotheses which seekto explain the variations in ozone levels in termsof circulation patterns in the atmosphere Certainobservations do support this For example at thetime the ozone level in the hole is at its lowest aring of higher concentration develops around thehole at between 40 and 50degS The hole begins tofill again in November seemingly at the expenseof the zone of higher concentration (Stolarski et

al 1986) Bowman (1986) has suggested that themain mechanism involved is the circumpolarvortex The Antarctic circumpolar vortex is aparticularly tight self-contained wind systemwhich is most intense during the southern winterwhen it permits little exchange of energy ormatter across its boundaries Thus it prevents anyinflow of ozone from lower latitudes where mostof it is produced allowing the cumulative effectsof the catalytic ozone-destroying chemicals tobecome much more obvious The breakdown ofthe vortex allows the transfer of ozone fromlower latitudes to fill the hole Observations showthat when the breakdown of the vortex is lateozone levels become very low and when thebreakdown is early the ozone hole is less well-marked The possibility also exists that thecooling of the atmosphere above the Antarcticresulting from ozone depletion will encouragethe vortex to persist longer become more intenseand perhaps even spread to lower latitudes thuscompounding the problem (Gribbin 1993)

Most authorities tend to place the hypotheseswhich attempt to explain the Antarctic ozonelevels into either chemical or dynamic categories(Rosenthal and Wilson 1987) but there areseveral hypotheses which might be placed in athird group combining both chemical anddynamic elements The paper which first drewattention to the increased thinning of theAntarctic ozone layer for example might beclassed in the chemical group since it attributesthe decline to CFCs (Farman et al 1985) It doeshowever have a dynamic element in itsconsideration of the polar vortex which appearsto be a necessary prerequisite for the ozonedepletion

Another hypothesis which combines dynamicand chemical elements is that proposed by Callisand Natarajan (1986) They suggest that thedepletion of the ozone in the Antarctic is a naturalphenomenon caused by elevated levels of NOx

in the atmosphere during periods of increasedsolar activity Sunspot activity did peak at aparticularly high level in 1979 and continuedinto the early 1980s but measurements by DrSusan Solomon released at a special meeting of

the Royal Meteorological Society in 1988indicated that NOx levels in the lowerstratosphere in the Antarctic were too low to havethe necessary catalytic effect (Shine 1988)

Comparison of the situation in Antarctica withthat in the Arctic provides indirect support forthe role of the circumpolar vortex in the thinningof the ozone layer The circumpolar vortex in thenorthern hemisphere is much less intense thanits southern counterpart which may explain inpart at least the absence of a comparable holein the ozone over the Arctic (Farman et al 1985)A smaller more mobile hole was identified in theozone above the Arctic in the late 1980s howeverand further investigation set in motion (Shine1988)

The European Arctic Stratospheric OzoneExperiment (EASOE)mdashinvolving scientistsfrom the European Community the UnitedStates Canada Japan New Zealand andRussiamdashwas undertaken during the northernwinter of 1991ndash92 using groundmeasurements balloons aircraft and a varietyof modelling techniques to establish the natureand extent of ozone depletion over the Arctic(Pyle 1991) Preliminary results tended toconfirm the absence of a distinct hole over theArctic but noted the higher than average ozoneloss in middle latitudes Because the Arcticstratosphere is generally warmer than itssouthern counterpart polar stratosphericclouds are less ready to form and ozonedestruction is less efficient The less developedArctic circumpolar vortex also allows the lossof ozone at the pole to be offset to some extentby the influx of ozone from more southerlylatitudes The relatively free flow of air out ofthe Arctic in the winter might also contributethe peculiar patterns of ozone depletion in thenorth Chemicals incapable of destroying ozonebecause of the lack of energy available duringthe Arctic winter night become energized whencarried south into the sunlight of mid-latitudesand cause greater thinning there than at the pole(Pyle 1991) By March 1992 the EASOE haddetected decreases of 10ndash20 per cent in Arcticozone (Concar 1992) Although this was less

than the 40 per cent reduction initiallypredicted by the middle of the year theCanadian Atmospheric Environment Servicehad become so concerned about the destructionof ozone in mid-latitudes that it began toinclude ultraviolet radiation warnings alongwith its regular weather forecasts and in March1993 it announced that the ozone layer aboveToronto and Edmontonmdashat 43degN and 53degNrespectivelymdashwas thinner than at any timesince records began (Environment Canada1993)

There is as yet no one theory which canexplain adequately the creation of the Antarcticozone hole or the thinning of the ozone layer inthe northern hemisphere The impact of CFCs isconsidered the most likely cause by manyhowever There can be little doubt that the linkbetween the ozone hole and the CFCs tenuousas it may have seemed to some scientists helpedto revive environmental concerns andcontributed to the speed with which the worldrsquosmajor industrial nations agreed in Montreal in1987 to take steps to protect the ozone layer

Agricultural fertilizers nitrous oxide and theozone layer

When concern for the ozone layer was at itsheight compounds other than CFCs wereidentified as potentially harmful These includednitrous oxide carbon tetrachloride and methylchloroform Methyl bromide has recently beenadded to the group It is used extensively as afumigant to kill pests in the fruit and vegetableindustry and may be responsible for as much as10 per cent of existing ozone depletion Howeverits actual impact is still a matter of dispute(MacKenzie 1992) Nitrous oxide (N2O) as oneof the oxides of nitrogen group known for theirability to destroy ozone has received mostattention It is produced naturally in theenvironment by denitrifying bacteria which causeit to be released from the nitrites and nitrates inthe soil It is an inert gas not easily removedfrom the troposphere Over time it graduallydiffuses into the stratosphere where the higher

energy levels cause it to be oxidized into NOleading to the destruction of ozone moleculesThis process was part of the earth atmospheresystem before human beings came on the sceneand with no outside interference natural ozonelevels adjust to the output of N2O from the soiland the oceans

One of societyrsquos greatest successes has beenthe propogation of the human species as aglance at world population growth will showThis has occurred for a number of reasons butwould not have been possible without agrowing ability to supply more and more foodas population numbers grew By the late 1940sand 1950s this ability was being challenged aspopulation began to outstrip food supply In anattempt to deal with the problem newagricultural techniques were introduced intoThird World countries where the need wasgreatest A central element in the process wasthe increased use of nitrogen fertilizers alongwith genetically improved grains whichtogether produced the necessary increase inagricultural productivity Since that timecontinued population growth has beenparalleled by the growth in the use of nitrogen-based fertilizers (Dotto and Schiff 1978)

The nitrogen in the fertilizer used by theplants eventually works its way through thenitrogen cycle and is released into the air asN2O to initiate the sequence which ultimatelyends in the destruction of ozone Thus intheory the pursuit of greater agriculturalproductivity through the increased applicationof nitrogen fertilizers is a threat to the ozonelayer There is however no proof that increasedfertilizer use has or ever will damage the ozonelayer through the production of N2O If proofdoes emerge it will create a situation notuncommon in humankindrsquos relationship withthe environment in which a developmentdesigned to combat one problem leads toothers unforeseen and perhaps undiscovereduntil major damage is done As Schneider andMesirow (1976) point out the dilemma lies inthe fact that nitrogen fertilizers are absolutelyessential to feed a growing world population

and improve its quality of life yet success mightlead to a thinning of the ozone layer withconsequent climatic change and biologicaldamage from increased ultraviolet radiation Ifmonitoring does reveal that N2O emissions areincreasing some extremely difficult judgementswill have to be made judgements which couldhave a far greater impact than the grounding ofSSTs or the banning of CFCs from use in aerosolspray cans

THE BIOLOGICAL ANDCLIMATOLOGICAL EFFECTS OFCHANGING OZONE LEVELS

Declining concentrations of stratosphericozone allow more ultraviolet radiation to reachthe earthrsquos surface Even after a decade and ahalf of research the impact of that increase isstill very much a matter of speculation butmost of the effects which have been identifiedcan be classified as either biological orclimatological

Biological effects

In moderate amounts ultraviolet radiation hasbeneficial effects for life on earth It is a powerfulgermicide for example and triggers theproduction of Vitamin D in the skin Vitamin Dallows the body to fix the calcium necessary forproper bone development lack of it may causerickets particularly in growing children Highintensities of ultraviolet radiation however areharmful to all forms of life

Lifeforms have evolved in such a way thatthey can cope with existing levels of ultravioletradiation They are also quite capable ofsurviving the increases in radiation caused byshort-term fluctuations in ozone levels Mostorganisms would be unable to cope with thecumulative effects of progressive thinning of theozone layer however and the biologicalconsequences would be far-reaching

The most serious concern is over rising levelsof UV-Bmdashthe radiation recognized as causingmost biological damage Intense UV-B rays alter

the basic foundations of life such as the DNAmolecule and various proteins (Crutzen 1974)They also inhibit photosynthesis Growth ratesin plants such as tomatoes lettuce and peas arereduced and experimental exposure of someplants to increased ultraviolet radiation hasproduced an increased incidence of mutation(Hammond and Maugh 1974) Insects whichcan see in the ultraviolet sector of the spectrumwould have their activities disrupted byincreased levels of ultraviolet radiation(Crutzen 1974)

Most of the concern for the biological effectsof declining ozone levels has been focused onthe impact of increased ultraviolet radiation onthe human species The potential effects includethe increased incidence of sunburn prematureageing of the skin among white populations andgreater frequency of allergic reactions caused bythe effects of ultraviolet light on chemicals incontact with the skin (Hammond and Maugh1974) These are relatively minor however incomparison to the more serious problems ofskin cancer and radiation blindness both ofwhich would become more frequent with higherlevels of ultraviolet radiation Skin cancer had aprominent role in the SST and CFC debates ofthe 1970s (Dotto and Schiff 1978) and itcontinues to evoke a high level of concern Thenumber of additional skin cancers to beexpected as the ozone layer thins is still a matterof debate but a commonly accepted estimate isthat a 1 per cent reduction in ozone allows anincrease in UV-B of 1ndash2 per cent This in turnleads to a 2ndash4 per cent increase in the incidenceof non-melanoma skin cancers (Concar 1992)Hammond and Maugh (1974) have suggestedthat a 5 per cent reduction in ozone levels wouldcause an additional 20000 to 60000 skincancers in the United States alone The USNational Academy of Sciences has alsoestimated that a 1 per cent decline in ozonewould cause 10000 more cases of skin cancerper year (Lemonick 1987) Many skin cancersare non-malignant and curable but only afterpainful and sometimes disfiguring treatment Arelatively small proportionmdashthe melanomasmdashare

malignant and usually fatal (Dotto and Schiff1978) The US Environmental Protection Agency(EPA) forecasts that 39 million more people thannormal could contract skin cancer within the nextcentury leading to more than 800000 additionaldeaths (Chase 1988)

Levels of skin cancer are currently risingamong the white-skinned peoples of the worldbut there is no direct evidence that the rise islinked to thinning ozone Rather it may be causedby lifestyle factors such as the popularity ofseaside holidays in sunny locations and fashiontrends which encourage a lsquohealthyrsquo tan InScotland between 1979 and 1989 there was an82 per cent increase in the occurrence ofmelanomas (Mackie et al 1992) (see Figure 67)and similar values apply across most of northernEurope an area not renowned for abundantsunshine and not particularly affected by thinningozone until relatively recently Rising skin cancer

totals there may reflect the impact of exposureto higher levels of ultraviolet radiation on thebeaches of southern Europe which have becomepopular vacation spots for northerners Sincethere is a time lag between exposure and thediscovery of cancer current increases mayrepresent the results of cell damage initiated 10ndash20 years ago It also follows that despiteincreasing attempts by health authorities toreduce public exposure to the sun the rising levelof skin cancers is likely to continue for some timeto come

The situation is particularly serious inAustralia where skin cancer is ten times moreprevalent than in northern Europe (Concar1992) Average summer receipts of UV-B haveincreased by 8 per cent since 1980 in southernAustralia and in New Zealand the amount ofultraviolet radiation reaching the earthrsquos surfaceis double that in Germany (Seckmeyer and

Source From Mackie et al 1992

Figure 67 Incidence of melanoma in Scotland 1979ndash89

McKenzie 1992) Since the Antarctic ozone holeextends over the southern continents at the endof spring the higher levels of ultraviolet radiationare probably associated with the thinner ozoneat that time However that in itself is consideredinsufficient to explain the high rates of skincancer Epidemiologists continue to place mostof the blame on social factors which encourageover-exposure to ultraviolet light rather than onozone thinning The consensus amongresearchers is that the latter will only begin tocontribute to skin cancer statistics in the secondhalf of the 1990s (Concar 1992) To counter thisboth the Australian and New Zealandgovernments have initiated campaigns todiscourage exposure to the sun by advocatingthe use of sunscreen lotions and wide-brimmed

hats and by promoting a variety of behaviouralchanges that would keep people out of the sunduring the high risk period around solar noon(Concar 1992 Seckmeyer and McKenzie 1992)A similar approach is being developed in Canadabut in the northern hemisphere concern over thedangers of excess ultraviolet radiation generallylags behind that in the south

The attention paid to skin cancer has causedother effects to be overshadowed Radiationblindness and cataracts were early identified aspotential problems (Dotto and Schiff 1978)More recently damage to the human immunesystem has been postulated and there is someevidence that ultraviolet light may be capable ofactivating the AIDS virus (Valerie et al 1988)

Concentration on the direct effects of ozone

Figure 68Schematicrepresentation ofthe radiation andtemperaturechangesaccompanying thedepletion ofstratosphereozone

depletion on people is not surprising orunexpected Much more research into otherbiological effects is required however Humanbeings are an integral part of the earthatmosphere system and as Dotto and Schiff(1978) suggest humankind may experience theconsequences of ozone depletion through itseffects on plants animals and climate The impactwould be less direct but perhaps no less deadly

Climatological effects

The climatological importance of the ozone layerlies in its contribution to the earthrsquos energy budget(see Figure 68) It has a direct influence on thetemperature of the stratosphere through its abilityto absorb incoming radiation Indirectly this alsohas an impact on the troposphere The absorptionof short-wave radiation in the stratospherereduces the amount reaching the loweratmosphere but the effect of this is limited tosome extent by the emission of part of theabsorbed short-wave energy into the troposphereas infrared radiation

Natural variations in ozone levels alter theamounts of energy absorbed and emitted butthese changes are an integral part of the earthatmosphere system and do little to alter its overallbalance In contrast chemically induced ozonedepletion could lead to progressive disruption ofthe energy balance and ultimately cause climaticchange The total impact would depend upon anumber of variables including the amount bywhich the ozone concentration is reduced and thealtitude at which the greatest depletion occurred(Schneider and Mesirow 1976)

A net decrease in the amount of stratosphericozone would reduce the amount of ultravioletabsorbed in the upper atmosphere producingcooling in the stratosphere The radiation nolonger absorbed would continue on to the earthrsquossurface causing the temperature there to riseThis simple response to declining ozoneconcentration is complicated by the effects ofstratospheric cooling on the system The lowertemperature of the stratosphere would cause lessinfrared radiation to be emitted to the

troposphere and the temperature of the loweratmosphere would also fall Since the coolingeffect of the reduction in infrared energy wouldbe greater than the warming caused by the extralong-wave radiation the net result would be acooling at the earthrsquos surface The magnitude ofthe cooling is difficult to assess but it is likely tobe small Enhalt (1980) has suggested that a 20per cent reduction in ozone concentration wouldlead to a global decrease in surface temperatureof only about 025degC

An increase in total ozone in the stratospherewould be likely to cause a rise in surfacetemperatures as a result of greater ultravioletabsorption and the consequent increase ininfrared energy radiated to the surface Sincecurrent concern is with ozone depletion thequestion of rising ozone levels has received littleattention However the possibility that naturalozoneenhancing processes might at times besufficiently strong to reverse the declining trendcannot be ruled out completely

Stratospheric ozone is not evenly distributedthrough the upper atmosphere Its maximumconcentration is 25 km above the surface(Crutzen 1972) Destruction of ozone does notoccur uniformly throughout the ozone layer andas a result the altitude of maximumconcentration may change A decrease in thataltitude will lead to a warming of the earthrsquossurface whereas an increase will have theopposite effect and lead to cooling (Schneiderand Mesirow 1976) CFCs begin to be mosteffective as ozone destroyers at about 25 kmabove the surface (Enhalt 1980) They thereforetend to push the level of maximum concentrationdown and promote warming

Thus any estimate of the impact of ozonedepletion on climate must consider not onlychanges in total stratospheric ozone but alsochanges in the altitude of its maximumconcentration The depletion of totalstratospheric ozone will always tend to causecooling but that cooling may be enhanced by anincrease in the altitude of maximumconcentration or retarded by a decrease inaltitude

Just as there are variations in the verticaldistribution of ozone there are also variationsin its horizontal distribution The latter areseasonal and associated with changing wind andpressure systems in the lower stratosphere(Crutzen 1974) Most ozone is manufacturedabove the tropics and is transported polewardsfrom there Increased levels of ozone have beenidentified regularly at middle and high latitudesin late winter and spring in the northernhemisphere (Crutzen 1972) and theredistribution of ozone by upper atmosphericwinds has been implicated by a number ofauthors in the development of the Antarctic ozonehole (Shine 1988) Such changes have local andshort-term effects which might reinforce orweaken the global impact of ozone depletion

Further complications are introduced by theability of several of the chemicals which destroyozone to interfere directly with the energy flowin the atmosphere Ramanathan (1975) hasshown that ozone-destroying CFCs absorbinfrared radiation and the resulting temperatureincrease might be sufficient to negate the coolingcaused by ozone depletion Similarly oxides ofnitrogen absorb solar radiation so effectively thatthey are able to reduce the cooling caused by theirdestruction of ozone by about half (Ramanathanet al 1976)

Changes in the earthrsquos energy budget initiatedby declining stratospheric ozone levels areintegrated with changes produced by otherelements such as atmospheric turbidity and thegreenhouse effect The specific effects of ozonedepletion are therefore difficult to identify andthe contribution of ozone depletion to climaticchange difficult to assess

THE MONTREAL PROTOCOL

In September 1987 thirty-one countries meetingunder the auspices of the United NationsEnvironment Program in Montreal signed anagreement to protect the earthrsquos ozone layer TheMontreal Protocol was the culmination of a seriesof events which had been initiated two yearsearlier at the Vienna Convention for the

Protection of the Ozone Layer Twenty nationssigned the Vienna Convention in September1985 promising international cooperation inresearch monitoring and the exchange ofinformation on the problem In the two-yearperiod between the meetings much time andeffort went into formulating plans to control theproblem with the countries of the EuropeanCommunity (EC) favouring a relatively gradualapproach compared to the more drasticsuggestions of the North Americans (Tucker1987) The Environmental Protection Agency(EPA) in the United States against a backgroundof an estimated 39 million additional cases ofskin cancer in the next century suggested a 95per cent reduction in CFC production within aperiod of 6ndash8 years (Chase 1988) When theMontreal Protocol was signed participantsagreed to a 50 per cent production cut by theend of the century although that figure isdeceptive since Third World countries were tobe allowed to increase their use of CFCs for adecade to allow technological improvements insuch areas as refrigeration The net result turnedout to be only a 35 per cent reduction in totalCFC production by the end of the century basedon 1986 totals (Lemonick 1987) This was ahistoric agreement but certain experts in thefield such as Sherwood Rowland felt that it isnot enough and that the original 95 per centproposed by the EPA was absolutely necessary(Lemonick 1987)

The signatories to the Montreal Protocol metagain in Helsinki in May 1989 At that timealong with participants from some 50 additionalcountries they agreed to the elimination of CFCproduction and use by the year 2000 Continuedreassessment of the issue led to a further meetingof the worldrsquos environment ministers inCopenhagen in 1992 at which deadlines for thecontrol of ozone-destroying gases were revisedIn most casesmdashCFCs carbon tetrachloride andmethyl chloroformmdashproduction bans werebrought forward from 2000 to 1996 but in thecase of halons the ban was to be implemented by1994 (see Table 62) HCFCsmdashwidely used assubstitutes for CFCsmdashwere to be phased out

progressively over a thirty-five-year period endingin 2030 Methyl bromide was added to the listof banned substances with emissions to be frozenat the 1991 level by 1995 (MacKenzie 1992)

In contrast to their attitudes in the 1970s theCFC manufacturers responded positively afterMontreal to the call for a reduction in themanufacture of the chemical The DuPontCompany for example pledged to reduce itsoutput by 95 per cent by the year 2000 (theoriginal EPA suggestion) although the initialsearch for appropriate substitutesmdashwhichincluded testing for health and environmentaleffectsmdashwas estimated to take up to five years(Climate Institute 1988c) The market forsubstitutes is large particularly in Europewhere CFCs were not banned in the 1970sAlthough the concern for the supply of energy isno longer at crisis levels the demand forresidential and industrial building insulationremains high and will undoubtedly rise ifenergy supplies are again threatened Since themanufacture of insulating materials accountsfor 28 per cent of worldwide CFC productionthe search for alternatives received urgentattention Results have been mixed DuPont hasdeveloped hydrochlorofluorocarbons (HCFCs)as potential replacements for CFCs in theproduction of polystyrene sheet which has beenused successfully in food packaging but is notsuitable for other forms of insulation since theproduct loses its insulating ability as the HCFCsbreak down (Webster 1988) HCFCs are 95 percent less damaging to ozone than normal CFCsbecause they are less stable and tend to breakdown in the troposphere before they can diffuseinto the ozone layer Although they are lessharmful they do have a negative impact on theozone layer and as a result they too willultimately need to be replaced An isocyanate-based insulation foam in which carbon dioxideis the foaming agent has produced promisingresults It cannot be produced in sheet formhowever and as a result its use remains limitedto situations where spray-on or foam-inapplication is possible

Substitutes are being sought in other areas

also For example a German company hasdeveloped a so-called lsquogreenrsquo refrigerator inwhich the normal CFC refrigerant is replaced bya propanebutane mixture Although the mixturehas a greater cooling capacity than the CFCscurrently used inefficiencies in the compressorsystem require attention before the refrigeratorcan compete with conventional units Bans onthe use of hydrocarbons in domestic refrigeratorsin some countries will also limit its adoption(Toro 1992) More damaging to the ozone thanmost CFCs are the halons used in fire-fightingmdashHalon 1301 for example is ten times moredestructive than CFC-11mdashand replacements arerequired to meet the 1994 ban on halonproduction agreed at Copenhagen in 1992 Tomeet that need a new gas mixture consisting ofnitrogen argon and carbon dioxide has beendeveloped in Britain None of these gasesdamages ozone and existing fire extinguishingsystems can be modified to use the mixture(Tickell 1992) Clearly CFC manufacturingcompanies and those utilizing gases hazardousto ozone are committed to the search foralternatives but it may be some time before theirgood intentions are translated into a suitableproduct

SUMMARY

There is an ever-increasing amount of evidencethat the earthrsquos ozone layer is being depletedallowing a higher proportion of ultravioletradiation to reach the earthrsquos surface If allowedto continue this would cause a serious increasein skin cancer cases produce more eye diseaseand change the genetic make-up of terrestrialorganisms In addition since ultraviolet radiationis an integral part of the earthrsquos energy budgetany increase in its penetration to the loweratmosphere could lead to climatic change Theeffects of a depleted ozone layer are widelyaccepted but the extent of the depletion and itscause are still not completely understood Thisreflects societyrsquos inadequate knowledge of theworkings of the earthatmosphere system ingeneral and the photochemistry of the

stratosphere in particular It may take many yearsof observation and research before this situationis altered but in the meantime a generalconcensus among scientists and politicians thatCFCs are the main culprits in the destruction ofthe earthrsquos ozone has led to the proscription ofthat group of chemicals By the end of thiscentury CFCs will no longer be produced buttheir effects will linger on until those presentlyin the atmosphere are gone

In dealing with the global aspects of airpollution involving such elements as acidprecipitation atmospheric turbidity and thethreat to the ozone layer it is quite clear thatalthough society now has the ability to cause allof these problems and may even possess thetechnology to slow down and reverse them its

understanding of their overall impact on theearthatmosphere system lags behind Until thatcan be changed the effects of human activitieson the system will often go unrecognizedresponse to problems will of necessity be reactiveand the damage done before the problem isidentified and analysed may be irreversible

Dotto L and Schiff H (1978) The Ozone WarGarden City Doubleday

Gribbin J (1993) The Hole in the Sky (revisededition) New York Bantam

Nance JJ (1991) What Goes Up the GlobalAssault on our Atmosphere New YorkWMorrow

The succession of exceptional years with recordhigh temperatures which characterized the1980s helped to generate widespread popularinterest in global warming and its manyramifications The decade included six of thewarmest years in the past century and the trendcontinued into the 1990s with 1991 the secondwarmest year on record All of this fuelledspeculationmdashespecially among the mediamdashthatthe earthrsquos temperature had begun an inexorablerise and the idea was further reinforced by theresults of scientific studies which indicated thatglobal mean temperatures had risen by about

05degC since the beginning of the century (seeFigure 71)

Periods of rising temperature are not unknownin the earthrsquos past The most significant of thesewas the so-called Climatic Optimum whichoccurred some 5000ndash7000 years ago and wasassociated with a level of warming that has notbeen matched since If the current global warmingcontinues however the record temperatures ofthe earlier period will easily be surpassedTemperatures reached during a later warm spell inthe early Middle Ages may well have beenequalled already More recently the 1930s

The greenhouse effect and global warming

Figure 71 Measured globally-averaged (ie land and ocean) surface air temperatures for this century

Source After Jones and Henderson-Sellers (1990)

THE GREENHOUSE EFFECT AND GLOBAL WARMING 145

provided some of the highest temperatures sincerecords began although that decade has beenrelegated to second place by events in the 1980sSuch warm spells have been accepted as part ofthe natural variability of the earth atmospheresystem in the past but the current warming isviewed in a different light It appears to be the firstglobal warming to be created by human activity

The basic cause is seen as the enhancement ofthe greenhouse effect brought on by rising levelsof anthropogenically-produced greenhouse gases(see Table 71) It is now generally accepted thatthe concentrations of greenhouse gases in theatmosphere have been increasing since the latterpart of the nineteenth century The increased useof fossil fuels has released large amounts of CO2and the destruction of natural vegetation hasprevented the environment from restoring thebalance Levels of other greenhouse gasesincluding CH4 N2O and CFCs have also beenrising Since all of these gases have the ability to

retain terrestrial radiation in the atmosphere thenet result should be a gradual increase in globaltemperatures The link between recent warmingand the enhancement of the greenhouse effectseems obvious Most of the media and many ofthose involved in the investigation and analysis ofglobal climate change seem to have accepted therelationship as a fait accompli There are only afew dissenting voices expressing misgivings aboutthe nature of the evidence and the rapidity withwhich it has been embraced A survey ofenvironmental scientists involved in the study ofthe earthrsquos changing climate conducted in thespring of 1989 revealed that many still haddoubts about the extent of the warming Morethan 60 per cent of those questioned indicated thatthey were not completely confident that thecurrent warming was beyond the range of normalnatural variations in global temperatures (Slade1990)

Table 71 Sources of greenhouse gas emissions

Source After Green and Salt (1992)

THE CREATION OF THE GREENHOUSEEFFECT

The greenhouse effect is brought about by theability of the atmosphere to be selective in itsresponse to different types of radiation Theatmosphere readily transmits solar radiationmdashwhich is mainly short-wave energy from theultraviolet end of the energy spectrummdashallowingit to pass through unaltered to heat the earthrsquossurface The energy absorbed by the earth isreradiated into the atmosphere but this terrestrialradiation is long-wave infrared and instead ofbeing transmitted it is absorbed causing thetemperature of the atmosphere to rise Some ofthe energy absorbed in the atmosphere is returnedto the earthrsquos surface causing its temperature torise also (see Chapter 2) This is consideredsimilar to the way in which a greenhouse worksmdashallowing sunlight in but trapping the resultingheat insidemdashhence the use of the namelsquogreenhouse effectrsquo In reality it is the glass in thegreenhouse which allows the temperature to bemaintained by preventing the mixing of thewarm air inside with the cold air outside Thereis no such barrier to mixing in the realatmosphere and some scientists have suggestedthat the processes are sufficiently different topreclude the use of the term lsquogreenhouse effectrsquoAnthes et al (1980) for example prefer to uselsquoatmospheric effectrsquo However the use of the termlsquogreenhouse effectrsquo to describe the ability of theatmosphere to absorb infrared energy is so wellestablished that any change would cause needlessconfusion The demand for change is not strongand lsquogreenhouse effectrsquo will continue to be usedwidely for descriptive purposes although theanalogy is not perfect

Without the greenhouse effect globaltemperatures would be much lower than theyaremdashperhaps averaging only -17degC compared tothe existing average of +15degC This then is avery important characteristic of the atmosphereyet it is made possible by a group of gases whichtogether make up less than 1 per cent of the totalvolume of the atmosphere There are abouttwenty of these greenhouse gases Carbon dioxide

is the most abundant but methane nitrous oxidethe chlorofluorocarbons and tropospheric ozoneare potentially significant although the impactof the ozone is limited by its variability and shortlife span Water vapour also exhibits greenhouseproperties but it has received less attention inthe greenhouse debate than the other gases sincethe very efficient natural recycling of waterthrough the hydrologic cycle ensures that itsatmospheric concentration is little affected byhuman activities Any change in the volume ofthe greenhouse gases will disrupt the energy flowin the earthatmosphere system and this will bereflected in changing world temperatures Thisis nothing new Although the media sometimesseem to suggest that the greenhouse effect is amodern phenomenon it is not It has been acharacteristic of the atmosphere for millions ofyears sometimes more intense than it is nowsometimes less

The carbon cycle and the greenhouse effect

Three of the principal greenhouse gasesmdashCO2methane (CH4) and the CFCsmdashcontain carbonone of the most common elements in theenvironment and one which plays a major rolein the greenhouse effect It is present in all organicsubstances and is a constituent of a great varietyof compounds ranging from relatively simplegases to very complex derivatives of petroleumhydrocarbons The carbon in the environment ismobile readily changing its affiliation with otherelements in response to biological chemical andphysical processes This mobility is controlledthrough a natural biogeochemical cycle whichworks to maintain a balance between the releaseof carbon compounds from their sources andtheir absorption in sinks The natural carboncycle is normally considered to be self-regulatingbut with a time scale of the order of thousandsof years Over shorter periods the cycle appearsto be unbalanced but that may be a reflection ofan incomplete understanding of the processesinvolved or perhaps an indication of the presenceof sinks or reservoirs still to be discovered (Mooreand Bolin 1986) The carbon in the system moves

Figure 73 CO2emissions percentage

share by region

Source Based on data inWorld Resources Institute(1992)

Figure 72 Schematic representation of the storage and flow of carbon in the earthatmosphere system

Source Compiled from data in Gribbin (1978) McCarthy et al (1986)

between several major reservoirs (see Figure 72)The atmosphere for example contains morethan 750 billion tonnes of carbon at any giventime while 2000 billion tonnes are stored onland and close to 40000 billion tonnes arecontained in the oceans (Gribbin 1978) Livingterrestrial organic matter is estimated to containbetween 450 and 600 billion tonnes somewhatless than that stored in the atmosphere (Mooreand Bolin 1986) World fossil fuel reserves alsoconstitute an important carbon reservoir of some5000 billion tonnes (McCarthy et al 1986) Theycontain carbon which has not been active in thecycle for millions of years but is now beingreintroduced as a result of the growing demandfor energy in modern society being met by themining and burning of fossil fuels It is beingreactivated in the form of CO

2 which is being

released into the atmospheric reservoir inquantities sufficient to disrupt the natural flowof carbon in the environment The greatestnatural flow (or flux) is between the atmosphereand terrestrial biota and between the atmosphere

and the oceans (Watson et al 1990) Althoughthese fluxes vary from time to time they haveno long-term impact on the greenhouse effectbecause they are an integral part of the earthatmosphere system In contrast inputs to theatmosphere from fossil fuel consumptionalthough smaller than the natural flows involvecarbon which has not participated in the systemfor millions of years When it is reintroducedthe system cannot cope immediately andbecomes unbalanced The natural sinks areunable to absorb the new CO

2 as rapidly as it is

being produced The excess remains in theatmosphere to intensify the greenhouse effectand thus contribute to global warming

The burning of fossil fuels adds more than 5billion tonnes of CO2 to the atmosphere everyyear (Keepin et al 1986) with more than 90 percent originating in North and Central AmericaAsia Europe and the republics of the formerUSSR (see Figure 73) Fossil fuel use remainsthe primary source of anthropogenic CO2 butaugmenting that is the destruction of natural

Figure 74 Rising levels of atmospheric CO2 at Mauna Loa Hawaii The smooth curve represents annualaverage values the zig-zag curve indicates seasonal fluctuations The peaks in the zig-zag curve represent thewinter values and the troughs represent the summer values

Source After Bolin et al (1986)

vegetation which causes the level of atmosphericCO2 to increase by reducing the amount recycledduring photosynthesis Photosynthesis is aprocess shared by all green plants by which solarenergy is converted into chemical energy Itinvolves gaseous exchange During the processCO2 taken in through the plant leaves is brokendown into carbon and oxygen The carbon isretained by the plant while the oxygen is releasedinto the atmosphere The role of vegetation incontrolling CO2 through photosynthesis is clearlyindicated by variations in the levels of the gasduring the growing season Measurements atMauna Loa Observatory in Hawaii showpatterns in which CO2 concentrations are lowerduring the northern summer and higher duringthe northern winter (see Figure 74) Thesevariations reflect the effects of photosynthesis inthe northern hemisphere which contains the bulkof the worldrsquos vegetation (Bolin 1986) Plantsabsorb CO2 during their summer growing phasebut not during their winter dormant period and

the difference is sufficient to cause semi-annualfluctuations in global CO2 levels

The clearing of vegetation raises CO2 levelsindirectly through reduced photosynthesis butCO2 is also added directly to the atmosphere byburning by the decay of biomass and by theincreased oxidation of carbon from the newlyexposed soil Such processes are estimated to beresponsible for 5ndash20 per cent of currentanthropogenic CO2 emissions (Waterstone 1993)This is usually considered a modernphenomenon particularly prevalent in thetropical rainforests of South America and South-East Asia (Gribbin 1981) (see Figure 75) butWilson (1978) has suggested that the pioneeragricultural settlement of North AmericaAustralasia and South Africa in the second halfof the nineteenth century made an importantcontribution to rising CO2 levels This issupported to some extent by the observation thatbetween 1850 and 1950 some 120 billion tonnesof carbon were released into the atmosphere as

Figure 75 Net regional release of carbon to the atmosphere as a result of deforestation during the 1980s(teragrams of carbon)

Source After Mintzer (1992)

a result of deforestation and the destruction ofother vegetation by fire (Stuiver 1978) Theburning of fossil fuels produced only half thatmuch CO2 over the same time period Currentestimates indicate that the atmospheric CO2

increase resulting from reduced photosynthesisand the clearing of vegetation is equivalent toabout 1 billion tonnes per year (Moore and Bolin1986) down slightly from the earlier valueHowever the annual contribution from theburning of fossil fuels is almost ten times what itwas in the years between 1850 and 1950

Although the total annual input of CO2 to theatmosphere is of the order of 6 billion tonnesthe atmospheric CO2 level increases by onlyabout 25 billion tonnes per year The differenceis distributed to the oceans to terrestrial biotaand to other sinks as yet unknown (Moore andBolin 1986) Although the oceans are commonlyconsidered to absorb 25 billion tonnes of CO2

per year recent studies suggest that the actualtotal may be only half that amount (Taylor 1992)The destination of the remainder has importantimplications for the study of the greenhouseeffect and continues to be investigated Theoceans absorb the CO2 in a variety of waysmdashsome as a result of photosynthesis inphytoplankton some through nutritionalprocesses which allow marine organisms to growcalcium carbonate shells or skeletons and someby direct diffusion at the airocean interface(McCarthey et al 1986) The mixing of the oceanwaters causes the redistribution of the absorbedCO2 In polar latitudes for example the addedcarbon sinks along with the cold surface watersin that region whereas in warmer latitudescarbon-rich waters well up towards the surfaceallowing the CO2 to escape again The turnoverof the deep ocean waters is relatively slowhowever and carbon carried there in the sinkingwater or in the skeletons of dead marineorganisms remains in storage for hundreds ofyears More rapid mixing takes place throughsurface ocean currents such as the Gulf Streambut in general the sea responds only slowly tochanges in atmospheric CO2 levels This mayexplain the apparent inability of the oceans to

absorb more than 40ndash50 per cent of the CO2

added to the atmosphere by human activitiesalthough it has the capacity to absorb all of theadditional carbon (Moore and Bolin 1986)

The oceans constitute the largest activereservoir of carbon in the earthatmospheresystem and their ability to absorb CO2 is not indoubt However the specific mechanismsinvolved are now recognized as extremelycomplex requiring more research into theinteractions between the atmosphere ocean andbiosphere if they are to be better understood(Crane and Liss 1985)

The changing greenhouse effect

Palaeoenvironmental evidence suggests that thegreenhouse effect fluctuated quite considerablyin the past In the Quaternary era for exampleit was less intense during glacial periods thanduring the interglacials (Bach 1976 Pisias andImbrie 1986) Present concern is with itsincreasing intensity and the associated globalwarming The rising concentration ofatmospheric CO2 is usually identified as the mainculprit although it is not the most powerful ofthe greenhouse gases It is the most abundanthowever and its concentration is increasingrapidly As a result it is considered likely to givea good indication of the trend of the climaticimpact of the greenhouse effect if not its exactmagnitude

Svante Arrhenius a Swedish chemist is usuallycredited with being the first to recognize that anincrease in CO2 would lead to global warming(Bolin 1972 Bach 1976 Crane and Liss 1985)Other scientists including John Tyndall in Britainand TCChamberlin in America (Jones andHenderson-Sellers 1990) also investigated the linkbut Arrhenius provided the first quantitativepredictions of the rise in temperature (Idso 1981Crane and Liss 1985) He published his findings atthe beginning of this century at a time when theenvironmental implications of the IndustrialRevolution were just beginning to be appreciatedLittle attention was paid to the potential impact ofincreased levels of CO2 on the earthrsquos radiation

climate for some time after that however and theestimates of CO2-induced temperature increasescalculated by Arrhenius in 1903 were not bettereduntil the early 1960s (Bolin 1972) Occasionalpapers on the topic appeared (eg Callendar 1938Revelle and Seuss 1957 Bolin 1960) but interestonly began to increase significantly in the early1970s as part of a growing appreciation of thepotentially dire consequences of human interferencein the environment Increased CO2 production andrising atmospheric turbidity were recognized as twoimportant elements capable of causing changes inclimate The former had the potential to causegreater warming whereas the latter was consideredmore likely to cause cooling (Schneider andMesirow 1976) For a time it seemed that thecooling would dominate (Calder 1974 Ponte1976) but results from a growing number ofinvestigations into greenhouse warming publishedin the early 1980s changed that (eg Idso 1980Manabe et al 1981 Schneider and Thompson1981 Pittock and Salinger 1982 Mitchell 1983NRC 1982 and 1983) They revealed that scientistshad generally underestimated the speed with whichthe greenhouse effect was intensifying and hadfailed to appreciate the impact of the subsequentglobal warming on the environment or on humanactivities

Worldwide concern coupled with a sense ofurgency uncommon in the scientific communityled to a conference on the lsquoInternationalAssessment of the Role of Carbon Dioxide andother Greenhouse Gases in Climate Variationsand Associated Impactrsquo held at Villach Austriain October 1985 To ensure the follow-up of therecommendations of that conference anAdvisory Group on Greenhouse Gases (AGGG)was established under the auspices of theInternational Council of Scientific Unions (ICSU)the United Nations Environment Program(UNEP) and the World MeteorologicalOrganization (WMO) (Environment Canada1986) The main tasks of the AGGG were tocarry out biennial reviews of international andregional studies related to the greenhouse gasesto conduct aperiodic assessments of the rates ofincreases in the concentrations of greenhouse

gases and to estimate the effects of such increasesBeyond this they also supported further studiesof the socio-economic impacts of climatic changeproduced by the greenhouse gases and identifiedareas such as the monsoon region of south-eastAsia the Great Lakes region of North Americaand the circumpolar Arctic as likely candidatesfor increased investigation The AGGG suggestedthat the dissemination of information on recentdevelopments to a wide audience was alsoimportant and in keeping with that viewpointEnvironment Canada began the production of aregular newsletter to highlight current events inCO2climate research Its annual reportsUnderstanding CO2 and Climate published in1986 and 1987 were also devoted to that themeThroughout the 1980s Environment Canadafunded research on the environmental and socio-economic impacts of global warming in suchareas as agriculture natural resourcedevelopment and recreation and tourism TheDepartment of Energy in the United States hasalso been active in the field with more broadlybased reports on the effects of increasing CO2

levels on vegetation (Strain and Cure 1985) andon climate (MacCracken and Luther 1985a and1985b) as well as the effects of future energy useand technology on the emission of CO2 (Edmondset al 1986 Cheng et al 1986)

In Europe Flohnrsquos (1980) study of the climaticconsequences of global warming caused byhuman activities for the International Institutefor Applied Systems Analysis (IIASA) includedconsideration of CO2 More recently theCommission of European Communities (CEC)funded research into the socio-economic impactsof climate changes which might be caused by adoubling of atmospheric CO2 (Meinl et al 1984Santer 1985) Most of these investigationsinvolved the use of GCMs The UKMeteorological Office five-layer GCM forexample provided information on CO2-inducedclimatic change over western Europe (Wilson andMitchell 1987) Several other Europeancountries including Germany and theNetherlands also launched researchprogrammes

With the worldwide increase in the numberof government agencies involved in the study ofglobal warming it became clear that an attempthad to be made to assess the overall state of theresearch This was made possible by the WMOand UNEP which together established theIntergovernmental Panel on Climate Change

(IPCC) in 1988 The IPCC was charged withassessing the status of the scientific informationon climate change so that potentialenvironmental and socio-economic impacts couldbe evaluated It was also asked to formulateappropriate response strategies With the co-operation of several hundred scientists from

Table 72 A condensation of the IPCC Executive Summary on Climate Change 1990

Source Houghton et al (1990)

around the world the IPCC produced the firstpart of its reportmdashthe scientific assessmentmdashin1990 (Houghton et al 1990) The reports onimpact assessment (Tegart et al 1990) andresponse strategies (IPCC 1991) followed shortlythereafter The scientific assessment included asummary of current knowledge of globalwarming as well as predictions for futuredevelopments (see Table 72) A supplementaryreport was issued in 1992 generally confirmingthe results of the earlier assessment but alsopaying greater attention to the effects of sulphateemissions and ozone depletion on global warmingtrends (Houghton et al 1992)

Concern over global warming was also anintegral part of the Framework Convention onClimate Change signed at the Earth Summit inRio de Janeiro in 1992 The convention wasintended to deal with the human contribution toglobal warming and create a vehicle forinternational action comparable to the MontrealProtocol on ozone depletion For most observershowever it was no more than a symbolicstatement with few specific targets and no properenforcement mechanisms (Clery 1992 Hulme1993) The refusal of the United States to agreeto even modest greenhouse gas emission targetsand the unwillingness of some Third Worldcountries to support the energy efficiencystandards necessary for reduced emissionsfurther weakened the convention (Pearce 1992c)

The perceived weakness of the convention isin part a reflection of the different views of theissue held by policymakers and scientists (Leggett1992) Despite the immense volume of researchon global warming many key elementsmdashsuchas the magnitude and timing of the warmingmdashremain imperfectly understood and thereforedifficult to predict with any accuracy Uncertaintyof this type is not uncommon in scientificresearch and scientists have come to accept itbut among planners and politicians it is oftenseen as undermining the arguments of thosecalling for immediate attention to the issue Sincepolicymakers have the ultimate say in where andwhen policy will be implemented to deal withthe warming it appears that they must shoulder

much of the blame for the delays However someenvironmentalists also claim that scientists havefailed by giving too much attention to theuncertainties and too little to the direconsequences of full-scale warming (Leggett1992)

No matter how the blame is apportioned oneof the results of the uncertainty has been to allowthose who remain unconvinced by the scientificassessment or perhaps have a vested interest inretaining the status quo to argue successfully thatno steps be taken to deal with the issue until thereal impact of global warming is revealed byadditional research That may yet take severaldecades and some of the researchers investigatingthe problem warn that by then it may be too late(Roberts 1989 Henderson-Sellers 1990)

Atmospheric carbon dioxide and temperaturechange

Although present concern with global warmingcentres on rising concentrations of atmosphericCO2 concentrations of that gas have variedconsiderably in the past Analysis of air bubblestrapped in polar ice indicates that the lowestlevels of atmospheric CO2 occurred during theQuaternary glaciations (Delmas et al 1980) Atthat time the atmosphere contained only 180 to200 parts per million by volume (ppmv) of CO2although there is some evidence that levelsfluctuated by as much as 60 ppmv in periods asshort as 100 years (Crane and Liss 1985) Levelsrose to 275 ppmv during the warm interglacialphases and that level is also consideredrepresentative of the pre-industrial era of the earlynineteenth century (Bolin 1986) CO2

measurements taken by French scientists in the1880s just as the effects of the IndustrialRevolution were beginning to be felt have beenreassessed by Siegenthaler (1984) who hasconcluded that levels in the northern hemisphereaveraged 285 to 290 ppmv at that time

When the first measurements were made atMauna Loa in 1957 concentrations had risento 310 ppmv and they continued to rise by justover 1 ppmv per year to reach 335 in 1980

Since then levels have risen at a rate of 2 to 4ppmv per year to reach a level of 345 ppmv bythe mid-1980s (Gribbin 1981 Bolin 1986)That represents an increase of 70 ppmv in lessthan 200 years The difference between glacialand inter-glacial periods was about the samebut then the time interval was measured in tensof thousands of years The current level of 353ppmv is 25 per cent higher than thepreindustrial volume and without precedent in

the past 100000 years of earth history (Watsonet al 1990) If the present rate of increasecontinues until the year 2050 it is estimatedthat the atmospheric CO2 concentration will be450 ppmv and by 2075 it will be 500ndash600ppmv more than twice the 1800 level (Bolin1986) (see Figure 76)

Predicting such trends is difficult Futuregrowth rates and concentrations will dependupon a variety of factors including for

Figure 76 Projected changes in atmospheric CO2 concentration The curves LB and UB indicate the range of

estimates for CO2 alone LBrsquo and UBrsquo indicate the range of estimates when other greenhouse gases are included

and expressed in CO2 equivalents The value 550 ppm corresponds to a doubling of pre-industrial CO

2concentrations

Figure 77 Changinggreenhouse gas levelscomparison of thelsquobusiness-as-usualrsquo scenariowith one involving majoremission controls

Source After Houghton et al (1990)

example the nature and rate of industrialdevelopment the extent to which the earthrsquosforests continue to be destroyed and the successof programmes aimed at reducing the output ofCO2 Most studies have based their predictionson several scenarios one of which is commonlya direct projection of the status quomdashthe IPCClsquobusiness-as-usualrsquo scenario for examplemdashwithothers based on either increase or decreases inCO2 and combinations of other gases (Bolin etal 1986 Houghton et al 1990) The net resultis that future greenhouse gas levels are usuallypresented as ranges of possibilities rather thandiscrete values (see Figure 77)

Since CO2 is a radiative forcing agent knownto warm the atmosphere the rising CO2 valuescan be translated into temperature increases Ithas been estimated for example that a 03ndash06degC increase in the earthrsquos surfacetemperature has taken place since 1900 at arate broadly consistent with that expected fromthe rising levels of greenhouse gases (Houghtonet al 1990) Schneider (1987) claims that theearth is 05deg warmer in the 1980s than it was inthe 1880s The change has not been evenhowever The main increase took place between1910 and 1940 and again after 1975 (Gadd1992) Between 1940 and 1975 despite risinggreenhouse levels global temperaturesdeclined particularly in the northernhemisphere In addition analysis of the recordssuggests that the relatively rapid warming priorto 1940 was probably of natural origin (Follandet al 1990) Such changes are well within therange of normal natural variations in globaltemperatures (Crane and Liss 1985) butHansen and Lebedeff (1988) have calculatedthat the warming between the 1960s and 1980swas more rapid than that between the 1880sand 1940s which suggests that the greenhousewarming may be beginning to emerge from thegeneral background lsquonoisersquo Hansen of theGoddard Institute of Space Studies claimedsubsequently that the global greenhouse signalis sufficiently strong that a cause-and-effectrelationship between the CO2 increase andglobal warming can be inferred (Climate

Institute 1988a) The controversy continueshowever Kheshgi and White (1993) concludedthat it will not be possible to separate agreenhouse warming signal from the overallnoise until more is known about the dimensionsand causes of natural climate variability Wigleyand Barnett (1990) in their contribution to theIPCC Scientific Assessment took the middleground They noted that there is as yet noevidence of an enhanced greenhouse effect inthe observational record but cautioned thatthis may be in part a function of theuncertainties and inadequacies in currentinvestigative techniques In short althoughgreenhouse-gas-induced warming may not havebeen detected it does not follow that it does notexist

Estimates of global warming are commonlyobtained by employing atmospheric modellingtechniques based on computerized GeneralCirculation Models (GCMs) (see Table 24) Toexamine the impact of an enhanced greenhouseeffect on temperature for example the CO2

component in the model is increased to aspecific level The computer program is allowedto run until equilibrium is established amongthe various climatic elements included in themodel and the new temperatures have beenreached (see Figure 78) This approach hadproduced a general consensus by the mid-1980s that a doubling of CO2 levels wouldcause an average warming of 13ndash4degC (Manabeand Wetherald 1975 Cess and Potter 1984Dickinson 1986 Bolin et al 1986) The IPCCassessment produced values of 15ndash45degC witha best estimate of 25degC These results comparewith the estimate of 4ndash6degC made by Arrheniusat the beginning of the century (Kellogg 1987)Smaller increases have been calculated byNewell and Dopplick (1979) who estimatedthat the temperatures above the tropical oceanswould increase by only 003degC on average andby Idso (1980) who estimated a global increaseof 026degC These lower values are generallyconsidered to be unrepresentative by mostscientists investigating the problem however(Cess and Potter 1984 Webster 1984)

Although the estimated temperature increasesare not particularly impressivemdashmainly becausethey are average global valuesmdashevidence frompast world temperature changes indicates thatthey are of a magnitude which could lead tosignificant changes in climate and climate-relatedactivities During the Climatic Optimummdashthe

warm epoch following the last Ice Agemdashsome5000 to 7000 years ago temperatures in NorthAmerica and Europe were only 2ndash3degC higherthan the present average but they producedmajor environmental changes (Lamb 1977)Evidence from that time period and from anotherwarm spell in the early Middle Ages 800 to 1000

Figure 78 Change in global surface temperature following a doubling of CO2 (a) December January and

February (b) June July and August

Source Compiled from data in Houghton et al (1990)

years ago also suggests that the greatest impactof any change will be felt in mid to high latitudesin the northern hemisphere

The models used to investigate global changeare usually considered to provide less accurateresults at the regional level but they do supportthese past geographical trends Following a

doubling of CO2 the Canadian Climate CentreGCM indicates a warming of almost 5degC for thesouthern part of the country summer and winterIn the north the seasonal differences would beconsiderable with a mean temperature increaseof 8ndash12degC in winter but less than 1degC in summer(Hengeveld 1991) Similar values are predicted

Figure 79 Change in global soil moisture levels following a doubling of CO2 (a) December January and

for northern Russia but in north-western European annual increase of only 2ndash4degC is consideredmore likely (Mitchell et al 1990) However inthe latter region climate is strongly influencedby the north Atlantic oceanic circulation andsince reliable oceanatmosphere models remainin the development stage the accurate predictionof temperature change for such areas is difficult

The estimated temperature increases at lowerlatitudes are generally less According topredictions from China temperatures will riseby 2ndash3degC in that country with increases exceeding3degC only in the western interior (NCGCC 1990)In southern Europe the Sahel south-east Asiaand Australiamdashall areas which receivedparticular attention in the IPCC assessmentmdashthepredicted increases generally fall between 1ndash2degCwith only southern Europe expecting an increaseof 3degC in the summer months (Mitchell et al1990)

Because the various elements in theatmospheric environment are closely interrelatedit is only to be expected that if temperaturechanges changes in other elements will occuralso Moisture patterns are likely to be alteredfor example (see Figure 79) In the mid-continental grasslands of North Americaprecipitation totals may be reduced by 10 to 20per cent while summer soil moisture levels mayfall by as much as 50 per cent (Manabe andWetherald 1986) IPPC estimates are slightly lesswith summer precipitation declining by 5ndash10 percent and soil moisture by 15ndash20 per cent butconfidence in these values is low (Mitchell et al1990) In northern and north-western Chinamdashwhere an average temperature increase of 2degC isexpectedmdashevaporation rates would increase byabout 20 per cent compounding existingproblems of aridity in these areas (NCGCC1990) Precipitation would be less frequent overmost of Europe in summer and autumn andthroughout the year in the south (Wilson andMitchell 1987) causing soil moisture levels todecline by as much as 25 per cent (Mitchell et al1990) According to some projections morerainfall is possible in parts of Africa and southeastAsia (Wigley et al 1986 Kellogg 1987) The

latter would benefit from increases of up to 10per cent in soil moisture during the summermonths but in parts of Africa such as the Sahelmdashwhere temperatures are expected to rise no morethan 2degCmdashwinter rainfall would decline by 5ndash10 per cent and summer precipitation increasesof as much as 5 per cent would be insufficient toprevent a decline in soil moisture (Mitchell et al1990) Changes such as these in moisture regimescoupled with changes in the length and intensityof the growing season would disrupt existingvegetation patterns and require major alterationsin agricultural activities in many areas

The reality of the situation may only becomeapparent when the changes have occurred forthere are many variables in the predictions Thehuman factors as always are particularlyunpredictable Technology politics socio-economic conditions and even demography cancontribute to changes in the concentration ofCO2 yet the nature and magnitude of thevariations in these elements is almost impossibleto predict

The contribution of other greenhouse gases

Most predictions of future changes in theintensity of the greenhouse effect are based solelyon changes in the CO2 content of the atmosphereTheir accuracy is therefore questionable sinceCO2 is not the only greenhouse gas nor is it themost powerful Methane (CH4) nitrous oxide(N2O) and the CFCs are the most important ofthe other greenhouse gases Tropospheric ozone(O3) is also capable of enhancing the greenhouseeffect but its present concentrations are veryvariable in both time and place and there is noclear indication of future trends (Bolle et al1986)

Methane is a natural component of the earthatmosphere system with its origin in theanaerobic decay of organic matter mainly inthe earthrsquos natural wetlands Significantamounts of CH4 are also produced by the globaltermite population (Crutzen et al 1986) Levelsof atmospheric CH4mdashat 172 ppmvmdashare verylow compared to those of CO2 Molecule for

molecule it is about twenty-one times moreeffective than CO2 however and itsconcentration is increasing at about 08ndash10 percent per year (Blake and Rowland 1988 Shineet al 1990) The most important causes of thisincrease are to be found in agriculturaldevelopment (see Table 71) Biomass burningto clear land for cultivation adds CH4 to theatmosphere as does the worldrsquos growingpopulation of domestic cattle pigs and sheepwhich release considerable amounts of CH4

through their digestive processes (Crutzen et al1986) By far the largest source ofagriculturally-produced CH4 however is ricecultivation Rice paddies being flooded andtherefore providing an anaerobic environmentfor at least part of the year act much likenatural wetlands Their total contribution torising CH4 levels is difficult to measure since 60per cent of the worldrsquos rice paddies are in Indiaand Chinamdashboth areas from which reliabledata are generally unavailable Howeverannual rice production has doubled over thepast 50 years and it is likely that CH4 emissionshave increased in proportion (Watson et al1990) although perhaps not by as much as wasonce thought (Houghton 1992)

The energy industry is another importantsource of anthropogenic CH4 As a by-productof the conversion of vegetable matter into coalit is trapped in coal-bearing strata to bereleased into the atmosphere when coal ismined It is also one of the main components ofnatural gas and escapes during drillingoperations or through leaks in pipelines and atpumping stations (Cicerone and Oremland1988) Together these sources may account for15 per cent of global CH4 emissions (Hengeveld1991) The disposal of organic waste in landfillsites where it undergoes anaerobic decay isalso considered to be a potentially significantsource of CH4 Attempts to provide accurateestimates of emissions however are hamperedby the absence of appropriate data on thenature and amounts of organic waste involved(Bingemer and Crutzen 1987)

The lifespan of CH4 in the atmosphere

averages 10 years It is removed by reaction withhydroxyl radicals (OH) which oxidize it to watervapour and CO2 both of which are greenhousegases but less potent than CH4 (Watson et al1990) Atmospheric OH levels are currentlydeclining as a result of reactions with otheranthropogenically produced gases such as carbonmonoxide (CO) causing a reduction in the rateof removal of CH4 (Hengeveld 1991) The totalimpact of decreased concentrations is difficult toassess but CH4 emissions into the atmospherecontinue to grow and it has been estimated thatan immediate reduction in emissions of 15ndash20per cent would be required to stabilizeconcentrations at their current levels (Watson etal 1990) The IPCC Supplementary Report notedsome evidence that the rate of growth in CH4

concentration in the atmosphere may be alreadybeginning to slow down (Houghton 1992) Evenwith this however potential feedbacks workingthrough such elements as soil moisture levels andrising high latitude temperatures could result insignificant increases in future CH4 emissions Allof these trends suggest that CH4 will continue tocontribute to the enhancement of the greenhouseeffect well into the future

The current atmospheric concentration ofN2Omdashat 310 parts per billion by volume(ppbv)mdashis about a thousand times less than thatof CO2 and it is increasing less rapidly than eitherCO2 or CH4 N2O is released naturally into theatmosphere through the denitrification of soilsand is removed mainly through photochemicaldecompostion in the stratosphere in a series ofreactions which contribute to the destruction ofthe ozone layer (see Chapter 6) It is thought toowe its present growth to the increased use offossil fuels and the denitrification of agriculturalfertilizers The IPCC assessment has concludedhowever that past estimates of the contributionof fossil fuel combustion to the increase are toolargemdashby perhaps as much as ten timesmdashandN2O production rates during agricultural activityare difficult to quantify Thus although the totalincrease of N2O can be calculated the amountsattributable to specific sources cannot bepredicted with any accuracy It is even possible

that there are sources yet to be identified (Watsonet al 1990) As a result the global N2O budgetremains poorly understood and its futureconcentration is therefore difficult to predict

CFCs and other halocarbons released fromrefrigeration units insulating foams aerosolspray cans and industrial plants are recognizedfor their ability to destroy the stratospheric ozonelayer but they are also among the most potentgreenhouse gases For example CFC-11 is about12000 times more effective than CO2 (Houghtonet al 1990) The CFCs are entirely anthropogenic

in origin and should therefore be much easier tomonitor and control than some of the other gasesTheir concentrations in the atmosphere rangefrom CFC-115 at 5 parts per trillion by volume(pptv) to CFC-12 at 484 pptv and have beengrowing at rates between 4ndash10 per cent perannum Other halocarbons such as Halon-1211and Halon-1301 used mainly in fireextinguishers with current concentrations of lessthan 2 pptv are growing at rates as high as 15per cent per year (Watson et al 1990) Recentinternational agreements to reduce the use of

Figure 710 Greenhouse gas contributions to global warming (a) 1880ndash1980 (b) 1980s

CFCs (see Chapter 6) are aimed at preventingfurther damage to the ozone layer but they willalso have some impact on the greenhouse effectHowever CFCs have a long residence time inthe atmospheremdashup to 400 years in the case ofCFC-13 and CFC-115mdashand even as emissionrates fall they will continue to contribute toglobal warming for some time to come

The presence of these other greenhouse gasesintroduces a number of uncertainties into thepredictions of future greenhouse levels None ofthem is individually as important as CO2 It hasbeen suggested however that their combinedinfluence on the greenhouse effect is alreadyequivalent to half that of CO2 alone (Bolle et al1986) and by early next century theircontribution to global warming could be equalto that of CO2 (Ramanathan et al 1985) Theirimpact would become increasingly important inthe low CO2 emission scenarios envisaged bysome investigators The involvement of the CFCsand N2O in the depletion of the ozone layer addsa further complication Attempts to mitigate theeffects of these gases on the ozone layer wouldalso impact on the greenhouse effect Thusalthough such gases as CH4 N2O and the CFCshave received much less attention than CO2 inthe past it is clear that plans developed to dealwith global warming must include considerationof all the greenhouse gases not just CO2 (seeFigure 710)

ENVIRONMENTAL AND SOCIO -ECONOMIC IMPACTS OF INCREASINGGREENHOUSE GASES

Given the wide range of possibilities presentedin the estimates of future greenhouse gas levelsand the associated global warming it is difficultto predict the environmental and socio-economiceffects of such developments However using acombination of investigative techniquesmdashranging from laboratory experiments with plantsto the creation of computer generated models ofthe atmosphere and the analysis of past climateanomaliesmdashresearchers have produced results

which provide a general indication of what theconsequences might be in certain key sectors

The impact of elevated CO2 levels on naturaland cultivated vegetation has receivedconsiderable attention Two elements areinvolved since elevated CO2 has an effect on bothphotosynthesis and on temperature Through itsparticipation in photosynthesis CO2 provides thecarbon necessary for proper plant growth andin laboratory experiments under controlledconditions it has been shown that increasedconcentrations of the gas enhance growth in mostplants The bulk of the earthrsquos vegetation can beclassified into two general groupsmdashC3 and C4mdashwhich differ in the biochemistry of theirphotosynthetic processes The C3 group makesup about 95 per cent of the earthrsquos biomass andincludes important grain crops such as wheatrice and soybeans while the smaller C4 group isrepresented by maize sorghum and millet Mosttrees belong to the C3 group Both C3 and C4

plants react positively to increased levels of CO2with the former being most responsive (Melilloet al 1990) A doubling of CO2 has increasedyields of maize sorghum millet and sugar caneby 10 per cent in controlled experiments andincreases of as much as 50 per cent have beenachieved with some C3 plants (EnvironmentCanada 1986) The response of naturalvegetation or field crops might be less becauseof a variety of non-climatic variables but sometrees do seem capable of responding quitedramatically For example Sveinbjornsson(1984) has estimated that a doubling of CO2

would double the rate of photosynthesis in theAlaskan paper birch Chinese experiments alsoindicate that higher concentrations of CO2mdashupto 400ndash700 ppmvmdashwould bring about increasesin the trunk weight diameter and height ofnursery stock as long as appropriate soil andmoisture conditions could be maintained(NCGCC 1990) Some investigators consider thepresent levels of CO2 to be suboptimal forphotosynthesis and primary productivity in themajority of terrestrial plants They suggesttherefore that the biological effects of enhancedCO2 would likely be beneficial for most plants

(Wittwer 1984) There is some concern howeverthat the improvement in the growth rates wouldbe accompanied by a reduction in the quality ofplant tissue (Melillo et al1990)

The predicted higher temperatures workingthrough the lengthening and intensification of thegrowing season would have an effect on the ratesof plant growth and crop yields The regionaldistribution of vegetation would changeparticularly in high latitudes where thetemperature increases are expected to be greatest(Shugart et al 1986) Across the northern regionsof Canada Scandinavia and Russia the trees ofthe boreal forest would begin to colonize thetundra as they have done during warmer spellsin the past (Viereck and Van Cleve 1984 Ball1986) at a rate of about 100 km for every 1degCof warming (Bruce and Hengeveld 1985) Thesouthern limit of the boreal forest would also

migrate northwards under pressure from thespecies of the hardwood forests and grasslandswhich would be more suited to the newconditions An expansion of the grassland inwestern Canada would push the southernboundary of the forest north by 250ndash900 km(Wheaton et al 1989) and ultimately the borealforest might disappear completely from northernAlberta Saskatchewan and Manitoba (see Figure711)

The changing nature and distribution of thenorthern forest biomes would be the clearestindication of prolonged warming Accompanyingthese changes and contributing to them wouldbe a series of less obvious factors Rising soiltemperatures for example would speed up theprovision of soil nutrients through the more rapiddecay of organic matter and contribute toimproved plant growth (Van Cleve et al 1983)

Figure 711 Changing vegetation patterns in Canada following a doubling of CO2

Source After Hengeveld (1991)

New or increased disease and insect infestationmight also follow the warming but that mightbe offset by a decline in existing infestationproblems (Melillo et al 1990) With warmer andpotentially drier conditions in the forest anincrease in the frequency of forest fires is a distinctpossibility One element in particular that requiresstudy is the rate at which the forests can respondto the warming If the temperatures change morerapidly than the forests can accommodate themthen the rapid die-off of large numbers of treeswould cause disruption to the northernecosystems for perhaps centuries to come Thisin turn would disrupt the patterns of economicactivity in the north and have a significant effecton those countries such as Canada SwedenFinland and Russia where national and regionaleconomies depend very much on the harvestingof softwoods from the boreal forest

In lower latitudes where the temperatureelement is less dominant and changes areexpected to be less the impact of global warmingwill often be experienced through changes in theamount and distribution of moisture Soilmoisture levels would decline in areasexperiencing a Mediterranean-type climatemdashsouthern Europe South Africa parts of SouthAmerica and Western Australiamdashfor exampleas a result of reduced precipitation and the higherevapotranspiration rates associated with thewarming (Mitchell et al 1990 Pittock andSalinger 1991) Although the vegetation in theseareas has adapted to seasonal drought theextension of the dryness into the normally wetwinter season would ultimately bring aboutchanges in the composition and distribution ofthe Mediterranean climate biome Extraprecipitation in monsoon areas and the increasedpoleward penetration of the monsoon rainsexpected to accompany global warming (Pittockand Salinger 1991) would allow the expansionof the tropical and sub-tropical vegetation ofareas such as northern Australia ElsewheremdashtheSahel for examplemdashthe impact of the extraprecipitation would be offset perhaps completelyby increased evapotranspiration rates at highertemperatures

The conditions likely to alter the regionaldistribution of natural vegetation are also likelyto change the nature and extent of cultivatedvegetation A significant expansion of agricultureis to be expected in mid to high latitudes wherethe greatest warming will be experienced In theinterior of Alaska for example a doubling ofCO2 levels would raise temperatures sufficientlyto lengthen the growing season by three weeks(Wittwer 1984) which would allow landpresently under forage crops or evenuncultivated to produce food crops such ascabbage broccoli carrots and peas The growingseason in Ontario Canada would be lengthenedby 48 days in the north and 61 in the south Thereduced frost risk at the beginning and end ofthe growing season would be a major benefit insome areas By 2050 in New Zealand and thecoastal areas of Australia for example the frost-free season may be 30ndash50 days longer than atpresent (Salinger and Pittock 1991) The greaterintensity of the growing season along with theeffects of increased CO2 on photosynthesiswould lead to increased crop yields In Europesimulations of the effects of a doubling of CO2

on crops using grass as a reference haveindicated an average increase in biomass potentialof 9 per cent with regional values ranging froman increase of 36 per cent in Denmark to adecrease of 31 per cent in Greece (Santer 1985)The increase in agricultural output in China isexpected to be 2 per cent brought about by thegreater production of rice maize and cotton athigher latitudes and a northward shift of 50ndash100 km in the cultivation of tropical and sub-tropical fruits (NCGCC 1990)

It may not always be possible foragriculturalists to take full advantage of thebenefits of global warming because of the effectson agricultural production of elements eitherunrelated or only indirectly related to climatechange Warmer climates would allow thenorthward expansion of cultivation on theCanadian prairies for example but the benefitsof that would be offset by the inability of thesoils in those areas to support anything other thanmarginal forage crops which are not profitable

under current economic conditions (Arthur1988) A major problem in China would be anincrease in insect pests such as rice and cornborers rice winged fleas army worms aphidsand locusts Dealing with these would raise pestcontrol costs by 1ndash5 per cent (NCGCC 1990)Few simulations take such variables into accountThe model employed by Santer (1985) to predictchanges in European biomass potential includedno provision for such important elements asinsects disease and additional fertilizers Untilsuch unknowns can be estimated the results ofthis and similar simulations must be treated withcaution

The most serious problem for agriculture isthe increasing dryness likely to accompany therising temperatures in many areas Reducedprecipitation following changes in circulationpatterns plus the increased rates ofevapotranspiration caused by highertemperatures would create severe moisture stressfor crops in many areas (Climate Institute 1988b)Less precipitation and higher temperatures in thefarmlands of southern Ontario might reduceyields sufficiently to cause losses of as much as$100 million per year (Smit 1987) The areashardest hit would be the worldrsquos grain producingareas which would become drier than they arenow following the global warming (Kellogg1987) Corn yields would be reduced in the mid-western plains of the United States and a majorincrease in the frequency and severity of droughtwould lead to more frequent crop failures in thewheat growing areas to the north in Canada(Williams et al 1988) The grain belt in Russiaand Ukraine already unable to meet the needsof these nations would suffer as badly as itsNorth American equivalent (Kellogg 1987)

Such developments would disrupt the patternof the worldrsquos grain trade which depends heavilyon the annual North American surplus Foodsupply problems would become serious in Russiaand famine would strike many Third Worldcountries In the early 1980s the picture was notconsidered completely hopeless however forrainfall was expected to increase in some tropicalareas and the combination of more rain higher

temperatures and more efficient photosynthesiswould lead to increased rice yields of as much as10 per cent (Gribbin 1981) Predictions for someof the grain growing areas in Australia alsoindicated increased precipitation and highertemperatures (Kellogg 1987) However a 1992study by Martin Parrymdasha leading analyst of theagricultural implications of global warmingmdashwas much more pessimistic Computer modelprojections for the middle of the twenty-firstcentury indicated a decline of 15ndash20 per cent ingrain yields in Africa tropical Latin America andmuch of India and south-east Asia leading tomajor famine in these areas (Pearce 1992d) Suchvariations are only to be expected The manyelements which together determine thedistribution of natural and cultivated vegetationand the complex interrelationships involved areimperfectly understood It is therefore verydifficult to depict them accurately in currentmodels Coupled with the limited ability of mostglobal models to cope with regional scaleprocesses to which ecosystems respond thisensures that the ultimate changes in natural andcultivated vegetation resulting from globalwarming must remain speculative

Most of the agricultural projections note theimportance of an adequate water supply if thefull benefits of global warming are to beexperienced Beyond that little work has beendone on the impact of an intensified greenhouseeffect on water resources Canadian studies haveexamined the implications of climate change forfuture water resources in the Great LakesmdashStLawrence River System (Sanderson 1987) andin northern Quebec (Singh 1988) In Australiascientists using a high-resolution nested model(see Chapter 2) have been able to simulate thehydrology of the south-eastern part of thecontinent more accurately than is possible withexisting global models With a resolution aboutten times finer that that of the global models thenested model provides a better representation ofthose regional factorsmdashsurface morphology forexamplemdashknown to influence strongly thedistribution and intensity of precipitation andrun-off The model has performed favourably in

simulating current hydrological events and withfurther development it should be able to providea more accurate representation of regionalhydrology following global warming than iscurrently possible Beyond such studies thegeneral lack of attention to the hydrologic cyclefollowing global warming is an important gapin current research

One aspect of global hydrology which hasbeen considered in some detail is the impact ofhigher world temperatures on sea level Warmingwould cause sea level to rise as a result of thethermal expansion of sea water and the returnof additional water to the oceans from meltingtemperate glaciers Global mean sea level hasalready been rising by about 15 cm per decadeover the past century and with global warmingthat rate is likely to accelerate to between 3ndash10cm per decade (Hengeveld 1991) The IPCC hasestimated that by 2030 sea level will be 18 cmhigher than at present and by 2070 the rise willbe 44 cm (Warrick and Oerlemans 1990) Earlierstudies indicated that mean sea level might riseby as much as a metre as early as 2050 (Titus1986) but the IPCC assessment does not foreseea rise of that amount during the next century(Warrick and Oerlemans 1990)

Although such increases are relatively minorcompared to past changes in sea level theywould be sufficient to cause serious floodingand erosion in coastal areas In low lyingregions such as the Netherlands alreadydependent upon major protective works even asea level rise of half that postulated would havemajor consequences (Hekstra 1986) Land closeto sea level in Britainmdasharound the Wash forexamplemdashwould be similarly vulnerable andstructures like the Thames Flood Barrier mightbe needed on other British rivers EnvironmentCanada has commissioned studies of the impactof sea level rises in the Maritime Provinceswhich show that flooding events or stormsurges would become more frequent and severepresenting serious problems for sewage andindustrial waste facilities road and rail systemsand harbour activities (Martex Ltd 1987Stokoe 1988)

A sea level rise of little more than 20 cmwould place some 11 m hectares in jeopardyalong the east coast of China from the PearlRiver in the south to Bohai Bay in the north(NCGCC 1990) In Bangladesh more than100000 hectares would be inundated by a 50cm rise in sea level (Mackay and Hengeveld1990) causing the loss of good agriculturalland and displacing tens of thousands of peopleSome of the island nations in the Pacific andIndian Oceans would face even more seriousproblems The highest point on the MaldiveIslands in the Indian Ocean for example isonly 6 m above present sea level (Hengeveld1991) and the average elevation of theMarshall Islands in the west central Pacific isless than 3 m (Climate Institute 1992a) Bothareas are already being threatened by floodingand increased coastal erosion and if sea levelcontinues to rise they will becomeuninhabitable

The impact of even small rises in sea level isincreased during high tides or storms and whenthe two combine the results can be disastrousIn 1953 for example high tides and a stormsurge in the North Sea led to major flooding ineastern England Belgium and the NetherlandsOver 2000 people drowned At that time suchan event was to be expected no more than oncein 150 years Since then with rising sea levelsthe odds have shortened to once in 35 yearsdespite the development of better coastaldefences (Simons 1992)

Whether storminess would increase ordecrease in a warmer world remains a matter ofcontroversy In mid-latitudes the storms thatdevelop in the North Atlantic and Pacific andacross the southern oceans are driven by a stronglatitudinal temperature gradient With the risingtemperatures projected for higher latitudes thatgradient would be reduced and mid-latitudestorminess might therefore be expected to decline(Houghton et al 1990) It is possible howeverthat those storms that do develop will be moreintense fuelled by greater evaporation rates overa warmer ocean and the consequent increase inthe energy flux between ocean and atmosphere

This could lead to an increase in the incidence ofthe intense low pressure systems that wroughthavoc in Britain and western Europe in 1987 andagain in 1990 (Simons 1992) Unfortunatelycurrent models are unable to resolve the processesinvolved in these relatively small scale or regionalphenomena and predictions on storminess inmiddle latitudes remain inconclusive

Similarly climate models are inconsistent intheir predictions of the frequency and intensityof tropical stormsmdashcyclones typhoons andhurricanesmdashfollowing global warming Thesestorms only develop over oceans where seasurface temperatures (SSTs) exceed 26ndash27degCWith global warming such temperatures wouldbe exceeded more frequently and over largerareas than at present leading to an increase inthe number of tropical storms In the southernhemisphere with an SST warming of only 2ndash4degCtropical cyclones would become 20 per cent moreintense and form perhaps 200ndash400 km furtherto the south than at present (Salinger and Pittock

1991) Such storms currently move out of thetropics to become intense extratropicaldepressions which bring heavy precipitation andstrong winds to mid-latitudes With warmingthese travelling storms would be expected tomove farther north and south sustained in partby energy transferred from the warmer oceansAreas such as Australia New Zealand and thecoasts of the north Pacific and north Atlanticwould suffer but some of the island nations ofthe south Pacific might not survive such a trainof events

Aware of the problems they might face in thefuture the nations most likely to be affected cametogether as the Alliance of Small Island States(AOSIS) and as such were successful in havingtheir situation addressed as part of the UNFramework Convention on Climate Change atRio in 1992 AOSIS is hoping for the creation ofa disaster insurance scheme funded by thedeveloped nationsmdashwhom they see as mainlyresponsible for the climate change that threatens

Figure 712 Environmental and economic changes expected in central and eastern Canada following a doublingof atmospheric CO

2 levels

themmdashwhich will help in the creation of disasterprevention programmes (Climate Institute1992b)

Even in those areas not completely inundatedthe relationship between sea level and regionalhydrology would allow the effects to be felt somedistance inland in the form of altered streamflowpatterns and groundwater levels In time theimpact of higher temperatures on the rate ofablation of ice caps and sea ice in polar regionsmight be sufficient to cause a rise in mean sealevel of between 3 and 4 m (Hoffman et al 1983)Should this ever come to pass most of the worldrsquosmajor ports would not survive without extensiveand costly protection

This disruption of commercial activities byrising sea-level in coastal regions is only one of aseries of economic impacts which wouldaccompany global warming Others range fromchanges in energy use particularly for spaceheating to the development of entirely newpatterns of recreational activity Because of itsnorthern location Canada is especiallysusceptible to such changes and as a resultEnvironment Canada has invested a great dealof time and effort in viewing the greenhouse effectfrom a Canadian perspective (see Figure 712)The results of the studies although specific toCanada may also give an indication of futuredevelopments in other northern regions such asScandinavia and Russia (Kemp 1991)

Alternative points of view and the problemsof GCMs

The global warming scenario in which adoubling of atmospheric CO2 would cause atemperature increase of 13ndash45degC is widelyaccepted This consensus has evolved from theresults of many experiments with theoreticalclimate models which indicate the considerablepotential for change in the earthatmospheresystem as concentrations of atmosphericgreenhouse gases increase Even the mostsophisticated General Circulation Models(GCMs) cannot represent the working of theatmosphere exactly however and the limitations

that this imposes on the results have long been asource of criticism One of the earliest and mostvociferous critics of the theoretical modellingapproach was Sherwood Idso who attacked theestablished view of future global warmingthrough numerous publications (eg Idso 19801981 1982 1987) He suggested that increasingCO2 levels would produce negligible warmingand might even cause global cooling Hisconclusions were based on so-called naturalexperiments in which he monitored temperaturechange and radiative heat flow during naturaleventsmdashsuch as dust storms for example Fromthese he estimated the temperature changeproduced by a given change in radiation Sincethe effects of increasing CO2 levels are feltthrough the disruption of the radiative heat flowin the atmosphere it was therefore possible toestimate the temperature change that would beproduced by a specific increase in CO2 Initially

Idso (1980) suggested that the effect of adoubling of atmospheric CO2 would be less thanhalf that estimated from the models Later heconcluded that increasing CO2 levels mightactually cause cooling (Idso 1983) Idso receivedsome support for his views (eg Gribbin 1982Wittwer 1984) but for the most part his ideaswere soundly criticized by the modellingcommunity sometimes in damning terms (NRC1982 Cess and Potter 1984) Although theyinitiated an intensemdashand sometimesacrimoniousmdashdebate on the methods ofestimating climate change in the long term Idsorsquosnatural experiments did little to slow the growingtrend towards the use of GCMs However evenwith the growing sophistication of the modelsthose who used them were often the first to notetheir limitations (eg Rowntree 1990) and manyof the criticisms of the estimated impact ofelevated CO2 levels have arisen out of theperceived inadequacies of the models used (Kerr1989)

Many of the problems associated with GCMsarise from their inability to deal adequately withelements that are integral to the functioning ofthe earthatmosphere system The roles of cloudsand oceans in global warming are poorly

understood for example The former are difficultto simulate in GCMs in part because theydevelop at the regional level whereas GCMs areglobal in scale Parameterization provides only apartial solution (see Chapter 2) and the IPCCsupplement has identified the problem of dealingwith clouds and other elements of theatmospheric water budget as one of the mainlimitations to a better understanding of climate(Houghton et al 1992)

Oceans create uncertainty in climate modelsmainly as a result of their thermal characteristicsThey have a greater heat capacity than theatmosphere and a built-in thermal inertia whichslows their rate of response to any change in thesystem Thus models which involve bothatmosphere and ocean have to include someconcessions to accommodate these differentresponse rates In representing the oceans it isnot enough to deal only with surface conditionsyet incorporating other elementsmdashsuch as thedeep ocean circulationmdashis both complex andcostly As a compromise many coupled oceanatmosphere climate models include only theupper well-mixed layer of the oceans These arethe so-called lsquoslabrsquo models in which the ocean isrepresented by a layer or slab of water about 70m deep While of limited utility in dealing withlong-term change this approach at least allowsthe seasonal variation in ocean surfacetemperatures to be represented (Gadd 1992) Amore realistic representation of the system wouldrequire coupled deep-oceanatmosphere modelsbut their development is constrained by thelimited observational data available from theworldrsquos oceans and the high demands that suchmodels place on computer capacity and costsExisting coupled models do provide resultsconsistent with current knowledge of thecirculation of the oceans but they are simplifiedrepresentations of reality lacking the detailrequired to provide simulations that can beaccepted with a high level of confidence(Bretherton et al 1990)

GCMs also have difficulty dealing withfeedback mechanisms which act to enhance ordiminish the thermal response to increasing

greenhouse gas levels (Rowntree 1990)Feedbacks are commonly classified as positiveor negative but in the earthatmosphere systemthey may be so intimately interwoven that theirultimate climatological impact might be difficultto assess For example the higher temperaturesassociated with an intensified greenhouse effectwould bring about more evaporation from theearthrsquos surface Since water vapour is a veryeffective greenhouse gas this would create apositive feedback to augment the initial rise intemperature With time however the rising watervapour would condense leading to increasedcloudiness The clouds in turn would reduce theamount of solar radiation reaching the surfaceand therefore cause a temperature reductionmdashanegative feedbackmdashwhich might moderate theinitial increase Such complexities are difficult tounravel in the real world Their incorporation inclimate models is therefore not easy but theimportance of atmospheric water vapourfeedback in climate change is well recognized byresearchers (Ramanathan et al 1983Ramanathan 1988) and at least some of themechanisms involved are represented in mostcurrent models

Feedbacks associated with global warming arepresent in all sectors of the earthatmospheresystem and some have the potential to causemajor change The colder northern waters of theworldrsquos oceans for example act as an importantsink for CO2 but their ability to absorb the gasdecreases as temperatures rise (Bolin 1986) Withglobal warming expected to be significant in highlatitudes there would be a reduction in the abilityof the oceans to act as a sink Instead of beingabsorbed by the oceans CO2 would remain inthe atmosphere thereby adding to the greenhouseeffect On land the feedbacks often work throughsoil and vegetation Increased organic decay insoils at higher temperatures would releaseadditional greenhouse gasesmdashsuch as CO2 andCH4mdashinto the atmosphere producing a positivefeedback This may be particularly effective inhigher latitudes where the tundra currently a sinkfor CO2 would begin to release the gas into theatmosphere in response to rising temperatures

(Webb and Overpeck 1993) The additional fluxof carbon from terrestrial storage might add asmuch as 200 billion tonnes of carbon to theatmosphere in the next century (Smith andShugart 1993) In contrast higher temperaturesand more efficient photosynthesis in low andmiddle latitudes would initiate a negativefeedback in which increased vegetation growthwould cause more CO2 to be recycled and stored(Webb and Overpeck 1993)

Feedback mechanisms are incorporated insome form in most GCMs However thenumber and complexity of the feedbacks

included varies from model to model andcurrent modelling techniques continue to havedifficulty dealing with them Such constraints inthe existing models must be recognized andappropriate allowances made when predictionsof global warming are used

A basic concern among some researchers isthe concentration on one variablemdashgreenhousegas levelsmdashwhich has allowed the role of otherelements in the system to be ignored It is well-known that the earthrsquos climate is not static buthas varied over the years (see eg Lamb 1977)Some of the variations have been major such as

Source AfterSchneider and Mass(1975)Note The lower linebetween 1900 and1990 indicates actualchange whereas theupper line indicatesthe estimatedtemperature if theenhancedgreenhouse effect isincluded Thetemperatureexpressed in K isequivalent totemperature indegrees Celsius plus27315degC It ispossible that thedifference has beencaused by thecooling effects ofincreasedatmosphericturbidity which haveprevented the fullimpact of theenhancedgreenhouse effectfrom being realized

Figure 713Changingglobal annualsurfacetemperature

the Ice Ages whereas others have only beendetectable through detailed instrumental analysisSome have lasted for centuries some only for afew years While it is relatively easy to establishthat climatic change has taken place it is quiteanother matter to identify the causes There area number of elements considered likely tocontribute to climatic change however

Since the earthatmosphere system receives thebulk of its energy from the sun any variation inthe output of solar radiation has the potential tocause the climatic change The links betweensunspot cycles and changes in weather andclimate have long been explored by climatologists(see eg Lamb 1977) and there are researcherswho claim that solar variability has a greaterimpact on global climate than the greenhouseeffect For example a report prepared for theMarshall Institutemdasha think-tank in WashingtonDCmdashsuggested that reduced solar output in thenear future might offset current global warmingsufficiently to initiate a new Ice Age The IPCCassessment considered this unlikely howeverpointing out that the estimated solar changes areso small that they would be overwhelmed by theenhanced greenhouse effect (Shine et al 1990)Even if the solar energy output remains the samechanges in earthsun relationships may alter theamount of radiation intercepted by the earthVariations in the shape of the earthrsquos orbit orthe tilt of its axis for example have beenimplicated in the development of the Quaternaryglaciations (Pisias and Imbrie 1986)

The present intensification of the greenhouseeffect is directly linked to the anthropogenicproduction of CO2 in the past however CO2

levels have increased with no human contributionwhatsoever During the Cretaceous periodmillions of years before the Industrial RevolutionCO2 concentrations were much higher than theyare today (Schneider 1987) Other changes in thecomposition and circulation of the atmospherehave to be considered also The impact ofincreased atmospheric turbidity is not clear Itmay add to the general warming of theatmosphere (Bach 1976) but it has also been usedto explain global cooling between 1940 and

1960 at a time when CO2 levels continued torise (see Figure 713) Although this cooling isusually acknowledged as a problem byresearchers it has not yet been adequatelyexplained (Wigley et al 1986) More recentlythe eruption of Mount Pinatubo in mid-1991caused cooling which appears to have beensufficient to reverse the global warming of the1980s and early 1990s There is also someevidence that the turbidity increase caused by theeruption may have contributed to regionalwarming (see Chapter 5)

Thus there are many elements in the earthatmosphere system capable of producingmeasurable climate change Given their pastcontribution to change it seems unlikely that theywill now remain quiescent while anthropogenicCO2 provides its input Despite this they havebeen ignored by most researchers or areconsidered of minor importance compared to thepotential impact of the greenhouse gases (Roberts1989)

Research into global warming is continuingat a high level and it is possible that a betterunderstanding of its interaction with otherelements in the earthatmosphere system willemerge to resolve some of the existinguncertainties If not society will have to deal withthe future environmental changes in much thesame way as it has done in the pastmdashby reactingto them after they have happened

SUMMARY

The earthrsquos greenhouse effect has beenintensifying since the latter part of the nineteenthcentury largely as a result of human activitiesThe increased use of fossil fuels has raised thelevel of CO2 in the atmosphere and thedestruction of natural vegetation has preventedthe environment from restoring the balanceLevels of other greenhouse gasesmdashincluding CH4N2O and the CFCsmdashhave also been rising andthe net result has been a gradual global warmingIf present trends continue a rise in mean globaltemperatures of 15degCndash45degC is projected by theearly part of the twenty-first century These values

are not accepted by all scientists studying thesituation but there is enough evidence to suggestthat a major global climate change is in progressThe ultimate magnitude of the change isuncertain but it has the potential to cause large-scale alterations to the natural environment andto global socio-economic and political systemsFor these reasons the search for a betterunderstanding of the situation is being givenpriority both nationally and internationallyThere has always been a high degree ofcooperation among the worldrsquos environmentalscientists but it seems likely that suchcooperation will have to extend to other physicalscientists social scientists and decision-makersif society is to be in the best possible position tocope with what could well be the greatest globaltemperature rise in history

Flavin C (1989) Slowing Global Warming AWorldwide Strategy (Worldwatch Paper 91)Washington DC Worldwatch Institute

Houghton JT Jenkins GJ and Ephraums JJ(1990) Climate Change The IPCC ScientificAssessment Cambridge CambridgeUniversity Press

Houghton JT Callender BA and Varney SK(1992) Climate Change 1992 TheSupplementary Report to the IPCC ScientificAssessment Cambridge CambridgeUniversity Press

Rosen L and Glasser R (1992) Climate Changeand Energy Policy New York AmericanInstitute of Physics

Public interest in the global environmentalissues described in the preceding chapters haswaxed and waned over the past decade (seeFigure 81) At present ozone depletion andglobal warming elicit a high level of concernwhereas drought and desertification acid rainand atmospheric turbidity have a much lowerprofile than they once had With the break-up ofthe Soviet Union the re-alignment of easternEurope and the end of the lsquoCold Warrsquo nuclearwinter is no longer considered a serious threatby most observers This situation reflectscurrent perceptions of the seriousness ofparticular problems Perceptions can changehowever Since few members of the generalpublic are in a position to read the originalscientific reports which address the issues theymust depend upon an intermediary to satisfytheir interest In modern society this interpretiverole has been filled by the media and publicperception of the issues is formed to a largeextent by their rendition of research resultsWithout them the general level ofunderstanding of the problems would be muchlower than it is but as a group the media havealso been accused of sensationalizing andmisinterpreting the facts supplied by thescientific community There can be no doubtthat some of the accusations are valid butscientists too may be partly to blame forallowing conclusions to be presented as firmbefore all of the facts are in Such was the casewith the initial investigation of nuclear winterand also with the discovery of the hole in theozone layer above the Antarctic in the mid-1980s Given the scale and complexity of

current environmental issues problems ofinterpretation and dissemination are inevitableThey must not be allowed to divert attentionfrom the main task however which is thesearch for solutions to the major issues

CURRENT STATE OF THE ISSUES

One of the first environmental issues to beconsidered in a global context was the risinglevel of atmospheric turbidity which was thecentre of concern in the mid-1970s It linked airpollution with the cooling of the earth Coolinghad been taking place since the 1940s and somewriters saw the world descending into a new IceAge It was clear a decade later that the coolinghad reversed and atmospheric turbidity beganto receive less attention Evidence also began toappear indicating that increased atmosphericturbidity might actually contribute toatmospheric warming Currently it raises littleconcern among the general public except underexceptional circumstances such as those createdby the Kuwaiti oil fires and the eruption ofMount Pinatubo Both of these events initiatedcooling and serve as a reminder that exceptionalconditions capable of augmenting turbiditycannot be ignored Although air pollution maybe serious in specific areas the humancontribution to atmospheric turbidity isgenerally smaller and the overall impact muchless than that from natural sourcesmdashthe coolingassociated with the oil fires in Kuwait forexample remained local whereas that causedby Mount Pinatubo was global Whether or not

Present problems future prospects

Figure 81 Changing levels of interest in environmental problems as reflected in coverage in The Times newspaperbetween 1967 and 1988

PRESENT PROBLEMS FUTURE PROSPECTS 175

that means that anthropogenic contributions toatmospheric turbidity are relativelyunimportant remains to be seen and scientistsinterested in the impact of human activities onthe atmosphere continue to search for ananswer

Nuclear winter

Interest in nuclear winter has also waned from apeak reached between 1983 and 1985 Intensiveinvestigation of the issue from 1983 on producedincreasing evidence that the climatic impact ofnuclear war had been overestimated in theoriginal study The downgrading of the estimatesin the scientific and academic community wasinevitably accompanied by a general decline inthe level of public interest in the topic and despitea new examination of the theory by the originalinvestigators (Turco et al 1990)mdashwhich tendsto reaffirm the basic findings of the originalworkmdashthere has been no revival of interest

As a result of recent political developmentsnuclear conflict does seem less likely and the issueof nuclear winter is regarded as irrelevant bysome Certainly large scale nuclear war betweenthe NATO and Warsaw Pact powers is no longera consideration The events which reduced thelikelihood of superpower nuclear conflict didnothing to reduce local and regional tensionshowever and in some cases may even haveexacerbated them With the dissolution of theUSSR into a number of separate states forexample central control over nuclear devices hasbeen lost and the possibility of these weaponsbeing used in local conflicts cannot be ruled outHowever in early 1994 Ukraine agreed to thecomplete destruction or removal of all the nuclearweapons it had inherited from the former USSRIn the Middle East Israel may already have anuclear capability while other nations in theregion may be working towards that end BothIndia and Pakistan also have nuclear ambitionsElsewhere in Asia North Korea has shown adistinct reluctance to abstain from furtherdevelopment of nuclear weapons The availabilityof nuclear weapon scientists thrown out of work

by the new political reality which has reducedthe military requirements of the superpowers isalso a concern Their ability to aid smallernationsmdashparticularly in politically unstable partsof the worldmdashto acquire or build nuclear devicescould lead to the proliferation of such weaponsAlthough conflicts in these regions would notmatch the size or severity of nuclear exchangeupon which the original nuclear winterhypothesis was based they would certainly havesignificant local and regional environmentalimpacts which in some cases could have globalconsequences

Drought famine and desertification are relatedproblems of long standing in many parts of theworld The disastrous droughts in the Sahel inthe 1960s and 1970s for example were only themost recent in a series which can be traced backseveral centuries The earlier droughts and theiraccompanying famines passed mostly unnoticedoutside the areas immediately affected Incontrast modern droughts have beencharacterized by a high level of concernparticularly in the developed nations of thenorthern hemisphere Concern is often media-driven rising rapidly but falling just as quicklywhen the drought breaks or the initial benefitsof food and medical aid become apparent Whenthe rains returned to the Sahel in the late 1970sinterest in the problems of the area declinedalthough the improvements were little more thanminimal Similarly the concern raised bytelevision reports of drought and famine inEthiopia in the mid-1980s peaked at a very highlevel in 1985 only to decline again within theyear Such fluctuations give a false impression ofthe problem Drought and famine are endemicin many parts of the world and do not go awaywhen the interest of the developed nationsdeclines In some areas the return of the rainsdoes bring periodic relief and the land producesa crop Where the relief is infrequent or short-lived the desert advances inexorably intopreviously habitable land This is desertification

in its most elemental form Accelerated by humaninterference it has become the most seriousenvironmental problem facing some of thecountries of the earthrsquos arid zones Climatologistsagronomists foresters and scientists from anumber of United Nations organizations havebeen wrestling with it for nearly forty years yeteven now it receives less public recognition thanthe drought and famine with which it isassociated Despite this lengthy investigationattempts at the prevention and reversal ofdesertification have met with only limited successIn part this appears to be the result ofmisinterpretation of the evidence and all aspectsof the problemmdashfrom identification throughcauses to responsemdashare currently undergoingrigorous reassessment Like most environmentalissues desertification is not just a physicalproblem It has socio-economic and politicalaspectsmdashranging from the depressed economiesof many Third World nations to the civil strife incountries like Ethiopia and Somaliamdashwhichcomplicate the search for appropriate solutions

Public interest in drought famine anddesertification will continue to fluctuateHeightened concern followed by increasedfinancial nutritional and food aid may help toalleviate some of the immediate effects of theproblems but it is the steady less volatile interestof the scientific community which has thepotential to bring about longer-term relief

Acid rain

The study of acid rain has declined remarkablysince the early 1980s when it was viewed bymany as the major environmental problem facingthe northern hemisphere This decline has led onewriter to wonder lsquoWhatever happened to acidrainrsquo (Pearce 1990) It certainly has notdisappeared but it is one environmental issue inwhich abatement programmes have met withsome success Sulphur dioxide levels continue todecline the rain in many areas is measurably lessacid than it was a decade ago and thetransboundary disputes which absorbed largeamounts of time energy and money in the 1980s

have been resolved There are indications thatlakes and forests are showing some signs ofrecovery in the most vulnerable areas of NorthAmerica and Europe although this is still amatter of some dispute

Developments such as these have created theperception that the acid rain problem is beingsolved Interest has declined and the researchfunds available have been diverted to causesmdashsuch as ozone depletion and global warmingmdashthat now appear more relevant Although acidrain is arguably a less serious problem than itonce was it has not been solved It has changedin nature and geographical extent however Inthe developed nations NOX is gradually makinga larger contribution to acidity as levels of SO2

decline In less developed areas from easternEurope to China and parts of the southernhemisphere the full impact of acid rain may stillbe in the future and the research activitiescurrently winding down may have to be revivedto deal with it

Ozone depletion and global warming

Ozone depletion and global warming currentlyenjoy a much higher profile than the otherenvironmental issues both in the media and inthe scientific community They are the focus ofmajor research efforts costing millions of dollarsaimed at deciphering the causes and effects ofthe problems so that solutions can be identifiedThe intensity of the research effort has ensuredsome success but in many cases the search forinformation has done little more than confirmthe complexity of the earthatmosphere systemInterest in the topics has been developed andmaintained through a continuing series of highlevel international conferences some of whichconcentrate on the technical aspects of theproblems while others involve policydevelopment and decision-making These arecomplemented by national and regionalconferences dealing with specific aspects of theproblems

Progress in the development of the topics hasnot been even Consideration of ozone depletion

has already reached the decision-making stagewhereas in the study of global warming thetechnical aspects of the issuemdashsuch as theestablishment of the nature extent and timingof the warmingmdashcontinue to receive much of theattention This may reflect the relative complexityof the two issues Controlling CFCs is relativelyeasy for example because their uses are limitedthe small number of producers can be easilyidentified and production can be monitoredSubstitutes for many CFCs have been developedquite easily In contrast greenhouse gasproduction is widespread with a mixture ofnatural and anthropogenic sources which aredifficult to monitor with any accuracy The sizeand complexity of the problem is such that therehas been little development of replacements forthe products and processes releasing greenhousegases The greater progress in dealing withthinning ozone is also an indication of thedifferent ways in which the problems areperceived Of the two ozone depletion is usuallyseen as having the most serious consequencesThe discovery of the Antarctic ozone hole wasfollowed closely by the initiation of internationalefforts to save the ozone layer which gainedmomentum with every new report of ozonedepletion

The Vienna Convention on the Protection ofthe Ozone Layer which emerged from a 1985conference was followed in 1987 by theMontreal Protocol Signatories to the Protocolagreed to reduce production of CFCs by 50 percent (of 1986 values) by 2000 The concludingstatement of the World Conference on theChanging Atmosphere held in Toronto Canadain mid-1988 also included reference to CFCs Itcalled for them to be phased out by the year 2000and later that year delegates at the WorldConference on Climate and Development inHamburg Germany recommended a global banon the production and use of CFCs by 1995 InMarch 1989 the members of the EuropeanCommunity agreed to eliminate production anduse of ozone-destroying chemicals by the end ofthe century The US government endorsed theeffort but stressed the importance of finding safe

substitutes for CFCs In Australia in 1989 thegovernment passed the Ozone Protection Actwhich with supplementary regulations wasaimed at eliminating CFC and halon use by 1994At about the same time major government-sponsored conferences in London and Parisprovided international support for a worldwideban on CFCs and other environmentally harmfulchemicals Subsequent meetings have confirmedthe willingness of the nations of the world to dealwith the issue and the second half of the 1990swill see the progressive elimination of CFCs andother ozone-destroying compounds There isalready evidence of a decrease in the growth rateof atmospheric halons (Butler et al 1992) andcertain CFCs (Elkins et al 1993) The growth ofCFC-11 and CFC-12 is slowing for example andif this continues as expected the volume of thesegases in the atmosphere will reach a peak by theend of the century and then begin to declineEven with such developments the ozone layerwill take time to recover and levels of UV-B raysreaching the earthrsquos surface will remain high InOctober 1993 for example the US NationalOceanic and Atmospheric Administrationannounced that the amount of ozone in theatmosphere above Antarctica had reached arecord low being virtually absent between 135km and 18 km above the surface Announcementssuch as this apparently confirming the progressivethinning of the ozone have promptedgovernment agencies from as far apart asAustralia and Canada to issue warnings aboutexposure to excess ultraviolet radiation and thesewarnings are currently among the most obviousmanifestations of the ozone problem

Like ozone depletion global warming has beenthe subject of a large number of conferences andworkshopsmdashfrom Villach in 1985 where theoriginal investigative framework was set up toRio in 1992 where it was considered as a majorelement in the broader study of global changeThe net effect has been the accumulation of aconsiderable body of knowledge on all aspectsof global warming and a recently publishedannotated bibliography lists several hundredpublications on the topic (Handel and Risbey

1992) Much of the material is complex writtenin the scientific jargon of research reports butperhaps more than any of the other issues thegreenhouse effect has been treated by the mediain such a way as to stimulate public interest inthe problem Government organizations havealso published the evidence from the researchreports in a simplified form for media use andpublic consumption The Climate Change Digestsof the Canadian Atmospheric EnvironmentService are a good example of this approach Inaddition private non-profit organizationsmdashsuchas the Climate Institute in the United Statesmdashhave been formed to advance publicunderstanding of the global warming producedby the enhanced greenhouse effect

The success of these endeavours is difficult tomeasure as yet but the public approach isbecoming more common It reflects the generalconsensus in the scientific community thatsolutions to current large-scale environmentalproblems can only be implemented successfullyif they have a high level of public support andthat such support is most likely to come from apublic kept well-informed about the nature andextent of the problems As the investigation ofglobal warming enters the decision-makingphase and solutions involving such elements astax increases and lifestyle changes are proposedmaintaining that public support will becomeincreasingly important

SOLUTIONS

Although there may be individuals and groupswho for various reasons are willing to continuewith the lsquodo nothingrsquo or lsquobusiness-as-usualrsquoapproaches to current global environmentalproblems they are a minority and the urgentneed to provide solutions is widely acceptedGiven the complexity of the problems beingaddressed it is not surprising that there is no oneapproach that satisfies all needs Most of theoptions currently being considered involve eitheradaptation or prevention and sometimes acombination of the two

Adaptation in its simplest form is already part

of the earthatmosphere system represented bythe adjustments required to maintain equilibriumamong the various elements in the environmentAs part of the environment human beings havealways had the ability to adapt to changingconditions and in many cases society has beenshaped by such adaptation Can society continueto adjust in the face of the major environmentalchanges currently taking place In the short-termthe answer is a qualified yes Adjustment isalready under way and takes many forms Theabandonment of drought-stricken land is oneform for example as is the addition of lime toacidified lakes Many individual lifestylechangesmdashsuch as wearing a hat spending lesstime in the sun or using sunscreen lotion to reducethe impact of higher UV-B levelsmdashare alsoadjustments to a changing environment This typeof approach may be necessary until appropriatepreventative measures are developed or the fulleffects of the solutions can work their waythrough the system

Adaptation often appears attractive becausein the short-term it is a relatively simple and low-cost approach With time and continuingenvironmental change however the cost ofadaptation may eventually exceed the costs ofproviding solutions In theory adaptation andthe development of preventative measures shouldtake place in phase so that solutions can be inplace before the cost-effectiveness of adaptationis lost In reality adaptation is a reactive approachinvolving little planning or consideration of suchelements as timing and cost-effectiveness Thusthe impact of adaptive policies is difficult topredict However considering the magnitude ofcurrent environmental problems it seems likelythat adaptation can only be a temporary measureUltimately the cumulative effects of the changeswill surpass the ability of society to adjust andsolutions will have to be found

As research into global change continues itbecomes increasingly clear that the only way toensure that environmental problems will notbecome progressively worse is to reduce andultimately halt the processes that cause themWhen considered qualitatively in the academic

isolation of the lecture hall or the comfort of anarmchair it appears that all of the current globalenvironmental problems can be solved in thatway They share the same overall causemdashhumaninterference in the environmentmdashand specificcauses are common to several of the issues Acidrain increased atmospheric turbidity theintensification of the greenhouse effect and ozonedepletion could all be reduced with the exerciseof greater control over anthropogenic emissionof dust smoke and gases Those problems whichcould not be solved completely might have theireffects mitigated It is unlikely for example thathumankind will ever be able to prevent or controldrought but through the management of humanactivities in areas prone to drought problems offamine and desertification might be alleviated

The technology exists to tackle most if not allof these problems but the gap between theoryand practice is immense perhapsinsurmountable It is maintained in large partby a combination of political intransigence andthe financial constraints under which governmentand non-government organizations are forced towork Much of the difficulty arises out of thesocio-economic and political consequences of therequired changes which are perceived by someto be even more detrimental to society than thecontinued existence of the problem In short thedisease is considered less damaging than the cureFor example a substantive diminution of acidrain could be accomplished by restricting the useof sulphur-rich bituminous coal but it wouldhave the effect of placing in jeopardy theeconomic viability of the areas producing thatcommodity The economic social and politicalimpact of the closure of dozens of minesaccompanied by thousands of redundancies maybe seen as much more serious than the death ofeven several hundred lakes Such concernscontinue to influence the methods adopted tocontrol acid rain Rather than replacing thesulphur-rich coal with a low-sulphur alternativethe problem has been dealt with at the post-combustion stage by the introduction ofscrubbers

Socio-economic and political considerations

also limit some of the solutions that might beapplied to drought and famine That problemcould be approached by letting nature take itscourse as was the norm in the not too distantpast It can be argued that the present system ofproviding food aid and drilling wells in areassuffering from famine and drought is the easiestway to ensure that the problems will continuewell into the future Cutting back on aid orremoving it completely would lead to largescalestarvation and death but it would also reducestress on the environment and allow therestoration of some form of equilibrium to thesystem Current moral and ethical attitudeswould presumably prevent the adoption of sucha policy and even the suggestion that it beconsidered would have far-reaching politicalimplications

Energy consumption is an element commonto many environmental problems and itstreatment illustrates quite well some of thedifficulties faced in the search for solutions Thereassessment of energy sources leadingeventually to a reduction in fossil fuel use is seenby many to provide the most likely solution tothe problems of atmospheric turbidity acid rainand global warming It is particularly attractivebecause it is a broad-spectrum solution whichdoes not require separate technology to bedeveloped for each issue Improved energyefficiency for example would have a wide-ranging impact It would reduce the output ofCO2 and CH4 the main greenhouse gases itwould bring about a decline in atmosphericacidity by reducing emissions of SO2 and NOx itwould create a cleaner and clearer atmosphereby reducing the release of particulate matter Allof these could be achieved without additionaltechnological developments With cooperationamong the developed and developing nations itis technically possible to lower atmospheric CO2

to 1976 levels through improved efficiency(Green 1992) In practical terms this is an unlikelyachievement however With all of theuncertainties inherent in the global warmingpredictions the developed nations might bereluctant to implement the necessary measures

because of the initial costs involved anddeveloping nationsmdashsuch as Chinamdashwhich seetheir future development tied to fossil fuels mightbe unwilling to make what they see as a majorsacrifice Attempts to improve energy efficiencywould be accompanied by widespread economicimpacts Although efficiency is usually associatedwith lower costs start-up costs would have tobe taken into account Supply and demandpatterns would change Greater efficiency wouldcause the demand for fuels to fall which in turnwould bring about a decline in prices Under thesecircumstances nations or groups of nations suchas OPEC economically dependent on fossil fuelproduction would be reluctant to participate insuch a scheme

Cooperation can be encouraged particularlyat the national level through fiscal measures suchas taxation A carbon tax paid on fuelconsumption has been suggested as a means ofreducing fossil fuel use for example Althoughmost often proposed as a scheme to slow globalwarming by slowing down CO2 emissions itwould have an impact on other issues also Itwould be relatively easy to set up and administerand the resulting higher fuel prices would intheory lead to improved energy efficiency Therevenue generated by the tax could be used tooffset existing environmental damage created byfossil fuel use or invested in research aimed atfinding solutions to environmental problems

The simplicity of the carbon tax conceptmakes it attractive but it is not without itsdrawbacks All tax increases have politicalconsequences and in a world dependent uponreadily available and relatively cheap energy theimposition of a carbon tax would have majorsocio-economic implications High cost energyproducers would suffer most Coal and oilproducers in North America and Europe wouldexperience a greater financial impact than theoil states of the Middle East for example If thetax was graduated according to the pollutingpotential of the fuel coalmdashwith its emissions ofCO2 SO2 and particulate mattermdashwould betaxed at a higher rate than oil or natural gasThe actual tax levy would vary according to CO2

emission targets and the timeframe proposedEstimates range from a tax of $20 per tonne ofcarbon to maintain CO2 emissions at 1990 levelsto $250ndash275 per tonne to reduce CO2 emissionsby 70 per cent from the current estimate for themid-twenty-first century (Green 1992) None ofthis would be achieved without internationalcooperation and ultimately it would lead tochanges in the composition and geographicaldistribution of the energy industry The overallhigher cost of energy might retard technologicaldevelopment particularly in the developingnations and it might be necessary to vary thetaxes not just by commodity but also by sourceso that the Third World would bear less of theburden Thus attractive as a carbon tax mightappear at first sight its implementation is notwithout some obvious and serious economic andpolitical constraints

Neither improved energy efficiency nor theimposition of an energy tax will halt the impactof fossil fuel use on the environment At best theycan reduce emissions of gases and particulatematter to manageable levels All of thesepollutants are eventually removed from thesystem by natural recycling processes butemission levels are now so high that the recyclingprocesses cannot keep up If they could berejuvenated perhaps they could contribute to thesolution or at least the amelioration of certainenvironmental problems Such an approach hasbeen proposed to counteract global warming byusing the natural ability of plants to remove CO2

from the atmosphere during photosynthesis Itwould involve the reforestation of large tracts ofland so that carbon could be removed from theatmosphere and sequestered or stored in thegrowing vegetation In Canadian studies it hasbeen estimated that after taking such factors asland availability soil conditions and climate intoconsideration the maximum possible increase incarbon sequestration would be only 9ndash10 percent at a cost of $6ndash23 per tonne of carbon stored(Van Kooten et al 1992) To absorb all of the 5billion tonnes of carbon emitted into theatmosphere annually would require the plantingof 13ndash17 billion acres of new forest every year

backed up by efficient harvesting and replantingschemes Clearly this is impossible and currentestimates based on moderate cost-effectivenesssuggest that no more than 4ndash5 per cent of totalcarbon emissions could be sequestered in this way(Green 1992)

Together these schemesmdashimproved energyefficiency carbon taxation and afforestationmdashwould indeed lessen the impact of fossil fuels onthe environment but no major improvement islikely until societyrsquos dependence on carbonbasedfuels is much reduced Alternative sources suchas the sun the wind the sea and the biospherecannot supply energy in the amounts and at therate demanded The other potential alternativemdashnuclear fissionmdashhas been so discredited by recentevents that it cannot be given seriousconsideration until problems of operationalsafety and radioactive waste disposal have beenresolved Thus although developments in theenergy sector have the potential to ameliorate anumber of existing global environmental issuesmajor improvements are unlikely under currenttechnological socio-economic and politicalconditions

It is increasingly apparent that solutions tothe global environmental problems presentlyfacing society must be economically socially andpolitically acceptable They must be moreCurrent global problems are multifacetedtherefore the solutions must be multifacetedThey must consider societal and environmentalconsequences equally rather than emphasizingthe former as is commonly done at present

The environment suffers because of the timescales followed by modern society Politiciansfor example tend to deal in short-term causeseffects and solutions living as they do fromelection to election Many environmentalproblems do not fit readily into such aframework Rapid sometimes catastrophicchange is an element in the environment butmore often than not change is accomplishedthrough the cumulative effects of relatively minorvariations over a long period of time As a resultpotentially important changes may not berecognized or if they are they are ignored

because of their seeming insignificance Evenwhen attempts are made to deal with suchchanges the results may only become apparentafter a considerable period of time In the case ofozone depletion for example it will take severaldecades following the complete abolition of CFCsbefore ozone levels return to their normal rangeIn politics where immediate and obvioussolutions tend to be the order of the day such atime lag is often considered politicallyunacceptable and no action is taken

Despite this concern for environmentalchange has been growing among politicians andgovernment bodies They have made fundsavailable for the investigation of globalproblems and encouraged the dissemination ofthe research results through conferences andpublications The next stage in the process mustbe the implementation of the recommendationscontained in the research reports That isproving to be difficult however since it requiresa long-range planning strategy and modernplanning policy is designed to provide solutionsto short-term problems Environmental impactprocedures for example seek to integrate theenvironmental and socio-economicconsiderations arising from the development ofa specific project and mitigate their effects atthe outset It is assumed that the decisions madeat that time will minimize environmentalimpact throughout the life of the project butthere is already evidence that environmentalchange is accelerating so rapidly that thisapproach is now inappropriate Currentenvironmental problems require long-rangeplanning extending several decades into thefuture and responsive to change if they are tobe solved Planners and policy-makers have notyet adjusted to that requirement For examplereforestation is going ahead on the assumptionthat current climatic conditions will prevailduring the life-span of the trees yet in the next50 years the intensification of the greenhouseeffect is likely to cause climate to change in theareas being planted Irrigation projects andhydro-electric schemes costing millions ofdollars are being developed with no thought to

the impact of current global problems onprecipitation patterns several decades fromnow Coastal and waterfront property is beingdeveloped as if the rise in sealevel projected toaccompany global warming is of noconsequence Few of the long-range plansnecessary to deal with the problems have beenput in place and there have been few importantgovernmental or industrial decisions whichhave paid more than lip-service to therecommendations of the research scientists

The implementation of measures to alleviatethe effects of global environmental disruption isfurther complicated by the scale of theproblems Most will require internationalcooperation if they are to be controlledsuccessfully The major conferences which haveaddressed such issues as acid rain ozonedepletion and the greenhouse effect have beeninternational in scope and have includedagreements in principal on the measuresrequired to reduce their impact Suchagreements are important but they provide noguarantee that the situation will improve Sincethey require ratification by individual nationsdelays in their implementation are commonOne year after the signing of the MontrealProtocol on the depletion of the ozone layeronly seven of the original thirty-sevensignatories had ratified the treaty Even whenthere is complete ratification the problem ofenforcement remains and the possibility alwaysexists that the worst offenders may refuse tobecome involved For example Britain and theUnited States declined membership in the lsquo30per cent clubrsquo when it was formed in 1979 tocombat rising levels of acid emissions Bothwere major contributors to acid rain and manyenvironmentalists felt that their lack ofcooperation would be disastrous It was only inthe mid-1980s that Britain began to acceptsome responsibility for downstream acid raindamage and began slowly to reduce acid gasemissions It took even longer in NorthAmericamdashmore than a decademdashbefore theUnited States instituted pollution abatementmeasures that would reduce the export of acid

rain north into Canada Continued high levelsof acid gas emissions during these lost yearssimply added to environmental deteriorationand retarded the necessary clean-up andrecovery

Similar problems arise with drought famineand desertification These were originally localissues in the Third World which became globalwhen the developed nations began to providerelief from drought and famine and sought tocombat desertification Some success has beenachieved against drought and famine usuallyby employing the developed worldrsquos advancedtechnology and long-established supply andtransport systems Desertification remainsrampant in many areas Steps must be taken toprevent further environmental damage and torehabilitate areas already damaged Since it isindependent of national boundaries howeverattempts to halt the spread of the desert in onearea may be negated if nothing is done in anadjacent area Success is only possible withinternational cooperation and economic orpolitical pressure may have to be applied toachieve that Even if cooperation is completehowever there is still no guarantee that theproblem will be solved Much will depend uponeconomic conditions in the developed nationsfor they will be required to provide much of thenecessary financial aid Any downturn in theworld economymdashsuch as the recession of theearly 1990smdashputs their contribution injeopardy and threatens the success of the fightagainst desertification

The great economic gap between rich andpoor nations adds to the difficulties of resolvingenvironmental problems at the internationallevel The developed nations are often accusedof asking the Third World to make sacrifices tosolve problems that they did little to create Intheory the benefits would be evenly spreadbecause of the integrated nature of the earthatmosphere system but the short-term impactoften appears detrimental rather thanbeneficial For example a reduction in theharvesting of tropical hardwoods from therainforest is the goal of a number of

environmental groups in North America andEurope It would reduce local environmentalproblems and contribute to the slowing downof global warming For many tropical nationshowever lumbering in the rainforest is animportant source of revenue They resentoutside interference and point out withjustification that their contribution to globalwarming is minimal compared to that of theindustrialized nations They question theemphasis on the rainforest when forestrypractices in mid to high latitudes also disruptnatural carbon recycling Similarly manynations in which petroleum production andexport is the main income earner resent theimposition of measuresmdashsuch as carbontaxesmdashdesigned to help the environment butalso likely to reduce their revenues and restrictfuture development

A major concern at the international level isthat measures aimed at preserving theenvironment will impose too high a cost onthose least responsible and least able to pay Thechallenge will be to define and present the issuesin such a way that the nations involved will seeit as in their own best interestsmdasheconomicsocial political and environmentalmdashtointroduce measures to prevent further damageto the environment and ameliorate existingproblems This will be no easy task andalthough the need for global cooperation tocombat global environmental problems iswidely recognized it seems likely that thevagaries of international politics and economicswill continue to frustrate attempts to implementsolutions

FURTHER STUDY NECESSARY OR NOT

Current global environmental problems areremarkably complex and despite anincreasingly intensive research effort they areeven now not completely understood Thatsituation must change if solutions are to befound Almost all individuals and organizationsstudying the problems have indicated thatfurther study is necessary In the past that was

often seen as a delaying tactic in that it wasoften easier to suggest further study than tomake a positive attack on a problem Attemptsto solve the acid rain problem in NorthAmerica were thwarted by these very tactics formany years

Such arguments are no longer possible nowthat sufficient data are available yet thereremains a very real need for further study ofmany aspects of the earthatmosphere systemThe roles of the various atmospheric processesrequire particular attention since they areintimately involved in all of the major problemscurrently confronting the environmentTraditionally the study of the atmosphere wasbased on the collection and analysis ofobservational data That approach is time-consuming and costly it is also of limitedoverall accuracy because of gaps in themeteorological network particularly in highlatitudes and over the oceans Modern attemptsat exploring the workings of the atmosphere arealmost exclusively dependent upon computermodels which range in complexity from simple1-D formats providing information on oneelement in the system to highly sophisticatedmodels employing as many as 100 variablesand including consideration of the oceans aswell as the atmosphere Studies of such topics asnuclear winter global warming and ozonedepletion have benefited greatly from the use ofcomputer modelling techniques The models arebecoming increasingly complex andcomprehensive but a high level ofsophistication is no guarantee of perfectionEven state-of-the-art 3-D general circulationmodels include some degree of simplificationand certain variablesmdashcloudiness forexamplemdashare very difficult to deal withwhatever the level of model employed In shortthere is as yet no model capable of simulatingexactly the conditions and processes in the realatmosphere As a result models using the samedata often generate different results (see Figure82) That should not be used as an excuse to donothing however It might be tempting to waitfor the perfect model but that may prove

Figure 82 GCMs and climate change Estimates of surface air temperature change over Australia and Indonesiaduring December January and February following a doubling of CO

Source From Henderson-Sellers (1991)

impossible to develop Existing models do haveflaws but they can be used provided theirlimitations are understood and despite theirinherent problems there is probably no betterway of studying environmental problemsinvolving the atmosphere

The major tasks facing scientists involved inthe study of environmental problems is thereduction of the uncertainty which still cloudssome of the issues despite the ever-increasingcomplexity and sophistication of current researchmethods Scientists can deal with some degree ofuncertainty because they have been trained toaccept different levels of confidence in theirresults In contrast planners politicians andpolicy-makers must have reliable estimates of theimpact of environmental change if they are tomake the decisions necessary to minimize itsnegative effects and maximize its positive effectsAs long as uncertainty exists it is all too easy todelay action

This has been a particular problem in thedevelopment of strategies for alleviating theeffects of global warming Decision-makers areunwilling to institute measures which will requiresocio-economic and political sacrifices until theyare sure that global warming is a realityResearchers unwilling to discard normalscientific caution cannot give that assurance Inan attempt to find out what would be requiredto resolve this dilemma Henderson-Sellers (1990)conducted a survey which included a questionabout the minimum level of certainty requiredbefore action should be initiated to reduce oradapt to global warming Respondents suggestedthat a 50 per cent confidence level would besufficient Although this is lower than mostresearchers would accept it remains difficult toattain It may take some time for the stand-offbetween decision-makers and scientists to beresolved and as an interim measure someresearchers have advocated the development ofapproaches which would alleviate the problemif global warming occurred as projected yet

would have no detrimental effects if it did not Awide-spectrum solution involving increasedenergy efficiency is one possibility for exampleCoupled with enhanced multidisciplinaryresearch which acknowledges the complexity ofthe problem and the wide range of expertiserequired to respond to it this approach shouldat least slow the change until the answer to theuncertainty question can be determined

Even as better models are developed andconfidence in their results improves there willbe surprises Current models tend to concentrateon the impact of human interference on the earthatmosphere system and generally ignore thepossibility that natural variations in the systemwill instigate change Most assume that thenatural elements in the system will remain benignEvidence from the earthrsquos past suggests that suchan assumption cannot be made Events such asthe eruption of Mount Pinatubo serve as timelyreminders that natural events can augment ordiminish environmental changes initiated byhuman activity It is important therefore thateven in an increasingly anthropocentric worldevery attempt be made to identify and understandnatural variations in the earthatmospheresystem both at present and in the past Only thenwill it be possible to place human interference inits broader environmental context and makerealistic projections of its future impact

Buchholz R Marcus AA and Post JE (1992)Managing Environmental Issues A CasebookEnglewood Cliffs Prentice-Hall

OECD (1991) Climate Change Evaluating theSocio-Economic Impacts Paris Organizationfor Economic Cooperation and Development

World Resources Institute (1992) WorldResources 1992ndash93mdashA Guide to the GlobalEnvironment New York Oxford UniversityPress

absorption coefficient A measure of the degree towhich a substance is capable of absorbing radiation

acid A compound containing hydrogen which onsolution in water produces an excess of hydrogenions An acid reacts with a base or alkali to form asalt and water

acid loading The addition of acids to waterbodies byway of deposition from the atmosphere

acid precipitation The wet or dry deposition of acidicsubstances of anthropogenic origin on the earthrsquossurface Commonly called acid rain but alsoincludes acid snow and acid fog

acid rain see acid precipitation and wet deposition

actual evapotranspiration see evapotranspiration

actuarial drought forecast An estimation of theprobability of drought based upon past occurrencesin the meteorological record

Advisory Group on Greenhouse Gases (AGGG) seeVillach Conference

aerosols Finely divided solid or liquid particlesdispersed in the atmosphere

Agenda 21 A blueprint for sustainable developmentinto the twenty-first century produced at the UnitedNations Conference on Environment andDevelopment (UNCED)

albedo A measure of the reflectivity of the earthrsquossurface Newly fallen snow reflects most of the solarradiation falling on it and has a high albedo a blackasphalt road surface which readily absorbsradiation has a low albedo

algae An order of aquatic plants occurring in bothfresh and salt water

alkali A compound which on solution with waterproduces an excess of hydroxyl ions (OH)

Alliance of Small Island States (AOSIS) A group ofstates mainly in the tropics who considerthemselves most likely to suffer the consequencesof rising sea level and increased storminess expectedto accompany global warming

anaerobic decay The breakdown of organic materialin the absence of oxygen Brought about byanaerobic bacteria the process commonly causesthe production of methane a greenhouse gas

Antarctic ozone hole Intense thinning of thestratospheric ozone layer above Antarctica firstreported in the early 1980s by scientists of theBritish Antarctic Survey at Halley Bay

anticyclone A zone of high atmospheric pressurecreated by the cooling of air close to the earthrsquossurface (cold anticyclone) or the sinking of air fromhigher levels in the atmosphere (warm anticyclone)The circulation of air in an anticyclone is clockwisein the northern hemisphere and counter clockwisein the southern hemisphere

aquifer A layer of rock beneath the earthrsquos surfacesufficiently porous and permeable to storesignificant quantities of water

Arctic haze The pollution of the Arctic atmospheremainly in winter by aerosols such as dust sootand sulphate particles originating in Eurasia

aridity Permanent dryness caused by low averagerainfall often in combination with hightemperatures

atmosphere The blanket of air which envelops the solidearth It consists of a mixture of gases and aerosols

atmospheric circulation The large scale movement ofair around and above the earth associated withcomplex but distinct patterns of pressure systemsand wind belts

atmospheric models Physical or mathematicalrepresentations of the workings of theatmosphere ranging from regional models such

Glossary

GLOSSARY 187

as the midlatitude cyclonic models used inweather forecasting to general circulation modelswhich attempt to represent global circulationpatterns

atmospheric turbidity A measure of the dustiness ordirtiness of the atmosphere as indicated by thereduction in solar radiation passing through it

atom The smallest unit of an element that retains thecharacteristics of that element

autovariation Environmental change produced whenone component of the environment respondsautomatically to change in another

Biodiversity Convention A document outlining policiesaimed at combining the preservation of naturalbiological diversity with sustainable developmentof biological resources A product of UNCED

biogeophysical feedback mechanism The hypothesisdeveloped to explain degradation induced droughtin the Sahel Overgrazing and woodcuttingincreased the surface albedo which in turndisrupted the regional radiation balance Reducedsurface heating retarded convective activity andlimited precipitation With less precipitationvegetation cover decreased and the albedo of thesurface was further enhanced This is an exampleof positive feedback

biomass The total living organic matter in a given area

biosphere The zone of terrestrial life including theearthrsquos surface plus the lowest part of theatmosphere and the upper part of the soil layer

bromofluorocarbon A group of chemicals containingbromine fluorine and carbon Similar in propertiesand use to chlorofluorocarbons They decomposeto release bromine which contributes to thedestruction of the ozone layer

Brundtland Commission see World Commission onEnvironment and Development

buffering agents Materials which reduce changes inpH when an acid or alkali is added to a solutionor mixture Alkaline or basic materials arecapable of reducing or neutralizing acidity forexample and natural buffering agents such aslimestone help to reduce the environmentalimpact of acid rain

lsquobusiness-as-usuarsquo scenario A scenario based on themaintenance of the status quo used to predict thefuture status of environmental issues

Carbon A non-metallic element which exists in avariety of forms in the environment It is chemicallymobile readily combines with other elements andis present in all organic substances

carbon cycle A natural bio-geochemical cycle whichworks to maintain a balance between the releaseof carbon compounds from their sources and theirabsorption in sinks

carbon cycle models Computer based models whichsimulate the workings of the carbon cycle

carbon dioxide One of the variable gases currentlymaking up 00353 per cent of the atmosphere byvolume but growing Despite its low volume it isimportant to life on earth because of itsparticipation in photosynthesis and its contributionto the greenhouse effect

carbon tax A policy which would tax fossil fuelsaccording to the amount of carbon they containedthe ultimate aim being to reduce carbon dioxideemissions and slow the enhancement of thegreenhouse effect

carbon tetrachloride A solvent and cleaning agentidentified as contributing to the depletion of theozone layer

carrying capacity The maximum number of organismsthat can be supported by a particular environment

cash cropping Growing crops for monetary returnrather than direct food supply (See also subsistencefarming)

catalyst A substance which facilitates a chemicalreaction yet remains unchanged when the reactionis over Being unchanged it can continue topromote the same reaction again and again as longas the reagents are available or until the catalystitself is removed This is a catalytic chain reaction

Central Electricity Generating Board (CEGB) TheEnglish public utility identified in the 1970s and1980s as the main source of acid rain falling inScandinavia

chemical models Models which simulate chemicalprocesses In studies of the atmosphericenvironment they are being developed to investigatethe role of trace gases in the circulation of theatmosphere

chlorine monoxide A compound containing chlorineand oxygen which has been implicated in thedestruction of stratospheric ozone

GLOSSARY188

chlorofluorocarbons (CFCs) A group of chemicalscontaining chlorine fluorine and carbon usedin refrigeration and air conditioning systems andin the production of polymer foams Inert atsurface temperature and pressure they becomeunstable in the stratosphere breaking down torelease chlorine which initiates a catalyticchemical reaction leading to the destruction ofozone

chloroform A volatile liquid solvent which contributesto ozone depletion through the release of chlorine

chlorophyll A green pigment in plants which makesphotosynthesis possible through its ability toabsorb solar energy

circumpolar vortex A band of strong winds circlingthe poles in the upper atmosphere The vortex ismainly a winter phenomenon and best developedaround the South Pole

Clear Air Legislation Various laws acts and ordinancesdesigned to bring about the reduction ofatmospheric pollution

climate The combination or aggregate of weatherconditions experienced in a particular area Itincludes both averages and extremes as measuredover an extended period of time

Climate Impact Assessment Program (CIAP) Aprogramme commissioned by the US Departmentof Transportation in the mid-1970s to study theeffects of supersonic transports (SSTs) on the ozonelayer

Climatic Optimum Period of major warming duringthe immediate post-glacial period between 5000and 7000 years ago

coal gasification The heating or cooking of coalmdashsometimes in placemdashto release volatile gases suchas methane a cleaner more efficient fuel than thecoal itself

coal liquefaction The conversion of coal into a liquidpetroleum-type fuel by a combination of heatingand the use of solvents and catalysts

Concorde The most successful of the two types ofsupersonic transport built in the 1970s

condensation nuclei Small particles in theatmosphere of natural or anthropogenic originaround which water vapour condenses to formliquid droplets

continental tropical air mass (cT) An air massoriginating over tropical to sub-tropical continentalareas and therefore hot and dry

contingent drought A type of drought characterizedby irregular and variable precipitation in areaswhich normally have an adequate supply ofmoisture to meet crop needs

convective circulation Circulation initiated by surfaceheating which causes the vertical movement ofheated air After cooling the air ultimately returnsto the surface to complete the circulation

cosmic rays High-energy radiation reaching the earthfrom outer space

coupled models Models which combine two or moresimulations of elements in the earthatmospheresystem Carbon cycle models for example havebeen combined with oceanic and atmosphericcirculation models in the search for a betterunderstanding of global warming

coupled oceanatmosphere climate models The mostcommon combination in coupled models The mainproblem with these models is the difference in timescales over which atmospheric and oceanicphenomena develop and respond to changeBecause of the relatively slow response time of theoceanic circulation the oceanic element in mostcoupled models is much less comprehensive thanthe atmospheric element

crumb structure The combination of individual soilparticles into loose aggregates or crumbs

cyclone An atmospheric low pressure systemgenerally circular in shape with the air-flowcounter-clockwise in the Northern Hemisphereand clockwise in the Southern Hemisphere andconverging towards the centre of the systemCommonly used to refer to intense tropicalstorms in the Indian Ocean which are theequivalent of Atlantic hurricanes or Pacifictyphoons

deforestation The clearing of forested areas as partof a commercial forestry enterprise or for someother economic purpose such as the expansionof settlement or the development of agriculture

degradation induced drought Drought promoted byenvironmental degradation usually initiated byhuman activity which disrupts the regional energyand water balances

demography (The study of) statistics on humanpopulations including such elements as growthrate age and sex and their effects on socio-economic and environmental conditions

GLOSSARY 189

Depression (The) A period of major economic declinein the 1930s associated with reduced industrialactivity high unemployment and the collapse ofinternational trade

desert An area of permanent aridity characterized bysparse xerophytic vegetation or in some areasby the complete absence of plant life Almost 30per cent of the earthrsquos surface exhibits desertcharacteristics of some form The worldrsquos largestdesert is the Sahara occupying some 85 millionsq km in North Africa

desertification The expansion of desert or desert-likeconditions into adjacent areas May be initiatedby natural environmental change by humandegradation of marginal environments or acombination of both

Desertification Convention A proposal presented atUNCED aimed at addressing the problems of thoseareas suffering from desertification

diatoms Microscopic unicellular aquatic organisms

dimethyl sulphide (DMS) A sulphur compoundemitted by phytoplankton during their seasonalbloom It is oxidized into sulphur dioxide andmethane sulphonic acid (MSA)

drought (A period of) reduced water availabilityoccurring when precipitation falls below normalor when near normal rainfall is made less effectiveby other weather conditions such as hightemperature low humidity and strong winds

drought prediction The attempt to forecast theoccurrence of drought so that responses can beplanned and consequences much reduced

dry deposition A form of acid precipitation consistingof dry acidic particles The particles are convertedinto acids when dissolved in surface water at whichtime their environmental impact is similar to thatof wet deposition

dry farming A technique which involves thepreservation of several years of precipitation to beused for the production of one crop It includesthe use of deep ploughing to provide a reservoirfor the rain that falls plus a combination oftechniques to reduce losses by evapotranspiration

dry sedimentation The fallout of dry particulate matterfrom the atmosphere under the effects of gravity

Dustbowl (The) An area of the Great Plains stretchingfrom Texas in the south to the Canadian Prairiesin the north which suffered the effects ofdesertification in the 1930s A combination ofdrought and inappropriate farming practices

caused the destruction of the topsoil and allowedit to be carried away by the wind

Dust Veil Index (DVI) A rating system developed byclimatologist HHLamb to provide an assessmentof the impact of volcanic eruptions on atmosphericturbidity and hence on global weather and climate

dynamic equilibrium see environmental equilibrium

earthatmosphere system see system

Earth Summit Popular name for the United NationsConference on Environment and Development(UNCED)

easterly tropical jet A rapidly moving easterly airstreamencountered in the upper atmosphere in

equatorial latitudes It is less persistent than jet streamsin higher latitudes

ecosystem A community of plants and animalsinteracting with each other and their environment

El Nintildeo A flow of abnormally warm water across theeastern Pacific Ocean towards the coast of Peru Itis associated with changing pressure patterns anda reversal of airflow in the equatorial Pacific aphenomenon referred to as the SouthernOscillation

electromagnetic spectrum The arrangement ofelectromagnetic radiation as a continuum accordingto wavelength It extends from high-energyshortwave radiation such as cosmic rays to themuch longer low-energy radio and electric powerwaves

energy budget The relationship between the amountof solar energy entering the earthatmospheresystem and the amount of terrestrial energy leavingIn theory these energy fluxes should balance inpractice it applies only in general terms to the earthas a whole over an extended time period It is notapplicable to any specific area over a short periodof time

ENSO An acronym for El NintildeomdashSouthernOscillation

environment A combination of the various physicaland biological elements that affect the life of anorganism Environments vary in scale frommicroscopic to global and may be subdividedaccording to their attributes The aquaticenvironment for example is that of rivers lakesand oceans the terrestrial environment that of the

GLOSSARY190

land surface The term lsquobuiltrsquo environment has beenapplied to areas such as cities created by humanactivity

environmental equilibrium The tendency of thecomponents of the environment to achieve somedegree of balance in their relationships with eachother The balance is never complete but takes theform of a dynamic equilibrium which involves acontinuing series of mutual adjustments to therelationships

environmental impact studies Analyses of the potentialimpact of various forms of human activities on theenvironment

environmental lapse rate The rate at whichtemperature falls with increasing altitude in thetroposphere Although the lapse rate varies withtime and place it is normally considered toaveragemdash65degC per 1000 m

Environmental Protection Agency (EPA) US agencyestablished in 1970 to coordinate governmentaction on environmental issues

equilibrium models A form of general circulationmodel (GCM) Change is introduced into such amodel representing existing climate conditions andthe model is allowed to run until a new equilibriumis established The new model climate can then becompared with the original to establish the overallimpact of the change

European Arctic Stratospheric Ozone Experiment(EASOE) An experiment undertaken during thenorthern winter of 1991ndash92 using groundmeasurements balloons aircraft and a variety ofmodelling techniques to establish the nature andextent of ozone depletion over the Arctic

evaporation The process of vaporization by whichwater changes from a liquid to a gaseous state

evapotranspiration The transfer of water from theterrestrial environment into the atmosphere Itcombines evaporation from the land surface withtranspiration from plants A distinction may bemade between actual evatranspirationmdasha specificmeasurementmdashand potential evapotranspirationmdashan estimate of the environmentrsquos capacity forevapotranspiration

fallout The deposition of particulate matter from theatmosphere on to the earthrsquos surface

fallow Arable land left unfilled or tilled but unsownfor a season Used in dry farming to provide a soil

reservoir for precipitation or in normal arableagriculture to allow the land to recover from theeffects of cropping

famine Acute food shortage leading to wide-spreadstarvation Usually associated with large scalenatural disasters such as drought flooding or plantdisease producing crop failure and disruption offood supply

feedback Occurs in integrated systems where changein one part of a system will initiate change elsewherein the system This involves the process ofautovariation The change may be fed back intothe system in such a way as to augment (positivefeedback) or diminish (negative feedback) theeffects of the original change

field capacity see soil moisture storage

flue gas desulphurization (FGD) The process by whichsulphur dioxide is removed from exhaust gasesproduced by the burning of coal (See alsoscrubbers)

fluidized bed combustion (FBC) The burning of amixture of crushed coal limestone and sand in thepresence of high pressure air which causes themixture to behave like a boiling liquid Continualmixing ensures that combustion is very efficientwith up to 90 per cent of the sulphur in the fuelbeing absorbed by the limestone

fossil fuels Fuels formed from organic material whichwas buried by sediments and retained its storedenergy It is that energy which is released when fossilfuels are burned Fossil fuels include coal oil andgas the main energy sources in advanced industrialsocieties Fossil fuel consumption is a majorcontributor to a number of current environmentalissues including acid rain atmospheric turbidityand global warming

Framework Convention on Climate Change One ofthe conventions signed at the Earth Summit It grewout of concern for global warming but was signedonly after much controversy and ended up as arelatively weak document lacking even specificemission reduction targets and deadlines

Freon The commercial name for chlorofluorocarbons(CFCs)

fuel desulphurization The reduction in the sulphurcontent of fuels such as coal and oil prior tocombustion (see also coal gasification and coalliquefaction)

fuel switching One of the simplest approaches to thecontrol of acid gas emissions It involves the

GLOSSARY 191

replacement of high sulphur fuels with low sulphuralternatives

fuelwood Wood products harvested for use as fuelThe removal of scrub woodland for fuelwood is amajor cause of desertification in the arid or semiaridparts of Africa and Asia

Gaia hypothesis Developed by James Lovelock thehypothesis views the earth as a superorganism inwhich the living matter is capable of manipulatingthe earthrsquos environment to meet its own needs

gas phase reaction The conversion of sulphur dioxideand oxides of nitrogen into sulphuric and nitricacid all of the reactions taking place with thevarious compounds remaining in a gaseous state

general circulation models (GCMs) Three-dimensionalmodels which incorporate major atmosphericprocesses plus local climate features predictedthrough the process of parameterization Theyinclude some representation of feedbackmechanisms and are able to cope with the evolvingdynamics of the atmosphere as change takes placeIn an attempt to emulate the integrated nature ofthe earthatmosphere system atmospheric GCMsare often coupled with other environmental modelsparticularly those representing the oceans

Glaciological Volcanic Index (GVI) An index basedon acidity levels in glacial ice as revealed by icecores This gives an indication of sulphur dioxidelevels associated with past volcanic eruptions anelement absent from both the Dust Veil Index andthe Volcanic Explosivity Index

Global Forum A conference of non-governmentorganizations (NGOs) held at the same time as theUnited Nations Conference on Environment andDevelopment (UNCED) It included a wide rangeof topicsmdashfrom the presentation of biologicaldiversity to sustainable developmentmdashwhichgenerally paralleled those included in the mainUNCED event

global warming The rise in global mean temperaturesof about 05degC since the beginning of the centuryThe basic cause of the warming is seen by manyresearchers as the enhancement of the greenhouseeffect over the same period brought on by risinglevels of anthropogenically-produced greenhousegases

Great American Desert Common perception of thewestern interior plains of North America in thenineteenth century The image grew out of the

reports of exploratory expeditions which visitedthe plains during one of the periods of droughtcommon to the area and observed the results ofnatural desertification

Great Plains An area of temperate grassland with asemi-arid climate in the interior of North Americastretching for some 2500 km from western Texasin the south along the flanks of the RockyMountains to the Canadian prairie provinces inthe north (See also Great American DesertDustbowl and Pueblo drought)

Green Parties Political organizations which aim toprotect the environment through the use ofestablished parliamentary procedures

greenhouse effect The name given to the ability of theatmosphere to be selective in its response todifferent types of radiation Incoming short-wavesolar radiation is transmitted unaltered to heat theearthrsquos surface The returning long-wave terrestrialradiation is unable to penetrate the atmospherehowever It is absorbed by the so-called greenhousegases causing the temperature of the atmosphereto rise Some of the energy absorbed is returned tothe earthrsquos surface and the net effect is to maintainthe average temperature of the earth atmospheresystem some 30degC higher than it would be withoutthe greenhouse effect The process has been likenedto the way in which a greenhouse worksmdashallowingsunlight in but trapping the resulting heat insidemdashhence the name

greenhouse gases The group of about twenty gasesresponsible for the greenhouse effect through theirability to absorb long-wave terrestrial radiationThey are all minor gases and together make up lessthan 1 per cent of the total volume of theatmosphere Carbon dioxide is the most abundantbut methane nitrous oxide thechlorofluorocarbons and tropospheric ozone alsomake significant contributions to the greenhouseeffect Water vapour also exhibits greenhouseproperties but has received less attention than theothers Since the beginning of the twentieth centuryrising levels of these gases in the atmosphereassociated with increasing fossil fuel use industrialdevelopment deforestation and agriculturalactivity have brought about an enhancement ofthe greenhouse effect and have contributed togradual global warming

grid-point models Climate models which provide fullspatial analysis of the atmosphere by means of athree-dimensional grid covering the earthrsquos surfaceand reaching as high as 30 km into the atmosphereThe progressive solution of thousands of equations

GLOSSARY192

at each of these points allows powerful computersto provide simulations of current and futureclimates

ground control The use of observation andmeasurement at the earthrsquos surface to verifyinformation provided by remote sensing fromsatellites or aircraft

groundwater The water which accumulates in the porespaces and cracks in rocks beneath the earthrsquos surfaceIt originates as precipitation which percolates downinto sub-surface aquifers The upper limit ofgroundwater saturation is the water-table

growing season The period of the year when mean dailytemperatures exceed the temperature at which plantgrowth takes place Since different plants mature atdifferent rates the length of the growing season willdetermine the mix of natural vegetation and the typesof crop that will grow in a particular area

Gulf Stream A warm ocean current originating in theeastern Caribbean which flows north along theeastern seaboard of the United States beforeswinging north-eastwards into the Atlantic Oceanwhere it becomes the weaker and cooler NorthAtlantic Drift

Hadley cells Convection cells which form in thetropical atmosphere north and south of the equatorNamed after George Hadley who in the eighteenthcentury developed the classic model of the generalcirculation of the atmosphere based on a simpleconvective circulation

halons Synthetic organic compounds containingbromine Commonly used in fire extinguishers theyare more effective in destroying the ozone layer thanchlorofluorocarbons (See alsobromofluorocarbons)

Harmattan A hot dry dusty wind which blows outof the Sahara Desert over the Sahel and much ofWest Africa during the northern hemisphere winterIt brings continental tropical air southwards whichcontributes to the seasonal drought characteristicof the area

heavy metals Metals such as mercury lead tin andcadmium which may be converted into a solubleorganic form or concentrated by hydrological orbiological processes so that they become hazardousto natural ecosystems and human health

heterogeneous chemical reactions Chemical reactionswhich take place on the surface of the ice particles

that make up polar stratospheric clouds leadingto the release of chlorine which attacks the ozonelayer Similar reactions have been identified on thesurface of stratosphere sulphate particles such asthose released during the eruption of MountPinatubo in 1991

humidity A measure of the amount of water vapourin the atmosphere It may be expressed as specifichumiditymdashthe ratio of the weight of water vapourin the air to the combined weight of the watervapour and the airmdashor as relative humiditymdashtheamount of water vapour in the air compared tothe amount of water vapour the air can hold atthat temperature

humus Partially decomposed organic matter which isan essential component of fertile soil

hydrocarbons Organic compounds composed ofhydrogen and carbon bound together in chains orrings The largest sources of hydrocarbons are fossilfuels such as petroleum and natural gas

hydrochlorofluorocarbons (HCFCs) A widely-usedsubstitute for chlorofluorocarbons (CFCs) Beingless stable than CFCs hydrochlorofluorocarbonsbegin to break down in the troposphere before theycan diffuse into the stratosphere and destroy theozone layer They are about 95 per cent lessdestructive than CFCs

hydrogen oxides A group of naturally occurringcompounds derived from water vapour methaneand molecular hydrogen which can destroy ozonethrough catalytic chain reactions (see catalyst)They include atomic hydrogen the hydroxyl radicaland the perhydroxyl radical referred to collectivelyas odd hydrogens

hydrologic cycle A complex group of processes bywhich water in its various forms is circulatedthrough the earthatmosphere system It is poweredby solar radiation which provides the energy tomaintain the flow by way of such processes asevaporation transpiration precipitation andrunoff Short- and long-term storage of water inlakes oceans ice sheets and the groundwaterreservoir is also part of the cycle

hydroxyl radical (See hydrogen oxides)

Ice Ages Periods in the geological history of the earthwhen glaciers and ice sheets covered large areas ofthe earthrsquos surface The Ice Ages occurred in seriesseparated by periods of temperate conditions calledinterglacials The most recent series began some

GLOSSARY 193

35 million years ago and persisted until 10000years ago causing major disruption of land-formsdrainage animal communities and vegetation

index cycle See zonal index

Industrial Revolution A period of rapid transition froman agricultural to an industrial society beginningin Britain in the mid-eighteenth century andspreading to other parts of the world in the nexttwo hundred years It was characterized by a majorexpansion in the use of coal as a fuel in the steamengine and in the iron industry Although theIndustrial Revolution was a period of greattechnical achievement and economic developmentit also marked the beginning of increasingly seriousenvironmental deterioration Currentenvironmental issues such as acid rain globalwarming and atmospheric turbidity have their rootsin activities initiated or expanded during theIndustrial Revolution

infrared radiation Low-energy long-wave radiationwith wavelengths between 07 and 1000 micromTerrestrial radiation is infrared It is captured bythe atmospheric greenhouse gases and as a resultis responsible for the heating of the earthatmosphere system

insolation Solar radiation entering the atmosphere

interactive models General circulation models whichare programmed to deal with the progressivechange set in motion when one or more of thecomponents of the atmosphere is altered

Intergovernmental Panel on Climate Change(IPCC) A group of eminent scientists broughttogether in 1988 by the World MeteorologicalOrganization (WMO) and the United NationsEnvironment Program (UNEP) It was chargedwith assessing the overall state of research onclimate change so that potential environmentaland socio-economic impacts might beevaluated and appropriate response strategiesdeveloped Three reports were produced in1990 and 1991

Intertropical Convergence Zone (ITCZ) A thermallow-pressure belt which circles the earth inequatorial latitudes It owes its origin to convectiveuplift caused by strong surface heating augmentedby converging air-flows from the northeast andsoutheast Trade Winds It lies between the tropicalHadley Cells positioned north and south of theequator The ITCZ moves north and south withthe seasons bringing the rains to areas of seasonaldrought to areas such as the Sahel India andnorthern Australia

invisible drought A form of drought that can beidentified only by sophisicated instrumentation andstatistical techniques There may be no obvious lackof precipitation but moisture requirements are notbeing met the crops are not growing at theiroptimum rate and the potential yield is thereforereduced

ion An atom or group of atoms that has becomeelectrically charged by picking up or losingelectrons Ions formed from metals are generallypositively charged (cations) those from non-metalsare negatively charged (anions)

IPCC Supplementary Report A report issued in 1992by the Intergovernmental Panel on Climate Changewhich generally confirmed the results of its earlierassessments

irrigation The provision of water for crops in areaswhere the natural precipitation is inadequate forcrop growth

isothermal layer An atmospheric layer in which thelapse rate is neutral that is the temperature remainsconstant with increasing altitude (See alsoenvironmental lapse rate)

jet stream A fast flowing stream of air in the upperatmosphere at about the level of the tropopauseTwo well-defined jet streams flowing from westto east in a sinuous path around the earth arelocated in sub-tropical latitudes (the sub-tropicaljet) and in mid to high latitudes (the polar frontjet) An easterly tropical jet has also beenidentified

Krakatoa A volcanic island in the Sunda StraitIndonesia which erupted explosively in 1883sending several tonnes of debris high into theatmosphere The volcanic dust circled the earthremaining in the upper atmosphere for several yearswhere it increased atmospheric turbidity

La Nintildea An intermittent cold current flowing fromeast to west across the equatorial Pacific Ocean inthose years when El Nintildeo is absent It is caused bystrong equatorial easterly winds pushing cold waterupwelling off the South American coast far outinto the ocean Its influence appears to extendbeyond the Pacific being associated with increasedprecipitation in the Sahel and India

GLOSSARY194

latent heat The quantity of heat absorbed or releasedby a substance during a change of state Latentheat of fusion is involved in the transformation ofa solid into a liquid (or liquid into a solid) as inthe ice-water-ice transformation Latent heat ofvaporization is the equivalent in changesinvolving liquids and gases such as water andwater vapour

leaching The removal of soluble minerals from soil bypercolating water Leaching is commonlyaccelerated in soils subject to acid precipitation

leguminous plants A group of pod (or legume) bearingplants of the pea family Nodules on the roots ofleguminous plants contain nitrogen-fixing bacteriawhich have an important role in the earthatmosphere nitrogen cycle

lime injection multi-stage burning (LIMB) A techniquedeveloped to reduce acid gas emissions from coalburning furnaces Fine lime is injected into thecombustion chamber where it fixes the sulphurreleased from the burning coal thus reducingsulphur dioxide emissions by 35ndash50 per cent

limestone A sedimentary rock consisting mainly ofcalcium carbonate When limestone is heatedcarbon dioxide is driven off and calcium oxide orlime is left Lime has been used for centuries tosweeten acid soils and as an alkali it is widely usedto neutralize the acid gas emissions responsible foracid rain

liquid phase reaction The conversion of acid gases intoliquid acids the reactions taking place in solution(See also gas phase reaction)

Little Ice Age A period of global cooling lasting forabout 400 years from the mid-fourteenth to themid-eighteenth century

Live Aid An organization set up in 1985 to help thevictims of drought and famine in Ethiopia It raisedmoney through two major concerts in England andthe United States at which the leading popularentertainers of the day performed

London Smog (1952) A major pollution event inLondon England caused by a combination ofmeteorological conditions (low temperatures highpressure poor ventilation) and energy use (theburning of high-sulphur coal) An estimated 4000deaths were attributed to the smog

Long Range Transportation of Air Pollution (LRTAP)The transportation of air pollution over greatdistancesmdashusually in excess of 500 kmmdashby theprevailing winds in the atmosphere Aggravated bythe introduction of the tall stacks policy which

increased the height of emissions encouragedLRTAP and contributed to the spread of acid raindamage

long-wave radiation Relatively low energy radiationfrom the infrared sector of the electromagneticspectrum Terrestrial radiation is long-wave

malnutrition A result of the consumption of essentialnutrients at levels inadequate to maintain goodhealth

maritime tropical air mass (mT) An air massoriginating over the oceans in tropical latitudesand therefore hot and moist mT air is alsoinherently unstable and capable of producing largeamounts of precipitation

melanoma A malignant normally fatal form of skincancer associated with over-exposure to ultraviolet-B radiation (See also skin cancer)

mesopause The boundary between the mesosphere andthermosphere lying some 80 km above the earthrsquossurface

mesosphere The layer of the atmosphere lying abovethe stratosphere In it temperatures decline withincreasing altitude from close to 0degC at thestratopause to-80degC at the mesopause

methane A simple hydrocarbon gas producedduring the decomposition of organic materialunder anaerobic conditions It is a powerfulgreenhouse gas being about twenty-one timesmore effective than carbon dioxide moleculefor molecule and increasing more rapidly

methane sulphonic acid (MSA) a gas produced by theoxidization of dimethyl sulphide (DMS) releasedby phytoplankton during their seasonal bloomMSA is ultimately converted into sulphate in theatmosphere therefore adding to naturalatmospheric acidity

methyl bromide A fumigant which may be responsiblefor as much as 10 per cent of existing ozonedepletion It is used to kill pests in the fruit andvegetable industry

mid-latitude frontal model A model of mid-latitudecyclonic circulation developed between 1915 and1920 by Norwegian weather forecasters Itidentified the interactions between the air massesin a mid-latitude cyclone and explained theresulting weather patterns It remained animportant forecasting tool in mid-latitudes for at

GLOSSARY 195

least 50 years and the elements of the modelmdashsuchas cold front warm front warm sectormdashremainvery much part of the current weather forecastingvocabulary

models Idealized and usually simplifiedrepresentations of complex phenomena models areused to describe and explain as well as forecast theeffects of change Models have been usedextensively in attempts to unravel the complexityof the earth atmosphere system (See also egcarbon cycle models climate models coupledmodels coupled oceanatmosphere generalcirculation models)

moisture deficit In theory a moisture deficit exists in aregion when evapotranspiration exceedsprecipitation However all soils have the ability tostore moisture which will offset the effects of anydeficit As a result a true deficit may not exist untilthe soil water storage has been used up and formost practical purposesmdashdrought evaluation forexamplemdashthe focus is on soil moisture deficit(SMD) rather than the simple relationship betweenprecipitation and evapotranspiration

moisture index A representation of moistureavailability in an area often used in drought andaridity studies Most indices incorporate acombination of meteorological elements includingprecipitation evapotranspiration solar radiationtemperature and wind speed

moisture surplus A moisture surplus exists in a regionwhen precipitation exceeds evapotranspiration andwhen the soil moisture storage is full Anyadditional precipitation will flow across the surfaceas run-off into rivers and lakes

molecule The smallest part of a compound whichretains the composition and chemical propertiesof the compound

Montreal Protocol An agreement reached inMontreal Canada in 1987 aimed at reducing thedestruction of the ozone layer The signatoriesagreed to a 50 per cent cut in the production ofchlorofluorocarbons by the end of the century

natural environment see environment

necrosis (of leaf tissue) The disintegration of leaf tissuecaused by the degeneration of cells in direct contactwith acid precipitation

nitric oxide (NO) One of the oxides of nitrogen inwhich each molecule consists of one atom of

nitrogen and one of oxygen An important naturalozone destroying gas responsible for perhaps asmuch as 50ndash70 per cent of the natural destructionof stratospheric ozone through a long catalyticchain reaction

nitrifying bacteria A group of bacteria found in soiland in the root nodules of leguminous plants whichare capable of fixing the atmospheric nitrogenessential for the creation of the complex nitrogen-based compounds found in all forms of life

nitrogen A colourless odourless gas which makes upabout 78 per cent of the volume of the atmosphereMolecular nitrogen is inert and may be consideredas a dilutant for the oxygen with which it sharesmost of the atmosphere When it does combine withoxygen it forms oxides of nitrogen a group of gasesinvolved in a number of current environmentalproblems

nitrogen dioxide (NO2) A red-brown toxic gas in which

each molecule consists of one atom of nitrogen andtwo of oxygen It is a major constituent ofautomobile exhaust gases and a commoncomponent of urban photochemical smog

nitrogen oxides see oxides of nitrogen

nitrous oxide (N2O) One of the oxides of nitrogen in

which each molecule consists of two atoms ofnitrogen and one of oxygen Nitrous oxide is anaturally produced greenhouse gas but owes itscurrent growth to the increased use of agriculturalfertilizers and the burning of fossil fuels

nomadism A way of life in which groups of people movefrom place to place in search of food for themselvesor their animals Before the development ofagriculture nomadic hunting groups were the norm

normals (climate) Data representing average climaticconditions Usually calculated over a thirty-yearperiod

North American Water and Power Alliance(NAWAPA) A scheme to divert Canadian riversflowing into the Arctic Ocean southwards into theUnited States by means of a continental-scalenetwork of reservoirs canals aqueducts andpumping stations The main purpose of the schemewas the provision of water for irrigation andmunicipal supply in the arid west and southwestof the United States It was not developed beyondthe planning stage

nuclear autumn (fall) see nuclear winter

nuclear winter The result of rapid cooling brought onby increased atmospheric turbidity and reduced

GLOSSARY196

insolation following a major nuclear war Modelsimulations predicted such a rapid decline ininsolation that temperatures would fall to winterlevels even in mid-summer One subsequentmodification suggested that cooling would be lessthan first expected with temperatures declining tovalues more common in autumn or fall than inwintermdashhence the term nuclear autumn (fall) (Seealso TTAPS scenario)

oceanic circulation The movement of water in theearthrsquos ocean basins The surface circulation iscaused mainly by wind setting the surface waterlayers in motion in the form of distinct currentswhereas the deeper ocean circulation is associatedmainly with differences in water density caused bychanging temperature and salinity

odd hydrogens see hydrogen oxides

open system A system in which there is free transferof energy and matter across the systemboundaries

overpopulation A situation in which the populationexceeds the carrying capacity of the environment

oxidation The combination of oxygen with anotherelement to form an oxide

oxides of nitrogen A group of gases formed by thecombination of oxygen and nitrogen Theyinclude nitric oxide nitrous oxide and nitrogendioxide

oxygen A colourless odourless gas which makes up21 per cent of the volume of the atmosphere Theamount of oxygen in the air is kept relativelyconstant through the process of photosynthesis Itis a highly reactive chemical combining readily withother elements to form oxides Oxygen is essentialfor life on earth being absorbed by animals duringrespiration and used to release energy in reactionswith other chemicals

ozone A form of oxygen in which each moleculecontains three atoms rather than the two of normalatmospheric oxygen It is a blue gas with a pungentodour and a very powerful oxidizing agent In thetroposphere it is normally considered a pollutantbut a layer of ozone in the stratosphere protectsthe earthrsquos surface by absorbing ultravioletradiation

ozone depletion The damage to the stratospheric ozonelayer caused by the growing volume of ozone-destroying chemicals in the atmosphere

Ozone Protection Act Legislation passed by theAustralian government in 1989 aimed ateliminating chlorofluorocarbon and halon use by1994 to protect the ozone layer from furtherdepletion

parameterization The method by which regionalscale processes are included in global climatemodels For example cloudiness which is verymuch a local factor can be represented usingtemperature and humidity values calculated at themodel grid points

particle coagulation An atmospheric cleansing processin which individual particles floating in theatmosphere combine together to form largerparticles which are too heavy to remain suspendedand under the effect of gravity fall back to thesurface

particulate matter A collective name for all forms ofmaterial added to the atmosphere by processes atthe earthrsquos surface

pastoral agriculture A form of agriculture based onthe herding of grazing animals such as cattle sheepgoats and camels common in semi-arid tropicaland temperate natural grasslands

permanent drought A form of drought under whichagriculture is not normally possible since there isinsufficient moisture for anything but thexerophytic vegetation which has adapted to the aridenvironment

pH (potential hydrogen) The representation of theacidity or akalinity of a substance on a logarithmicscale of 0 to 14 based on hydrogen ionconcentration Acid substances have pH valuesbetween 0 and 7 alkaline substances range between7 and 14 and a pH of 7 is considered neutral

photochemical processes Chemical processes inducedby the presence of sunlight which provides theenergy required for the reactions involved

photochemical smog Smog produced by the action ofphotochemical processes on primary combustionproducts particularly the hydrocarbons and oxidesof nitrogen produced by the internal combustionengine

photosynthesis A biochemical process in which greenplants absorb solar radiation and convert it intochemical energy It is made possible by chlorophyllDuring the process carbon dioxide and water areconsumed carbohydrates are produced and stored

GLOSSARY 197

and oxygen is released helping to maintain theoxygencarbon dioxide balance in the atmosphere

phytoplankton Microscopic plants which live in theupper layers of the oceans

polar stratospheric clouds Clouds of ice particles whichform in the stratosphere above the poles duringthe winter (see also heterogeneous chemicalreactions)

pollution (environmental) The contamination of thephysical and biological components of the earthatmosphere system to such an extent that normalenvironmental processes are adversely affected

polymer foams Synthetic foam produced by bubblingchlorofluorocarbons through liquid plastic

potential evapotranspiration (PE) The amount ofevaporation and transpiration that will take placeif sufficient moisture is available to fill theenvironmentrsquos capacity for evapotranspirationMeasurable evaporation ceases when water is nolonger available but the environment may retainthe ability to cause additional evapotranspirationthrough such elements as temperature radiationhumidity and wind Potential evapotranspirationis the theoretical value that represents that ability

pre-Cambrian Shield An area of ancient acidic rocksmainly igneous and metamorphic in origin formedperhaps as early as 45 billion years ago Longexposure to erosion has worn them down to thesubdued rounded landscapes found in northernCanada Sweden and Finland

precipitation Any solid or liquid water particles fallingto the earthrsquos surface from the atmosphere Itincludes rain snow hail and sleet

precipitation scavenging A process by which rain andsnow wash particulate matter out of theatmosphere thus helping to cleanse it

primary aerosols Large particles with diametersbetween 1 and 100 microm They include soil dust anda variety of industrial emissions formed by thebreakup of material at the earthrsquos surface

Pueblo drought A serious drought which struck theterritory of the Pueblo group of Indian tribes in thesouth-western United States in the thirteenth century

radiation absorption The intake of radiant energy byan object causing the temperature of the object torise and allowing it to become a radiating body inits own right

radiation blindness Loss of sight caused by damage tothe eye from exposure to excess solar radiation Itusually takes the form of cataracts in which thenormally clear lens of the eye becomes opaquecausing reduced light transmission and loss of visualperception

radiation scattering The disruption of the smooth flowof radiation through the atmosphere usually as aresult of paniculate matter in the energy path

radiation spectrum see electromagnetic spectrum

radiative-convective models One-dimensional climatemodels incorporating global-scale radiative andconvective processes at different levels in theatmosphere used to estimate temperature changeinitiated by changing atmospheric aerosol levels

radiative forcing agent Any factor capable ofdisturbing the energy balance of the earthatmosphere system

RAINS A computer model developed to study acidrain in Asia Similar to a model developed for theEuropean Community it will allow researchers toalert Asian governments to the extent and intensityof the problem

recharge rate The rate at which water withdrawn fromthe groundwater system is replaced byprecipitation

recycling The recovery of waste material forreprocessing into new products or the reuse ofdiscarded products

remote sensing The observation of the surface of theearth from a distance by means of sensors Aerialphotography was the earliest form of remotesensing but satellite observation is now mostcommon involving the creation of simplephotographic images or the collection of data indigital form

retrofitting The adaptation of an existing structure orappliance to meet needs which did not exist whenthe structure or appliance was first constructed

Rio Declaration A declaration of global principles onthe theme of economically and environmentallysound development Along with Agenda 21 itrepresented the culmination of the activities of theUnited Nations Conference on Environment andDevelopment (UNCED)

Rossby waves Longwaves in the circumpolar westerlyairflow in the upper atmosphere first described byCarl Rossby in the 1930s The flow patternfollowed by the waves is quite variable and difficult

GLOSSARY198

to forecast but it makes a major contribution tomeridional energy transfer in mid to high latitudes(see also zonal index)

Run-off See moisture surplus

Sahel A semi-arid to arid area subject to seasonaland long-term drought in Africa south of theSahara Desert The Sahel proper consists of thesix nations Senegal Mauretania Mali BurkinaFaso Niger and Chad but the name has cometo include adjacent nations which suffer fromproblems of drought famine and desertificationwhich are characteristic of the Sahel

salinization The build-up of salts in soils as a result ofthe evaporation of irrigation water

scrubbers Structures used to reduce acid-gas emissionsfrom industrial plants Wet or dry techniques canbe used but all involve bringing the gases in contactwith alkaline or basic substances which neutralizetheir acidity

sea-ice models Models which attempt to simulate therole of sea-ice in global climates They may beincorporated in ocean models or coupled directlyto general circulation models

sea-surface temperatures (SSTs) A source ofinformation on potential change in the earthatmosphere system Anomalous warming orcooling of the sea surface for example may befollowed some time later by changing pressurewind and precipitation patterns Once the initialchange has been identified such time-lags allowsubsequent events to be predicted

seasonal drought Regularly occurring droughtrestricted to one season of the year and offset by adistinct rainy season Seasonal drought is commonin the tropical and sub-tropical grasslands of AfricaIndia and Australia where dry conditions arebrought on by the arrival of continental tropicalair as the Intertropical Convergence Zone advancesequatorwards

secondary aerosols Aerosols formed as a result ofchemical and physical processes in the atmosphereThey are concentrated in the size range of 01ndash1microm and may make up as much as 64 per cent oftotal global aerosols

selective catalytic reduction (SCR) An efficient butcostly process developed to reduce the emission ofoxides of nitrogen from power plants With the

help of a catalyst the oxides of nitrogen are brokendown into harmless nitrogen and oxygen

sensible heat Heat which can be felt or sensed andwhich causes the temperature of a body to changeshifting sands A popular image of desertificationin which desert sand dunes migrate into an areacovering arable land and pasture and sometimessettlements thus creating a desert

short-wave radiation Radiation from the high energyend of the electromagnetic spectrum The term iscommonly applied to solar radiation which consistsof ultraviolet and visible light rays

sink Natural reservoir or store for materials circulatingthrough the earthatmosphere system The oceansare a major natural sink for many substances fromheavy metals to carbon

skin cancer A disease indicated by the alteration ofskin cells and associated with damage to the geneticmake-up of the cells Levels of skin cancer havebeen rising since the late 1970s apparently inparallel with the thinning of the ozone layer whichhas allowed the ultraviolet radiation reaching theearthrsquos surface to increase (See also melanoma)

lsquoslabrsquo models Interactive atmosphere-ocean circulationmodels in which the ocean is represented by onlythe uppermost layer or lsquoslabrsquo of water This isnecessary to accommodate the different responsetimes of atmosphere and ocean

smog A combination of smoke and fog which createsatmospheric pollution

soil erosion The removal of topsoil by water windand gravity at a rate greater than it can be formed

soil moisture deficit (SMD) A measure of moistureavailability used in drought evaluation Itincorporates not only traditional elements such asevapotranspiration and precipitation but alsoconsiders the nature and state of cropdevelopmentmdashwhich influences moisturerequirementsmdashand the storage capacity of thespecific soil

soil moisture storage Water held in the pore spaces ofthe soil In arid areas it offsets the water deficitcaused when evapotranspiration exceedsprecipitation and delays the onset of droughtWhen the soil moisture storage is full the soil issaid to be at field capacity

soil structure The form of the aggregates producedwhen individual soil particles clump togetherAggregates may be crumbs blocks or plates forexample

GLOSSARY 199

soil texture A measure of the proportions of sand siltand clay in a soil

solar radiation Radiant energy given off by the sunSince the sun is a very hot body the bulk of theradiation is high energy at ultraviolet and visiblelight wavelengths

soot Finely divided particles of carbon formed duringcombustion which readily combine with each otherinto clusters or strings and are effective atabsorbing radiation across the entire spectrum

Southern Oscillation The periodic reversal of pressurepatterns and wind directions in the atmosphereabove the equatorial Pacific Ocean Part of theWalker Circulation and responsible for thedevelopment of El Nintildeo (See also ENSO)

spatial resolution An indication of the detail availablefrom weather and climate models determined bythe horizontal and vertical distribution of the gridpoints for which data are available and at whichthe appropriate equations are solved

spectral models Atmospheric circulation models whichfocus on the representation of atmosphericdisturbances or waves by a finite number ofmathematical functions The progressive solutionof a series of equations allows the development ofthe atmospheric disturbances to be predicted

sphagnum A type of moss which is a commoncomponent of the plant community in temperatepeat bogs Being acid tolerant it colonizes themargins of acid lakes

spring flush The rapid run-off of water from meltingsnow and ice common in mid to high latitudes atthe end of winter which carries the winterrsquosaccumulation of acidity into rivers and lakes in amatter of days rapidly reducing the pH of thesewaterbodies

Statement of Forest Principles A general statementfrom the Earth Summit which acknowledges theneed to balance exploitation and conservation offorests but with no provision for internationalmonitoring or supervision

steady state system A system in which inputs andoutputs are equal and constant and in which thevarious elements are in equilibrium Any changealters the relationships among the components ofthe system creating imbalance and setting in traina series of reponses which attempt to restorebalance

steppe A semi-arid area characterized by short-grassvegetation Considered transitional between desert

and sub-humid climates In areas closer to the lattersteppe may include woody shrubs

stratopause The boundary between the stratosphereand mesosphere located at about 50 km above theearthrsquos surface

stratosphere The layer of the atmosphere lying abovethe tropopause It is characterized by anisothermal layer (temperatures remain constant)up to about 20 km above the earthrsquos surfacebeyond which temperatures rise again fromabout-50degC to reach close to 0degC at thestratopause This is the result of the presence ofozone which absorbs incoming ultravioletradiation causing temperatures to rise

stratospheric ozone See ozone

strip-cropping The practice of cultivating land in longnarrow strips of different crops to ensure that theland always retains some vegetation cover and istherefore protected from soil erosion

Study of Critical Environmental Problems (SCEP)Produced in 1970 this was the first major studyto draw attention to the global extent of human-induced environmental issues

Study of Manrsquos Impact on Climate (SMIC) A 1971report which grew out of issues raised originally inthe SCEP It focused on inadvertent climatemodification

subsidence The sinking of air in the atmosphereSubsidence may be associated with the coolingof air close to the surfacemdashas in coldanticyclonesmdashor with the larger scalecirculationmdashin the descending arm of aconvection cell for example

subsistence farming The production of sufficient foodand other necessities to meet the requirements of afarm unit leaving no surplus for sale and little forstorage

sulphate (particle) A negatively charged ion containingone atom of sulphur and four of oxygen

sulphur A non-metallic element present in all livingmatter Its release as sulphur dioxide during thecombustion of fossil fuels is a precursor of acidrain

sulphur dioxide An acid gas in which each moleculecontains one atom of sulphur and two of oxygenIt is a product of the combustion of materialscontaining sulphur

sunspots Dark spots that appear on the surface of thesun associated with strong electromagnetic activity

GLOSSARY200

supernova A star which over a short period of a fewdays becomes exceptionally bright and emits highlevels of cosmic radiation before declining again

supersonic transports (SSTs) Commercial aircraft thatroutinely fly faster than the speed of sound and athigher altitudes than subsonic airliners Flying highin the stratosphere they inject ozone destroyingpollutants such as oxides of nitrogen and oddhydrogens directly into the ozone layer

sustainable development Development which is botheconomically and environmentally sound so thatthe needs of the worldrsquos current population can bemet without jeopardizing those of futuregenerations

Sustainable Development Commission An institutionestablished as a result of the Earth Summit aimedat monitoring and promoting the approachtowards sustainable development identified at theSummit

system An assemblage of interrelated objects organizedas an integrated whole The earthatmospheresystem for example includes the various physicaland biological components of the environmentThese components are closely interrelated and worktogether for the benefit of all

tall stacks policy An approach to the problem of localair pollution which involved the building of tallsmokestacks to allow the release of pollutantsoutside the local atmospheric boundary layer Whilethis reduced local pollution it introducedpollutants into the larger scale circulation andcontributed to the long range transportation of airpollution (LRTAP)

teleconnection The linking of environmental eventsin time and place The linkages usually involve atime-lag and include locations that may bewellseparated from each other For example an ElNintildeo in the eastern Pacific late in one year may belinked to the failure of the Indian monsoon in thefollowing year

temperature inversion The reversal of the normaltemperature decline with altitude in thetroposphere In an inversion the temperature riseswith altitude because of the presence of a layer ofwarm air above the cooler surface air

terpene One of a group of hydrocarbons found in theessential oils of some plants particularly conifersReleased from these plants terpenes may beresponsible for the haze common over such areas

as the Blue Mountains of Virginia in the UnitedStates

terrestrial environment That part of the environmentthat includes the components of the land surfacemdashsuch as rock and soilmdashand the plants and animalsthat live on it

terrestrial radiation Low energy long-wave radiationfrom the infrared sector of the spectrum emittedby the earthrsquos surface

thermal low Low pressure system produced by theheating of the earthrsquos surface and the airimmediately above it The process is particularlyeffective in areas of high insolation

thermonuclear device A powerful bomb in which theexplosive force is created by the fusion(combination) of the nuclei of hydrogen atomsmdashhence the name lsquohydrogen bombrsquo

thermosphere The outermost layer of the atmospherebeyond the mesopause some 80 km above theearthrsquos surface

Third World A term commonly applied to thedeveloping and non-aligned nations of Africa Asiaand Latin America to distinguish them from thelsquowesternrsquo nations of the lsquofirst worldrsquo with theirdeveloped capitalist economies and thecommunist nations of the lsquosecond worldrsquo with theircentrally-planned economies

30 per cent club A group of 21 nationsmdashmainly fromEurope but including Canadamdashwhich agreed in1985 to reduce trans-boundary emissions ofsulphur dioxide by 30 per cent (of the 1980 level)by 1993 in an attempt to deal with the growingproblem of acid rain

topsoil The uppermost layer of the soil containing thebulk of its organic material nutrients and livingorganisms

tornado An intense rotating storm usually no morethan 100 to 500 m in diameter originating wherecold and warm moist air masses collide andaccompanied by winds that commonly exceed 200kph

transient models General circulation models whichattempt to provide information at intermediatestages during the model run unlike equilibriummodels which provide only one final result

transpiration The loss of water from vegetation to theatmosphere by its evaporation through leaf poresin individual plants

tree dieback The gradual wasting of a tree from the

GLOSSARY 201

outermost leaves and twigs inwards leading to thedeath of the tree over several seasons Dieback hasbeen linked to the effects of acid precipitation butthere is no conclusive proof that this is the onlyfactor involved (See also Waldsterben)

triatomic oxygen The gas ozone in which eachmolecule consists of three atoms of oxygen

tropopause The upper boundary of the troposphereIt varies in height from about 8 km at the poles to16 km at the equator

troposphere The lowest layer of the atmosphere inwhich temperatures decrease with altitude at a rateof about 65degC per kilometre to reach between-50degC and -60degC at the tropopause

TTAPS scenario The scenario developed to explainthe onset of nuclear winter TTAPS is an acronymbased on the first letters of the names of thescientists who developed the original hypothesis-Turco Toon Ackerman Pollack and Sagan

Tu-144 A supersonic transport developed by Tupolevin the Soviet Union in the 1970s

ultraviolet radiation High energy short-wave solarradiation most of which is absorbed by the ozonelayer in the stratosphere Thinning of the ozonelayer has increased the proportion of ultravioletradiation reaching the earthrsquos surface giving riseto fears of an increasing incidence of skin cancerand other radiation related problems

United Nations Conference on Desertification(UNCOD) A conference held in Nairobi Kenya in1977 which established the modern approach tothe problem of desertification

United Nations Conference on Environment andDevelopment (UNCED) An internationalconferencemdashthe Earth Summitmdashheld in Rio deJaneiro Brazil in 1992 with the theme ofsustainable development based on economicallyand environmentally sound principles (See alsoAgenda 21 Global Forum and RioDeclaration)

United Nations Conference on the HumanEnvironment (UNCHE) Held in StockholmSweden in 1972 this was the first conference todraw worldwide attention to the immensity ofenvironmental problems thus ushering in themodern era in environmental studies

United Nations Environment Program (UNEP) Aprogramme designed to combat the environmental

damage caused by development projects indeveloping nations

upper westerlies Westerly winds that blow in the upperatmosphere close to the tropopause in mid to highlatitudes (See also jet streams and Rossby waves)

Villach Conference The first of the major modernenvironmental conferences to deal with the risinglevels of greenhouse gases in the atmosphere andtheir impact on climate held at Villach Austria in1985 It established an Advisory Group onGreenhouse Gases (AGGG) to ensure that therecommendations of the conference were followedup

visible light Radiation from that part of the spectrumto which the human eye is sensitive Visible lightoccupies a position between ultraviolet and infraredradiation in the spectrum and varies in colour theshorter wavelengths being blue and the longerwavelengths red

vitamin D The vitamin essential for the uptake ofcalcium by the human body It is important for theproper growth and maintenance of bone andinsufficient vitamin D consumption can lead torickets a disease which causes bones to becomeprone to bending and fracturing

Volcanic Explosivity Index (VEI) An index forcomparing individual volcanic eruptions It is basedon volcanological criteria such as the intensitydispersive power and destructive potential of theeruption as well as the volume of material ejected(See also Dust Veil Index (DVI) and GlaciologicalVolcanic Index (GVI))

Waldsterben The destruction of the forests A termcoined in Germany to describe the damage causedto forests by acid rain (See also tree dieback)

Walker circulation A strong latitudinal circulation inthe equatorial atmosphere which contrasts with thenormal meridional circulation It is particularly wellmarked in the Pacific Ocean where it is linked withthe Southern Oscillation

water balance A book-keeping approach to themoisture budget It involves the comparison ofmoisture input (precipitation) and output(evapotranspiration) to provide a value for thenet surplus or deficit of water at a specificlocation

GLOSSARY202

weather The current or short-term state of theatmosphere expressed in terms of such variablesas temperature precipitation airflow andcloudiness

weather forecasting models Models which usefundamental equations representing atmosphericprocess to predict short-term changes inmeteorological elements

weather modification Any change in weatherconditions caused by human activity whether bydesign or accident

weathering The physical and chemical breakdown ofrocks at the earthrsquos surface brought about byexposure to air water temperature change andorganic activity

wet deposition The most common form of acidprecipitation in which the acids are present insolution in the atmosphere and reach the earthrsquossurface in rain snow hail and fog (See also drydeposition)

windbreak A row of trees or shrubs planted at rightangles to the airflow The consequent reduction inwindspeed helps to protect sensitive plants reduces

the rate of evapotranspiration and helps to preventsoil erosion

World Commission on Environment and DevelopmentA commission chaired by Gro Harlem Brundtlandthe Norwegian Prime Minister in 1987 Itpromoted the concept of sustainable developmentand led directly to the UNCED

World Meteorological Organization (WMO) A UnitedNations sponsored organization based in GenevaSwitzerland which coordinates worldwideweather data collection and analysis

xerophytic vegetation Plants adapted to life in aridconditions

zonal index A measure of the intensity of the

midlatitude atmospheric circulation patternDuring a period of high zonal index the airflow iswest to east or latitudinal A low zonal indexinvolves a south-north-south or meridionalcomponent as a result of waves forming in theairflow

Abrahamsen G Seip HM and Semb A (1989)lsquoLong-term acidic precipitation studies in Norwayrsquoin DCAdriano and MHavas (eds) AcidicPrecipitation Volume 1 Case Studies New YorkSpringer-Verlag

Adetunji J McGregor J and Ong CK (1979)lsquoHarmattan hazersquo Weather 34430ndash6

Ahrens CD (1993) Essentials of MeteorologyMinneapolisSt Paul West Publishing Company

Almer B Dickson W Ekstrom C and HornstromE (1974) lsquoEffects of acidification in Swedish lakesrsquoAmbio 330ndash6

Anon (1983) lsquoOne-third of German trees hit by acidrainrsquo New Scientist 100 (1381) 250

mdashmdash(1984) lsquoBalancing the risks what do we knowrsquoWeatherwise 37240ndash9

Anthes RA Cahir JJ Fraser AB and PanofskyHA (1980) The Atmosphere (3rd edition)Columbus Ohio Merrill

Appleby L and Harrison RM (1989)lsquoEnvironmental effects of nuclear warrsquo Chemistryin Britain 251223ndash8

Arthur LM (1988) The Implication of ClimateChange for Agriculture in the Prairie ProvincesCCD 88ndash01 Ottawa Atmospheric EnvironmentService

Ashford OM (1981) lsquoThe dream and the fantasyrsquoWeather 36323ndash5

mdashmdash(1992) lsquoDevelopment of weather forecasting inBritain 1900ndash40rsquo Weather 47394ndash402

Austin J (1983) lsquoKrakatoa Sunsetsrsquo Weather 38226ndash31

Bach W (1972) Atmospheric Pollution New YorkMcGraw-Hill

mdashmdash(1976) lsquoGlobal air pollution and climatic changersquoReviews of Geophysics and Space Physics 14429ndash74

mdashmdash(1979) lsquoShort-term climatic alterations caused byhuman activities status and outlookrsquo Progress inPhysical Geography 355ndash83

mdashmdash(1986) lsquoNuclear war the effects of smoke anddust on weather and climatersquo Progress in PhysicalGeography 10315ndash63

Baker JP and Schofield CL (1985) lsquoAcidificationimpacts on fish populations a reviewrsquo in DDAdams and WPPage (eds) Acid DepositionEnvironmental Economic and Political IssuesNew York Plenum Press

Balchin WGV (1964) lsquoHydrologyrsquo in JWWatsonand JBSissons (eds) The British Isles A SystematicGeography London Nelson

Ball T (1986) lsquoHistorical evidence and climaticimplications of a shift in the boreal forest-tundratransition in Central Canadarsquo Climatic Change8 121ndash34

Balling RC (1991) lsquoImpact of desertification onregional and global warmingrsquo Bulletin of theAmerican Meteorological Society 72232ndash4

Bark LD (1978) lsquoHistory of American droughtsrsquo inNJRosenberg (ed) North American DroughtsBoulder Colorado Westview Press

Barrie LA (1986) lsquoArctic air pollution An overviewof current knowledgersquo Atmospheric Environment20643ndash63

Barrie LA and Hoff RH (1985) lsquoFive years of airchemistry observations in the Canadian ArcticrsquoAtmospheric Environment 191995ndash2010

Barry RG and Chorley RJ (1992) AtmosphereWeather and Climate London Routledge

Barton IJ Prata AJ Watterson IG and YoungSA (1992) lsquoIdentification of the Mount Hudsonvolcanic cloud over SEAustraliarsquo GeophysicalResearch Letters 191211ndash14

Berkofsky L (1986) lsquoWeather modification in aridregions the Israeli experiencersquo Climatic Change9103ndash12

Bingemer HG and Crutzen PJ (1987) lsquoTheproduction of methane from solid wastesrsquo Journalof Geophysical Research 922181ndash7

Biswas AK (1974) Energy and the EnvironmentOttawa Environment Canada

Blake DR and Rowland FS (1988) lsquoContinuingworldwide increase in tropospheric methanersquoScience 2391129ndash31

Blank LW Roberts TM and Skeffington RA

Bibliography

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Bolin B (1960) lsquoOn the exchange of carbon dioxidebetween the atmosphere and the searsquo Tellus 12274ndash81

mdashmdash(1972) lsquoAtmospheric chemistry andenvironmental pollutionrsquo in DPMclntyre (ed)Meteorological Challenges A History OttawaInformation Canada

mdashmdash(1986) lsquoHow much CO2 will remain in the

atmospherersquo in BBolin BRDoos JJager andRAWarrick (eds) The Greenhouse Effect ClimateChange and Ecosystems SCOPE 29 New YorkWiley

Bolin B Jager J and Doos BR (1986) lsquoThegreenhouse effect climatic change and ecosystemsa synthesis of present knowledgersquo in B BolinBRDoos JJager and RAWarrick (eds) TheGreenhouse Effect Climatic Change andEcosystems SCOPE 29 New York Wiley

Bolle HJ Seiler W and Bolin B (1986) lsquoOthergreenhouse gases and aerosolsrsquo in BBolin BRDoos JJager and RAWarrick (eds) TheGreenhouse Effect Climatic Change andEcosystems SCOPE 29 New York Wiley

Borchert JR (1950) lsquoThe climate of the central NorthAmerican grasslandrsquo Annals of the Association ofAmerican Geographers 401ndash39

Bowman KP (1986) lsquoInterannual variability of totalozone during the breakdown of the Antarcticcircumpolar vortexrsquo Geophysical Research Letters131193ndash6

Bradley RS and Jones PD (1992) Climate since AD1500 London Routledge

Brakke DF Landers DH and Eilers JM (1988)lsquoChemical and physical characteristics of lakes inthe northeastern United Statesrsquo EnvironmentalScience and Technology 22155ndash63

Brasseur G and Granier C (1992) lsquoMount Pinatuboaerosols chlorofluorocarbons and ozonedepletionrsquo Science 2571239ndash42

Bretherton PP Bryan K and Woods JD (1990)Time-dependent greenhouse-gas-induced climatechangersquo in JTHoughton GJJenkins and JJEphraums (eds) Climate Change The IPCCScientific Assessment Cambridge CambridgeUniversity Press

Bruce J and Hengeveld HG (1985) lsquoOur changingnorthern climatersquo Geography 141ndash6

Bryson RA (1968) lsquoAll other factors being constanthellip a reconciliation of several theories of climaticchangersquo Weather 2156ndash61 and 94

mdashmdash(1973) lsquoDrought in the Sahel who or what is toblamersquo Ecologist 3366ndash71

Bryson RA and Dittberner GJ (1976) lsquoAnonequilibrium model of hemispheric mean surface

temperaturersquo Journal of Atmospheric Sciences 332094ndash106

Bryson RA and Murray TJ (1977) Climates ofHunger Madison University of Wisconsin Press

Bryson RA and Peterson JT (1968) lsquoAtmosphericaerosols increased concentrations during the pastdecadesrsquo Science 162120ndash1

Bryson RA Lamb HH and Donley DL (1974)lsquoDrought and the decline of the MycenaersquoAntiquity 4846ndash50

Burdett NA Cooper JRP Dearnley S Kyte WSand Turnicliffe MF (1985) lsquoThe application ofdirect limestone injection to UKpower stationsrsquoJournal of the Institute of Engineering 5864ndash9

Burroughs WJ (1981) lsquoMount St Helens a reviewrsquoWeather 36238ndash40

Butler JH Elkins JW Hall BD Cummings SOand Montzka SA (1992) lsquoA decrease in thegrowth rates of atmospheric halon concentrationsrsquoNature 359403ndash5

Calder N (1974) The Weather Machine and theThreat of Ice London BBC Publications

Callaghan TV Bacon PJ Lindley DK and elMoghraby AI (1985) lsquoThe energy crisis in theSudan alternative supplies of biomassrsquo Biomass8217ndash32

Callendar GS (1938) lsquoThe artificial production ofcarbon dioxide and its influence on temperaturersquoQuarterly Journal of the Royal MeteorologicalSociety 64223ndash37

Callis LB and Natarajan M (1986) lsquoOzone andnitrogen dioxide changes in the stratosphere during1979ndash1984rsquo Nature 323772ndash7

CIDA (Canadian International Development Agency)(1985) Food Crisis in Africa Hull Quebec CIDA

Capot-Rey R (1951) lsquoUne carte de lrsquoindice drsquoariditeacuteau Sahara Franccedilaisrsquo Bulletin de lrsquoAssociationGeacuteographique Francaise 216773ndash6

Carpenter R (1968) Discontinuity in GreekCivilization New York WWNorton

Catchpole AJW and Milton D (1976) lsquoSunnierprairie citiesmdasha benefit of natural gasrsquo Weather31348ndash54

Caulfield C and Pearce F (1984) lsquoMinisters rejectclean-up of acid rainrsquo New Scientist 104 (1433) 6

Cess RD and Potter GL (1984) lsquoA commentary onthe recent CO

2-climate controversyrsquo Climatic

Change 6365ndash76Chapman S (1930) lsquoA theory of upper atmospheric

ozonersquo Quarterly Journal of the RoyalMeteorological Society 3103

Charles DF Battarbee RW Reuberg I VanDamH and Smot JP (1990) rsquoPalaeoecological analysisof lake acidification trends in North America andEurope using diatoms and clirysophytesrsquo inSANorton SELindberg and AL Page (eds)

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Acidic Precipitation Volume 4 Soils AquaticProcesses and Lake Acidification New YorkSpringer-Verlag

Charlson RJ Schwartz SE Hales JM Cess RDCoakley JA Hansen JE and Hofmann DJ(1992) lsquoClimate forcing by anthropogenic aerosolsrsquoScience 255423ndash30

Charney J (1975) lsquoDynamics of deserts and droughtin the Sahelrsquo Quarterly Journal of the RoyalMeteorological Society 101193ndash202

Chase A (1988) lsquoThe ozone precedentrsquo Outside 1337ndash40

Cheng HC Steinberg M and Beller M (1986)Effects of Energy Technology on Global CO

2Emissions Washington DC US Department ofEnergy

Chester DK (1988) lsquoVolcanoes and climate recentvolcanological perspectivesrsquo Progress in PhysicalGeography 121ndash35

Cho HR Iribarne JV Grabenstetter JE and TamYT (1984) lsquoEffects of cumulus cloud systems onthe vertical distribution of air pollutantsrsquo inPJSamson (ed) The Meteorology of AcidDeposition Pittsburgh Air Pollution ControlAssociation

Cicerone RJ and Oremland R (1988)lsquoBiogeochemical aspects of atmospheric methanersquoGlobal Biogeochemical Cycles 2299ndash327

Cicerone RJ Stolarski RS and Walters S (1974)lsquoStratospheric ozone destruction by man-madechlorofluoromethanesrsquo Science 1851165ndash7

Clery D (1992) lsquoA climate of confidencersquo NewScientist 134 (1823) 12ndash13

Climate Institute (1988a) lsquoNASA scientist testifiesgreenhouse warming has begunrsquo Climate Alert 11ndash2

mdashmdash(1988b) lsquoIs there a droughtgreenhouse effectconnectionrsquo Climate Alert 19

mdashmdash(1988c) lsquoCFC producers plan phaseoutrsquo ClimateAlert 11

mdashmdash(1992a) lsquoMarshall Islands study measures tothwart ocean risersquo Climate Alert 51 and 4

mdashmdash(1992b) lsquoAre we heading into an era of moreintense hurricanesrsquo Climate Alert 51 and 6ndash7

Cocks A and Kallend T (1988) lsquoThe chemistry ofatmospheric pollutionrsquo Chemistry in Britain 24884ndash8

Concar D (1992) lsquoThe resistible rise of skin cancerrsquoNew Scientist 134 (1821) 23ndash8

Cortese T (1986) lsquoThe scientific and economicimplications for acid rain controlrsquo in ConferenceProceedings Intergovernmental Conference onAcid Rain Quebec April 10 11 and 12 1985Quebec City Ministry of the Environment

Crane A and Liss P (1985) lsquoCarbon dioxide climateand the searsquo New Scientist 108 (1483) 50ndash4

Crawford M (1985) lsquoSub-Sahara needs quick helpto avert disasterrsquo Science 230788

Critchfield HJ (1983) General ClimatologyEnglewood Cliffs Prentice-Hall

Cronan CS and Schofield CL (1979) lsquoAluminumleaching response to acid precipitation effects onhigh elevation watersheds in the north-eastrsquo Science204304ndash6

Cronin JF (1971) lsquoRecent volcanism and thestratospherersquo Science 172847ndash9

Cross M (1985a) lsquoAfricarsquos drought may last manyyearsrsquo New Scientist 105 (1439) 9

mdashmdash(1985b) lsquoWaiting and hoping for the big rainsrsquoNew Scientist 105 (1443)3ndash4

Crutzen PJ (1972) lsquoSSTsmdashA threat to the earthrsquosozone shieldrsquo Ambio 141ndash51

mdashmdash(1974) lsquoEstimates of possible variations in totalozone due to natural causes and human activitiesrsquoAmbio 3201ndash10

Crutzen PJ and Arnold F (1986) lsquoNitric acid cloudformation in the cold Antarctic stratosphere amajor cause for the springtime ldquoozone holerdquorsquoNature 324651ndash5

Crutzen PJ Aselmann I and Seiler W (1986)lsquoMethane production by domestic animals wildruminants other herbivorous fauna and humansrsquoTellus 38271ndash84

Cubasch U and Cess RD (1990) lsquoProcesses andmodellingrsquo in JTHoughton GJJenkins and JJEphraums (eds) Climate Change The IPCCScientific Assessment Cambridge CambridgeUniversity Press

Cunningham WP and Saigo BW (1992)Environmental Science A Global ConcernDubuque Iowa WCBrown

Delmas R Ascencio JM and Legrand M (1980)lsquoPolar ice evidence that atmospheric CO

2 20000

yr BP was 50 of presentrsquo Nature 284155ndash7Deshler T Adriani A Gobbi GP Hofmann DJ

Di Donfrancesco G and Johnson BJ (1992)lsquoVolcanic aerosol and ozone depletion within theAntarctic polar vortex during the austral springof 1991rsquo Geophysical Research Letters 191819ndash22

Detwyler TR (ed) (1971) Manrsquos Impact onEnvironment New York McGraw-Hill

di Sarra A Cacciani M Di Girolamo P FioccoG Fua D Knudsen B Larsen N andJoergensen JS (1992) lsquoObservations of correlatedbehaviour of stratospheric ozone and aerosol atThule during winter 1991ndash1992rsquo GeophysicalResearch Letters 191823ndash6

Dickinson RE (1986) lsquoHow will climate changersquoin BBolin BRDoos JJager and RAWarrick(eds) The Greenhouse Effect Climatic Changeand Ecosystems SCOPE 29 New York Wiley

BIBLIOGRAPHY206

Dotto L and Schiff H (1978) The Ozone WarGarden City New York Doubleday

Dutton EG and Christy JR (1992) lsquoSolar radiativeforcing at selected locations and evidence for globallower stratosphere cooling following the eruptionsof El Chichon and Pinatuborsquo Geophysical ResearchLetters 192313ndash16

Dyer AJ and Hicks BB (1965) lsquoStratospherictransport of volcanic dust inferred from solarradiation measurementsrsquo Nature 94545ndash54

Eagleman JR (1985) Meteorology The Atmospherein Action Belmont California Wadsworth

Edmonds JA Reilly JM Gardner RH andBrenkert A (1986) Uncertainty in Future GlobalEnergy Use and Fossil Fuel CO

2 Emission 1975

to 2075 Washington DC US Department ofEnergy

Ehrlich PR et al (1983) lsquoLong-term biologicalconsequences of nuclear warrsquo Science 2221293ndash1300

Elkins JW Thompson TM Swanson TH ButlerJH Hall BD Cummings SO Fisher DA andRaffo AG (1993) lsquoDecrease in the growth ratesof atmospheric chlorofluorocarbons 11 and 12rsquoNature 364780ndash3

Ellis EG Zeldin MD Brewer RL and ShepardLS (1984) lsquoChemistry of winter type precipitationin southern Californiarsquo in PJSamson (ed) TheMeteorology of Acid Deposition Pittsburgh AirPollution Control Association

Ellis EG Erbes RE and Grott JK (1990)lsquoAbatement of atmospheric emissions in NorthAmerica progress to date and promise for thefuturersquo in SELinberg ALPage and SANorton(eds) Acidic Precipitation Volume 3 SourcesDeposition and Canopy Interactions New YorkSpringer-Verlag

Ellis HT and Pueschel RF (1971) lsquoSolar radiationabsence of air pollution trends at Mauna LoarsquoScience 172845ndash6

Elwood JW and Mulholland PJ (1989) lsquoEffects ofacidic precipitation on stream ecosystemsrsquo in DCAdriano and AHJohnson (eds) AcidicPrecipitation Volume 2 Biological and EcologicalEffects New York Springer-Verlag

Enhalt DH (1980) lsquoThe effects ofchlorofluoromethanes on climatersquo in WBachJPankrath and JWilliams (eds) Interactions ofEnergy and Climate Dordrecht Reidel

Environment Canada (1986) Understanding CO2 and

ClimatemdashAnnual Report 1985 OttawaAtmospheric Environment Service

mdashmdash(1987) Understanding CO2 and Climatemdash

Annual Report 1986 Ottawa AtmosphericEnvironment Service

mdashmdash(1989) Preserving the Ozone Layer a StepBeyond Ottawa Commercial Chemicals Branch

mdashmdash(1991) A Report on Canadarsquos Progress toward aNational Set of Environmental Indicators SOEReport No 91ndash1 Ottawa Environment Canada

mdashmdash(1992) Stratospheric Ozone Depletion SOEBulletin 92ndash1 Ottawa State of the EnvironmentReporting

mdashmdash(1993) State of the Environment Newsletter No9 Ottawa State of the Environment Reporting

Farman JC Gardiner BC and Shanklin JD (1985)lsquoLarge losses of total ozone in Antarctica revealseasonal ClO

x interactionrsquo Nature 315

207ndash10Felch RE (1978) lsquoDrought characteristics and

assessmentrsquo in NJRosenberg (ed) NorthAmerican Droughts Boulder Colorado WestviewPress

Fennelly PF (1981) lsquoThe origin and influence ofairborne particulatesrsquo in BJSkinner (ed) ClimatesPast and Present Los Altos CaliforniaKauffmann

Fernandez IJ (1985) lsquoAcid deposition and forest soilspotential impacts and sensitivityrsquo in DD Adamsand WPPage (eds) Acid DepositionEnvironmental Economic and Political IssuesNew York Plenum Press

Findley R (1981) lsquoThe day the sky fellrsquo NationalGeographic 15950ndash65

Fischer WH (1967) lsquoSome atmospheric turbiditymeasurements in Antarcticarsquo Journal of AppliedMeteorology 6958ndash9

Flohn H (1980) Possible Climatic Consequences ofa Man-Made Global Warming RR-80ndash30Laxenburg Austria International Institute forApplied Systems Analysis

Foley HM and Ruderman MA (1973)lsquoStratospheric NO production from past nuclearexplosionsrsquo Journal of Geophysical Research 784441ndash50

Folland CK Palmer TN and Parker DE (1986)lsquoSahel rainfall and worldwide sea temperatures1901ndash85 observational modelling and simulationstudiesrsquo Nature 320602ndash7

Folland CK Karl T and Vinnikov KY (1990)lsquoObserved climate variations and changersquo in JTHoughton GJJenkins and JJEphraums (eds)Climate Change The IPCC Scientific AssessmentCambridge Cambridge University Press

Forster BA (1985) lsquoEconomic impact of aciddeposition in the Canadian aquatic sectorrsquo inDDAdams and WPPage (eds) Acid DepositionEnvironmental Economic and Political IssuesNew York Plenum Press

Fowler MM and Barr S (1984) lsquoA long rangeatmospheric tracer field testrsquo in PJSamson (ed)

BIBLIOGRAPHY 207

The Meteorology of Acid Deposition PittsburghAir Pollution Control Association

Fritts H (1965) lsquoTree-ring evidence for climaticchanges in western North America MonthlyWeather Review 93421ndash43

Gadd AJ (1992) lsquoScientific statements and the RioEarth Summitrsquo Weather 47294ndash315

Garnett A (1967) lsquoSome climatological problems inurban geography with reference to air pollutionrsquoTransactions of the Institute of British Geographers4221ndash43

Gates WL Rowntree PR and Zeng Q-C (1990)lsquoValidation of climate modelsrsquo in JDHoughtonGJJenkins and JJEphraums (eds) ClimateChange The IPCC Scientific AssessmentCambridge Cambridge University Press

Glaeser B (1990) rsquoThe environmental impact ofeconomic development problems and policiesrsquo inTCannon and AJenkins (eds) The Geography ofcontemporary China London Routledge

Glantz MH (ed) (1977) DesertificationEnvironmental Degradation in and around AridLands Boulder Colorado Westview Press

Gobbi GP Congeduti F and Adriani A (1992)lsquoEarly stratospheric effects of the Pinatuboeruptionrsquo Geophysical Research Letters 19997ndash1000

Goldman MI (1971) lsquoEnvironmental disruption inthe Soviet Unionrsquo in TRDetwyler (ed) ManrsquosImpact on Environment New York McGraw-Hill

Gorham C and Gordon AG (1960) lsquoThe influenceof smelter fumes upon the chemical compositionof lake waters near Sudbury Ontario and uponthe surrounding vegetationrsquo The Canadian Journalof Botany 38477ndash87

Green C (1992) lsquoEconomics and the ldquoGreenhouseEffectrdquorsquo Climatic Change 22265ndash91

Green O and Salt J (1992) lsquoLimiting climate changeverifying national commitmentsrsquo EcodecisionDecember 9ndash13

Gribbin J (1978) lsquoFossil fuel future shockrsquo NewScientist 79 (1117)541ndash3

mdashmdash(1981) lsquoThe politics of carbon dioxidersquo NewScientist 90 (1248)82ndash4

mdashmdash(1982) Future Weather and the GreenhouseEffect New York Delacorte

mdashmdash(1992) lsquoArctic ozone threatened by greenhousewarminghellipand dust from last yearrsquos volcanoesrsquoNew Scientist 136 (1849)16

mdashmdash(1993) The Hole in the Sky (revised edition) NewYork Bantam

Groisman PY (1992) lsquoPossible regional climateconsequences of the Pinatubo eruption anempirical approachrsquo Geophysical Research Letters191603ndash6

Grove AT (1986) lsquoThe state of Africa in the 1980srsquoThe Geographical Journal 152193ndash203

Haas PM Levy MA and Parson EA (1992) lsquoTheEarth SummitmdashHow should we judge UNCEDrsquossuccessrsquo Environment 347ndash11 and 26ndash33

Haines BL and Carlson CL (1989) lsquoEffects of acidprecipitation on treesrsquo in DCAdriano and AHJohnson (eds) Acidic Precipitation Volume 2Biological and Ecological Effects New YorkSpringer-Verlag

Hameed S and Dignon J (1992) lsquoGlobal emissionsof nitrogen and sulphuric oxides in fossil-fuelcombustionrsquo JournalmdashAir and Waste ManagementAssociation 42159ndash63

Hamilton RA and Archbold JW (1945)lsquoMeteorology over Nigeria and adjacent territoryrsquoQuarterly Journal of the Royal MeteorologicalSociety 71231ndash62

Hammond AL and Maugh TH (1974)lsquoStratospheric pollution Multiple threats to earthrsquosozonersquo Science 186335ndash8

Hampson J (1974) lsquoPhotochemical war on theatmospherersquo Nature 250189ndash91

Handel MD and Risbey JS (1992) lsquoAn annotatedbibliography on the greenhouse effect and climaticchangersquo Climatic Change 2197ndash255

Hansen J and Lebedeff S (1988) lsquoGlobal surface airtemperatures update through 1987rsquo GeophysicalResearch Letters 15323ndash6

Hansen JE Lacis A Ruedy R and Sato M (1992)lsquoPotential climatic impact of Mount Pinatuboeruptionrsquo Geophysical Research Letters 19215ndash18

Hare FK (1973) lsquoThe atmospheric environment ofthe Canadian Northrsquo in DHPimlott KMVincent and CEMcKnight (eds) ArcticAlternatives Ottawa Canadian Arctic ResourcesCommittee

Hare FK and Thomas MK (1979) Climate CanadaToronto John Wiley

Harvey HH (1989) lsquoEffects of acidic precipitationon lake ecosystemsrsquo in DCAdriano and AHJohnson (eds) Acidic Precipitation Volume 2Biological and Ecological Effects New YorkSpringer-Verlag

Harwell MA and Hutchinson TC (1985)Environmental Consequences of Nuclear WarVolume II Ecological and Agricultural EffectsSCOPE 28 New York Wiley

Hauhs M and Ulrich B (1989) lsquoDecline of Europeanforestsrsquo Nature 339265

Havas M (1990) lsquoRecovery of acidified and metalcontaminated lakes in Canadarsquo in SANortonSELindberg and ALPage (eds) AcidicPrecipitation Volume 4 Soils Aquatic Processesand Lake Acidification New York Springer-Verlag

BIBLIOGRAPHY208

Hayashida S and Sasano Y (1993) lsquoStratosphericaerosol change in the early stage of volcanicdisturbance by the Pinatubo eruption observed overTsukuba Japanrsquo Geophysical Research Letters20(7) 575ndash8

Heathcote RL (1987) lsquoImages of a desertPerceptions of arid Australiarsquo AustralianGeographical Studies 253ndash25

Hekstra GP (1986) lsquoWill climatic changes flood theNetherlands Effects on agriculture land-use andwell-beingrsquo Ambio 15316ndash26

Hekstra GP and Liverman DM (1986) lsquoGlobal foodfutures and desertificationrsquo Climatic Change 959ndash66

Henderson-Sellers A (1990) lsquoGreenhouse guessingwhen should scientists speak outrsquo ClimaticChange 165ndash8

mdashmdash(1991) lsquoGlobal climate change the difficulties ofassessing impactsrsquo Australian Geographical Studies29202ndash25

Hendrey GR (1985) lsquoAcid deposition a nationalproblemrsquo in DDAdams and WPPage (eds) AcidDeposition Environmental Economic andPolitical Issues New York Plenum Press

Hengeveld HG (1991) Understanding AtmosphericChange SOE Report 91ndash2 Ottawa EnvironmentCanada

Henriksen A and Brakke DF (1988) lsquoSulphatedeposition to surface waterrsquo Environmental Scienceand Technology 228ndash14

Hepting GH (1971) lsquoDamage to forests from airpollutionrsquo in TRDetwyler (ed) Manrsquos Impact onEnvironment New York McGraw-Hill

Hidore JJ and Oliver JE (1993) Climatology AnAtmospheric Science New York Macmillan

Hoffman JS Keyes D and Titus JG (1983)Projecting Future Sea Level Rise US EnvironmentalProtection Agency Washington DC

Houghton JT (1992) lsquoGlobal warming a 1992updatersquo Ecodecision June 8ndash9

Houghton JT Jenkins GJ and Ephraums JJ (eds)(1990) Climate Change The 1PCC ScientificAssessment Cambridge Cambridge UniversityPress

Houghton JT Callender BA and Varney SK(1992) Climate Change 1992 The SupplementaryReport to the IPCC Scientific AssessmentCambridge Cambridge University Press

Howard R and Perley M (1991) Poisoned SkiesToronto Stoddart

Hulme M (1989) lsquoIs environmental degradationcausing drought in the Sahel An assessment fromrecent empirical researchrsquo Geography 7438ndash46

mdashmdash(1993) lsquoGlobal warmingrsquo Progress in PhysicalGeography 1781ndash91

Hulme M and Kelly M (1993) lsquoExploring the links

between desertification and climate changersquoEnvironment 354ndash11 and 39ndash45

Hunt P (1992) lsquoPutting Asian acid rain on the maprsquoNew Scientist 136 (1851) 6

Idso SB (1980) lsquoThe climatological significance ofdoubling of Earthrsquos atmospheric carbon dioxideconcentrationrsquo Science 2071462ndash3

mdashmdash(1981) lsquoCarbon dioxidemdashan alternative viewrsquoNew Scientist 92 (1279)444ndash6

mdashmdash(1982) Carbon Dioxide Friend or Foe TempeArizona IBR Press

mdashmdash(1983) lsquoDo increases in atmospheric CO2 have a

cooling effect on surface air temperaturersquoClimatological Bulletin 1722ndash6

mdashmdash(1987) lsquoA clarification of my position on the CO2

climate connectionrsquo Climatic Change 10 81ndash6IPCC (Intergovernmental Panel on Climate Change)

(1991) Climate Change The IPCC ResponseStrategies Washington DC Island Press

Israelson D (1987) lsquoAcid rain updatersquo Toronto StarApril 4 A1-A3

Jacobsen T and Adams RM (1971) lsquoSalt and silt inancient Mesopotamian agriculturersquo in TRDetwyler (ed) Manrsquos Impact on Environment NewYork McGraw-Hill

Jeffries DS (1990) lsquoSnowpack storage of pollutantsrelease during melting and impact on receivingwatersrsquo in SANorton SELindberg and AL Page(eds) Acidic Precipitation Volume 4 Soils AquaticProcesses and Lake Acidification New YorkSpringer-Verlag

Jenkins I (1969) lsquoIncreases in averages of sunshinein Greater Londonrsquo Weather 2452ndash4

Jensen KW and Snekvik E (1972) lsquoLow pH levelswipe out salmon and trout populations insouthernmost Norwayrsquo Ambio 1223ndash5

Johnson AH and Siccama TG (1983) lsquoAciddeposition and forest declinersquo EnvironmentalScience and Technology 17294ndash305

Johnson AH Siccama TG Silver WL and BattlesJJ (1989) lsquoDecline of red spruce in highelevationforests of New York and New Englandrsquo inDCAdriano and MHavas (eds) AcidicPrecipitation Volume 1 Case Studies New YorkSpringer-Verlag

Johnson DW Kilsby CG McKenna DSSaunders RW Jenkins GJ Smith FB and FootJS (1991) lsquoAirborne observations of the physicaland chemical characteristics of the Kuwait oilsmoke plumersquo Nature 353617ndash21

Johnson FS (1972) lsquoOzone and SSTsrsquo BiologicalConservation 4220ndash2

Johnston HS (1971) lsquoReduction of stratosphericozone by nitrogen oxide catalysts from SSTexhaustrsquo Science 173517ndash22

Jones MDH and Henderson-Sellers A (1990)

BIBLIOGRAPHY 209

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Kamara SI (1986) lsquoThe origins and types of rainfallin West Africarsquo Weather 4148ndash56

Karl TR and Quayle RG (1981) lsquoThe 1980 summerheat and drought in historical perspectiversquo MonthlyWeather Review 1092055

Katz RW and Glantz MH (1977) lsquoRainfallstatistics droughts and desertification in the Sahelrsquoin MHGlantz (ed) Desertification EnvironmentalDegradation in and around Arid Lands BoulderColorado Westview Press

Keepin W Mintzer I and Kristoferson L (1986)lsquoEmission of CO

2 into the atmospherersquo in B Bolin

BRDoos JJager and RAWarrick (eds) TheGreenhouse Effect Climatic Change andEcosystems SCOPE 29 New York Wiley

Kellogg WW (1980) lsquoAerosols and climatersquo in WBach JPankrath and JWilliams (eds) Interactionsof Energy and Climate Dordrecht Reidel

mdashmdash(1987) lsquoMankindrsquos impact on climate theevolution of an awarenessrsquo Climatic Change 10113ndash36

Kellogg WW and Schneider SH (1977) lsquoClimatedesertification and human activitiesrsquo in MHGlantz (ed) Desertification EnvironmentalDegradation in and around Arid Lands BoulderColorado Westview Press

Kemp DD (1982) lsquoThe drought of 1804ndash1805 inCentral North Americarsquo Weather 3734ndash40

mdashmdash(1991) lsquoThe greenhouse effect a Canadianperspectiversquo Geography 76121ndash31

Kerr JB Wardle DI and Tarasick DW (1993)lsquoRecord low ozone values over Canada in early1993rsquo Geophysical Research Letters 201979ndash82

Kerr RA (1989) lsquoGreenhouse skeptic out in the coldrsquoScience 2461118ndash19

Keys JG Johnston PV Blatherwick RD andMurcray FJ (1993) lsquoEvidence for heterogeneousreactions in the Antarctic autumn stratospherersquoNature 36149ndash51

Kheshgi HS and White BS (1993) lsquoDoes recentglobal warming suggest an enhanced greenhouseeffectrsquo Climatic Change 22121ndash39

Kiernan V (1993) lsquoAtmospheric ozone hits a newlowrsquo New Scientist 138 (1871) 8

Kleinbach MH and Salvagin CE (1986) EnergyTechnologies and Conversion Systems EnglewoodCliffs Prentice Hall

Komhyr WD Grass RD and Leonard RK (1986)lsquoTotal ozone decrease at the South Pole 1964ndash1985rsquo Geophysical Research Letters 13 1248ndash51

Krug EG and Frink CR (1983) lsquoAcid rain in acidsoilmdasha new perspectiversquo Science 221520ndash5

Kyte WS (1981) lsquoSome chemical and chemicalengineering aspects of flue gas desulphurizationrsquo

Transactions of the Institution of ChemicalEngineers 59219ndash29

mdashmdash(1986a) lsquoSome aspects of possible controltechnologies for coal-fired power stations in theUnited Kingdomrsquo Mine and Quarry 1526ndash9

mdashmdash(1986b) lsquoPossible emissions control technologiesfor coal-fired power stationsmdashthe UnitedKingdomrsquo paper presented at the Institution ofChemical Engineers Conference on lsquoThe Problemof Acid Emissionrsquo 22ndash23 September 1986University of Birmingham

mdashmdash(1988) lsquoA programme for reducing SO2 emissions

from UKpower stationsmdashpresent and futurersquoCEGB Corporate Environment Unit Paper 1ndash2

LaBastille A (1981) lsquoAcid rainmdashhow great amenacersquo National Geographic 160652ndash81

Lacis A Hansen J and Sato M (1992) lsquoClimateforcing by stratospheric aerosolsrsquo GeophysicalResearch Letters 191607ndash10

Lamb HH (1970) lsquoVolcanic dust in the atmospherewith a chronology and assessment of itsmeteorological significancersquo PhilosophicalTransactions of the Royal Society A 266435ndash533

mdashmdash(1972) Climate Present Past and Future VolumeI Fundamentals and Climate Now LondonMethuen

mdashmdash(1977) Climate Present Past and Future Volume2 Climatic History and the Future LondonMethuen

Landsberg HH (1986) lsquoPotentialities and limitationsof conventional climatological data fordesertification monitoring and controlrsquo ClimaticChange 9123ndash8

Last FT (1989) lsquoAcidic deposition case studyScotlandrsquo in DCAdriano and MHavas (eds)Acidic Precipitation Volume 1 Case Studies NewYork Springer-Verlag

Last FT and Nicholson IA (1982) lsquoAcid rainrsquoBiologist 29250ndash2

Lawson MP and Stockton C (1981) lsquoThe desertmyth evaluated in the context of climatic changersquoin CDSmith and MParry (eds) Consequences ofClimatic Change Nottingham University ofNottingham

Le Houerou HN (1977) lsquoThe nature and causes ofdesertizationrsquo in MHGlantz (ed) DesertificationEnvironmental Degradation in and around AridLands Boulder Colorado Westview Press

Leggett J (1992) lsquoRunning down to Riorsquo NewScientist 134 (1819)38ndash42 Legrand M andDelmas RJ (1987) lsquoA 220-year continuous recordof volcanic H

4 in the Antarctic ice sheetrsquo Nature

327671ndash6Leighton PA (1966) lsquoGeographical aspects of air

pollutionrsquo Geographical Review 56151ndash74

BIBLIOGRAPHY210

Lemonick MD (1987) lsquoThe heat is onrsquo Time 13062ndash75

Levenson T (1989) Ice Time Climate Science andLife on Earth New York Harper amp Row

Lippmann M (1986) lsquoThe effects of inhaled acid onhuman healthrsquo in Conference ProceedingsIntergovernmental Conference on Acid RainQuebec April 10 11 and 12 1985 Quebec CityMinistry of the Environment

Lockwood JG (1979) Causes of Climate LondonEdward Arnold

mdashmdash(1984) lsquoThe Southern Oscillation and El NintildeorsquoProgress in Physical Geography 8102ndash10

mdashmdash(1986) lsquoThe causes of drought with particularreference to the Sahelrsquo Progress in PhysicalGeography 10110ndash19

Lourenz RS and Abe K (1983) lsquoA dust storm overMelbournersquo Weather 38272ndash4

Lovelock JE (1972) lsquoGaia as seen through theatmospherersquo Atmospheric Environment 6579ndash80

mdashmdash(1986) lsquoGaia the world as living organismrsquo NewScientist 112 (1539)25ndash8

Lovelock JE and Epton S (1975) lsquoThe quest forGaiarsquo New Scientist 65 (939)304ndash6

Lovelock JE and Margulis L (1973) lsquoAtmospherichomeostasis by and for the biosphere the Gaiahypothesisrsquo Tellus 262

Ludlum DM (1971) Weather Record BookPrinceton New Jersey Weatherwise Inc

Lutgens FK and Tarbuck EJ (1982) TheAtmosphere Englewood Cliffs Prentice-Hall

McCarthey JJ Brewer PG and Feldman G (1986)lsquoGlobal ocean fluxrsquo Oceanus 2916ndash26

McCormick MP (1992) lsquoInitial assessment of thestratospheric and climatic impact of the 1991Mount Pinatubo eruptionrsquo Geophysical ResearchLetters 1949

McCormick RA and Ludwig JL (1967) lsquoClimatemodification by atmospheric aerosolsrsquo Science1561358ndash9

MacCracken MC and Luther FM (1985a)Detecting the Climatic Effects of Increasing CarbonDioxide Washington DC US Department ofEnergy

MacCracken MC and Luther FM (1985b)Projecting the Climatic Effects of IncreasingCarbon Dioxide Washington DC US Departmentof Energy

Mackay GA and Hengeveld HG (1990) lsquoThechanging atmospherersquo in CMungall and DJMacLaren (eds) Planet under Stress TorontoOxford University Press

McKay GA Maybank J Mooney OR and PeltonWL (1967) The Agricultural Climate ofSaskatchewan Climatological Studies No 10

Toronto Canada Dept of TransportMeteorological Branch

MacKenzie D (1987a) lsquoEthiopia plunges towardsanother faminersquo New Scientist 115 (1573)26

mdashmdash(1987b) lsquoCan Ethiopia be savedrsquo New Scientist115 (1579)54ndash8

mdashmdash(1992) lsquoAgreement reduces damage to ozonelayerrsquo New Scientist 136 (1850)10

Mackie R Hunter JAA Aitchison TC Hole DMcLaren K Rankin R Blessing K Evans ATHutcheon AW Jones DH Soutar DS WatsonACH Cornbleet MA and Smith JF (1992)lsquoCutaneous malignant melanoma Scotland1979ndash89rsquo The Lancet 339971ndash5

Maddox J (1984) lsquoNuclear winter not yetestablishedrsquo Nature 30811

Manabe S and Wetherald RT (1975) lsquoThe effectsof doubling the CO

2 concentration on the climate

of a general circulation modelrsquo Journal ofAtmospheric Science 323ndash15

Manabe S and Wetherald RT (1986) lsquoReductionin summer soil wetness induced by an increase inatmospheric carbon dioxidersquo Science 232626ndash8

Manabe S Wetherald RT and Stouffer RJ (1981)lsquoSummer dryness due to an increase of atmosphericCO

2 concentrationrsquo Climatic Change 3347ndash86

Marcus AA and Rands GP (1992) lsquoChangingperspectives on the environmentrsquo in RA BuchholzAAMarcus and JE Post (eds) ManagingEnvironmental Issues A Casebook EnglewoodCliffs Prentice-Hall

Martec Ltd (1987) Effects of a One metre Rise in SeaLevel at St John New Brunswick and the LowerReaches of the St John River CCD 87ndash04 OttawaAtmospheric Environment Service

de Martonne E (1926) lsquoUne nouvelle fonctionclimatologique lrsquoindice drsquoariditeacutersquo Meacuteteacuteorologie 2449ndash59

Mason BJ (1990) lsquoAcid rainmdashcause andconsequencersquo Weather 4570ndash9

Meinl H Bach W Jager J Jung HJ KnottenbergH Marr G Santer BD and Schwieren G (1984)The Socio-Economic Impacts of Climatic Changesdue to a Doubling of Atmospheric CO

2 Content

Brussels Commission of European CommunitiesReport

Meko DM (1992) lsquoDendroclimatic evidence fromthe Great Plains of the United Statesrsquo in RSBradley and PDJones (eds) Climate since AD 1500London Routledge

Melillo JM Callaghan TV Woodward FI SalatiE and Sinha SK (1990) lsquoEffects of ecosystemsrsquoin JTHoughton GJJenkins and JJEphraums(eds) Climate Change The IPCC ScientificAssessment Cambridge Cambridge UniversityPress

BIBLIOGRAPHY 211

Miller JM (1984) lsquoAcid rainrsquo Weatherwise 37234ndash9

Miller M (1992) lsquoGetting grounded at RiorsquoEcodecision June 55ndash7

Mintzer IM (1992) lsquoNational energy strategies andthe greenhouse problemrsquo in LRosen and R Glasser(eds) Climate Change and Energy Policy NewYork American Institute of Physics

Mitchell JFB (1983) lsquoThe seasonal response of ageneral circulation model to changes in CO

sea temperaturersquo Quarterly Journal of the RoyalMeteorological Society 109113ndash52

Mitchell JFB Manabe S Tokioka T andMeleshko V (1990) lsquoEquilibrium climate changersquoin JTHoughton GJJenkins and JJ Ephraums(eds) Climate Change The IPCC ScientificAssessment Cambridge Cambridge UniversityPress

Mitchell JM (1970) lsquoA preliminary evaluation ofatmospheric pollution as a cause of globaltemperature fluctuations of the past centuryrsquo inSFSinger (ed) Global Effects of EnvironmentalPollution Dordrecht Reidel

mdashmdash(1975) lsquoA reassessment of atmospheric pollutionas a cause of long-term changes of globaltemperaturersquo in SFSinger (ed) The ChangingGlobal Environment Boston Reidel

Molina MJ and Rowland FS (1974) lsquoStratosphericsink for chlorofluoromethanes chlorine atom-catalysed destruction of ozonersquo Nature 249 810ndash12

Mooley DA and Parthasarathy B (1983) lsquoIndiansummer monsoon and El Nintildeorsquo Pure and AppliedGeophysics 121339ndash52

Moore B and Bolin B (1986) lsquoThe oceans carbondioxide and global climate changersquo Oceanus 299ndash15

Morales C (1986) lsquoThe airborne transport of Saharandust a reviewrsquo Climatic Change 9219ndash42

More RJ (1969) lsquoWater and cropsrsquo inRJChorley (ed) Water Earth and ManLondon Methuen

Mortimer M (1989) Adapting to Drought FarmersFamines and Desertification in West AfricaCambridge Cambridge University Press

Mossop SC (1964) lsquoVolcanic dust collected at analtitude of 20 kmrsquo Nature 203824ndash7

Murphey R (1973) The Scope of GeographyChicago Rand McNally

Musk L (1983) lsquoOutlook-changeablersquo GeographicalMagazine 55532ndash3

NCGCC (1990) An Assessment of the Impact ofClimate Change Caused by Human Activities onChinarsquos Environment Beijing NationalCoordinating Group on Climate Change

Nelson R (1990) Dryland management the

lsquodesertificationrsquo problem World Bank TechnicalPaper No 16 Washington DC World Bank

Newell RE and Dopplick TG (1979) lsquoQuestionsconcerning the possible influence of anthropogenicCO

2 on atmospheric temperaturersquo Journal of

Applied Meteorology 18822ndash5Newhall GG and Self S (1982) lsquoThe volcanic

explosivity index (VEI) an estimate of the explosivemagnitude for historical volcanismrsquo Journal ofGeophysical Research 871231ndash8

Nicholson SE (1981) lsquoThe historical climatology ofAfricarsquo in TMLWigley MJIngram and GFarmer (eds) Climate and History CambridgeCambridge University Press

mdashmdash(1989) lsquoLong-term changes in African rainfallrsquoWeather 4447ndash56

Norton P (1985) lsquoDecline and Fallrsquo Harrowsmith 924ndash43

NRC (National Research Council) (1982) CarbonDioxide and Climate A Second AssessmentWashington DC National Academy Press

mdashmdash(1983) Changing Climate Washington DCNational Academy Press

Oguntoyinbo J (1986) lsquoDrought predictionrsquo ClimaticChange 979ndash90

Okada K Ikegami M Uchino O Nikaidou YZaizen Y Tsutsumi Y and Makino Y (1992)lsquoExtremely high proportions of soot particles inthe upper troposphere over Japanrsquo GeophysicalResearch Letters 19921ndash4

Ontario Ministry of the Environment (1980) The CaseAgainst the Rain Toronto Information ServicesBranch Ministry of the Environment

Owen JA and Ward MN (1989) lsquoForecasting Sahelrainfallrsquo Weather 4457ndash64

Palmer WC (1965) Meteorological DroughtResearch Paper 45 Washington DC US WeatherBureau

Palutikof JP (1986) lsquoDrought strategies in EastAfrica the climatologistrsquos rolersquo Climatic Change967ndash78

Park CC (1987) Acid Rain Rhetoric and RealityLondon Methuen

mdashmdash(1991) lsquoTrans-frontier air pollution somegeographical issuesrsquo Geography 7621ndash35

Parry ML (1978) Climatic Change Agriculture andSettlement Folkestone Dawson

Parson EA Hass PM and Levy MA (1992) lsquoAsummary of the major documents signed at theearth summit and the global forumrsquo Environment3412ndash15 and 34ndash6

Pearce F (1982a) lsquoWarning cones hoisted as acidrain-clouds gatherrsquo New Scientist 94(1311)828

mdashmdash(1982b) lsquoScience and politics donrsquot mix at acidrain debatersquo New Scientist 95 (1312)3

BIBLIOGRAPHY212

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mdashmdash(1982d) lsquoThe menace of acid rainrsquo New Scientist95 (1318)419ndash23

mdashmdash(1984) lsquoMPs back curbs on acid rainrsquo NewScientist 103(1420)3

mdashmdash(1990) lsquoWhatever happened to acid rainrsquo NewScientist 124 (1736)57ndash60

mdashmdash(199la) lsquoDesert fires cast a shadow over AsiarsquoNew Scientist 129 (1754)30ndash1

mdashmdash(1991b) lsquoA sea change in the Sahelrsquo New Scientist130 (1757)31ndash2

mdashmdash(1992a) lsquoLast chance to save the planetrsquo NewScientist 134 (1823)24ndash8

mdashmdash(1992b) lsquoDespondency descends on Riorsquo NewScientist 134 (1824)4

mdashmdash(1992c) lsquoHow green was our summitrsquo NewScientist 134 (1827)12ndash13

mdashmdash(1992d) lsquoGrain yields tumble in a greenhouseworldrsquo New Scientist 134 (1817)4

mdashmdash(1992e) lsquoWill Britain fail the acid testrsquo NewScientist 136 (1850)11

mdashmdash(1992f) lsquoMiracle of the Shifting Sandsrsquo NewScientist 136 (1851)38ndash42

Peacutegorieacute J (1990) lsquoOn-farm agroforestry research casestudy from Kenyarsquos semi-arid zonersquo AgroforestryToday 24ndash7

Peterson JT and Junge CE (1971) lsquoSources ofparticulate matter in the atmospherersquo in WHMatthews WWKellogg and GDRobinson (eds)Manrsquos Impact on the Climate Cambridge MassMITPress

Phillips DW (1982) Climatic Anomalies and UnusualWeather in Canada during 1981 DownsviewOntario Atmospheric Environment Service

Piette J (1986) lsquoLes initiatives des provinces de lrsquoEstdu Canada en matiere de precipitations acidesrsquo inConference Proceedings IntergovernmentalConference on Acid Rain Quebec April 10 11and 12 1985 Quebec City Ministry of theEnvironment

Pisias NG and Imbrie J (1986) lsquoVariations in theearthrsquos orbit influenced past climate changesrsquoOceanus 2943ndash9

Pitelka LF and Raynal DJ (1989) lsquoForest declineand acidic precipitationrsquo Ecology 702ndash10

Pittock AB and Salinger MJ (1982) lsquoTowardsregional scenarios for a CO

2-warmed earthrsquo

Climatic Change 423ndash40Pittock AB and Salinger MJ (1991) lsquoSouthern

hemisphere climate scenariosrsquo Climatic Change18205ndash22

Pollack JB and Ackerman TP (1983) lsquoPossibleeffects of the El Chichoacuten volcanic cloud on theradiation budget of the northern tropicsrsquoGeophysical Research Letters 101057ndash60

Ponte L (1976) The Cooling Englewood CliffsPrentice-Hall

Porcella DB Schefield CL Depinto JV DriscollCT Bukaveckas PA Gloss SP and Young TC(1990) lsquoMitigation of acidic conditions in lakes andstreamsrsquo in SANorton SELindberg and ALPage(eds) Acidic Precipitation Volume 4 Soils AquaticProcesses and Lake Acidification New YorkSpringer-Verlag

Pueschel RF Blake DF Snetsinger KG HansenADA Verma S and Kato K (1992) lsquoBlackcarbon (soot) aerosol in the lower stratosphere andupper tropospherersquo Geophysical Research Letters191659ndash62

Pyle J (1991) lsquoClosing in on Arctic ozonersquo NewScientist 132 (1794)49ndash52

Quinn WH and Neal VT (1992) lsquoThe historicalrecord of El Nintildeo eventsrsquo in RSBradley and PDJones (eds) Climate since AD 1500 LondonRoutledge

Ramage J (1983) Energy A Guidebook OxfordOxford University Press

Ramanathan V (1975) lsquoGreenhouse effect due tochlorofluorocarbons climatic implicationsrsquo Science19050ndash1

mdashmdash(1988) lsquoThe greenhouse theory of climate changea test by inadvertent global experimentrsquo Science240293ndash9

Ramanathan V Callis LB and Boughner RE(1976) lsquoSensitivity of surface temperature andatmospheric temperature to perturbations in thestratospheric concentration of ozone and nitrogendioxidersquo Journal of Atmospheric Science 331092ndash1112

Ramanathan V Pitcher GJ Malone RC andBlackmon ML (1983) lsquoThe response of a spectralGCM to refinements in radiative processesrsquo journalof Atmospheric Science 40 605ndash30

Ramanathan V Singh HB Cicerone RJ and KiehlJT (1985) lsquoTrace gas trends and their potentialrole in climate changersquo Journal of GeophysicalResearch 905547ndash56

Rampino MR and Self S (1984) lsquoThe atmosphericeffects of El Chichoacutenrsquo Scientific American 250 48ndash57

Rasmusson EM and Hall JM (1983) lsquoEl NintildeorsquoWeatherwise 36166ndash75

Revelle R and Seuss HE (1957) lsquoCarbondioxide exchange between the atmosphere andocean and the question of an increase ofatmospheric CO

2 during the past decadesrsquo

Tellus 918ndash27Ridley M (1993) lsquoCleaning up with cheap

technologyrsquo New Scientist 137 (1857)26ndash7Riefler RF (1978) lsquoDrought an economic

perspectiversquo in NJRosenberg (ed) North

BIBLIOGRAPHY 213

American Droughts Boulder Colorado WestviewPress

Roberts L (1989) lsquoGlobal warming blaming the sunrsquoScience 246992ndash3

Robitaille L (1986) lsquoLe deacuteperissement des eacuterabliegraveresau Queacutebec probleacutematique et eacutetat des recherchesrsquoin Conference Proceedings IntergovernmentalConference on Acid Rain Quebec April 10 11and 12 1985 Quebec City Ministry of theEnvironment

Robock A (1981) lsquoA latitudinally dependent volcanicdust veil index and its effect on climate simulationsrsquoJournal of Vulcanological and GeothermalResearch 1167ndash80

Rodriguez JM (1993) lsquoProbing atmospheric ozonersquoScience 2611128ndash9

Rosen JM Kjome NT and Fast H (1992)lsquoPenetration of Mt Pinatubo aerosols into thenorth polar vortexrsquo Geophysical Research Letters191751ndash4

Rosenberg NJ (ed) (1978) North AmericanDroughts Boulder Colorado Westview Press

Rosenthal H and Wilson JS (1987) An UpdatedBibliography (1945ndash1986) on Ozone its BiologicalEffects and Technical Applications Halifax NovaScotia Ministry of Supply and Services Canada

Rowntree PR (1990) lsquoEstimates of future climatechange over Britainrsquo Weather 4538ndash42

Ruderman MA (1974) lsquoPossible consequences ofnearby supernova explosions for atmosphericozone and terrestrial lifersquo Science 1841079ndash81

Sage B (1980) lsquoAcid drops from fossil fuelsrsquo NewScientist 85 (1197)743ndash5

Salinger MJ and Pittock AB (1991) lsquoClimatescenarios for 2010 and 2050 AD Australia andNew Zealandrsquo Climatic Change 18259ndash69

Sanders DW (1986) lsquoDesertification processes andimpact in rainfed agricultural regionsrsquo ClimaticChange 933ndash42

Sanderson M (1987) Implications of Climatic Changefor Navigation and Power Generation in the GreatLakes CCD 87ndash03 Ottawa AtmosphericEnvironment Service

Santer B (1985) lsquoThe use of general circulation modelsin climate impact analysismdasha preliminary study ofthe impact of CO

2 -induced climatic change on West

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Environment Study of Critical EnvironmentalProblems Cambridge Mass MITPress

Scheider W Adamski J and Paylor M (1975)Reclamation of Acidified Lakes near SudburyOntario Toronto Ontario Ministry of theEnvironment

Schindler DW and Bayley SE (1990) lsquoFresh watersin cyclersquo in CMungall and DJMcLaren (eds)

Planet under Stress Toronto Oxford UniversityPress

Schneider SH (1978) lsquoForecasting future droughtsis it possiblersquo in NJRosenberg (ed) NorthAmerican Droughts Boulder Colorado WestviewPress

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mdashmdash(1987) lsquoClimatic modelingrsquo Scientific American25672ndash89

Schneider SH and Mass C (1975) lsquoVolcanic dustsunspots and temperature trendsrsquo Science 190741ndash6

Schneider SH and Mesirow L (1976) The GenesisStrategy New York Plenum Press

Schneider SH and Thompson SL (1981)lsquoAtmospheric CO

2 and climate importance of the

transient responsersquo Journal of GeophysicalResearch 863135ndash47

Schoeberl MR and Kreuger AJ (1986) lsquoOverviewof the Antarctic ozone depletion issuersquo GeophysicalResearch Letters 131191ndash2

SCOPE-ENUWAR (1987) lsquoEnvironmentalconsequences of nuclear war an updatersquoEnvironment 294ndash5 and 45

Seager J (1991) lsquoOperation Desert DisasterrsquoEcodecision September 42ndash6

Seckmeyer G and McKenzie RL (1992) lsquoIncreasedultraviolet radiation in New Zealand (45degS) relativeto Germany (48degN) Nature 359135

Sellers WD (1973) lsquoA new global climatic modelrsquoJournal of Applied Meteorology 12241ndash54

Semazzi FHM Melita V and Sud YC (1988) lsquoAninvestigation of the relationship between sub-Saharan rainfall and global sea-surfacetemperaturesrsquo Atmosphere-Ocean 26118ndash38

Shaw GE (1980) lsquoArctic Hazersquo Weatherwise 33219ndash21

Shaw RW (1987) lsquoAir pollution by particlesrsquoScientific American 25796ndash103

Shaw WS (1992) lsquoSmoke at Bahrain during theKuwaiti oil firesrsquo Weather 47220ndash6

Shine K (1988) lsquoAntarctic OzonemdashAn extendedmeeting reportrsquo Weather 43208ndash10

Shine K Derwent RG Wuebbles DJ andMorcrette JJ (1990) lsquoRadiative forcing of climatersquoin JTHoughton GJJenkins and JJ Ephraums(eds) Climate Change The IPCC ScientificAssessment Cambridge Cambridge UniversityPress

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BIBLIOGRAPHY214

Shugart HH Antonovsky PG Jarvis PG andSandford AP (1986) lsquoCO

2 climatic change and

forest ecosystemsrsquo in BBolin BRDoos JJager andRAWarrick (eds) The Greenhouse EffectClimatic Change and Ecosystems SCOPE 29 NewYork Wiley

Siegenthaler U (1984) lsquo19th century measurementsof atmospheric CO

2mdasha commentrsquo Climatic

Change 6409ndash11Simons P (1992) lsquoWhy global warming could take

Britain by stormrsquo New Scientist 136 (1846)35ndash8

Singer SF (1984) lsquoIs the ldquonuclear winterrdquo realrsquoNature 310625

Singh B (1988) The Implications of Climate Changefor Natural Resources in Quebec CCD 88ndash08Ottawa Atmospheric Environment Service

Slade DH (1990) lsquoA survey of informed opinionregarding the nature and reality of ldquoGlobalGreenhouse Warmingrdquorsquo Climatic Change 161ndash4

SMIC (1971) Inadvertent Climate ModificationReport of the Study of Manrsquos Impact on ClimateCambridge Mass MITPress

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Smith JW (1920) lsquoRainfall of the Great Plains inrelation to cultivationrsquo Annals of the Associationof American Geographers 1069ndash74

Smith TM and Shugart HH (1993) lsquoThe transientresponse of terrestrial carbon storage to a perturbedclimatersquo Nature 361523ndash6

Smith ZA (1992) The Environmental PolicyParadox Englewood Cliffs Prentice-Hall

Spry IM (1963) The Palliser Expedition TorontoMacmillan

Starr VP (1956) lsquoThe general circulation of theatmospherersquo Scientific American 19540ndash5

Steila D (1976) The Geography of Soils EnglewoodCliffs Prentice-Hall

Stevenson AC (1993) lsquoEnvironmental acidificationa review of surface water acidification during 1991rsquoProgress in Physical Geography 1769ndash75

Stewart J and Tiessen H (1990) lsquoGrasslands intodesertsrsquo in CMungall and DJMcLaren (eds)Planet under Stress Toronto Oxford UniversityPress

Stokes PM Howell ET and Krantzberg G (1989)lsquoEffects of acidic precipitation on the biota offreshwater lakesrsquo in DCAdriano and AHJohnson (eds) Acidic Precipitation Volume 2Biological and Ecological Effects New YorkSpringer-Verlag

Stokoe P (1988) Socio-economic Assessment of thePhysical and Ecological Impacts of Climate Changeon the Marine Environment of the Atlantic Region

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Stolarski RS and Cicerone RJ (1974) lsquoStratosphericchlorine Possible sink for ozonersquo Canadian journalof Chemistry 521610ndash15

Stolarski RS Krueger AJ Schoeberl MRMcPeters RD Newman PA and Alpert JC(1986) lsquoNimbus 7 satellite measurements of thespringtime Antarctic ozone decreasersquo Nature 322808ndash11

Strain BR and Cure JD (eds) (1985) Direct Effectsof Increasing Carbon Dioxide on VegetationWashington DC US Department of Energy

Stuiver M (1987) lsquoAtmospheric carbon dioxide andcarbon reservoir changesrsquo Science 199253ndash8

Sveinbjornsson B (1984) lsquoAlaskan plants andatmospheric carbon dioxidersquo in JHMcBeath (ed)The Potential Effects of Carbon DioxidemdashInducedClimatic Change in Alaska ConferenceProceedings Fairbanks School of Agriculture andLand Resources Management University ofAlaska

Sweeney J (1985) lsquoThe 1984 drought on the Canadianprairiesrsquo Weather 40302ndash9

Swift J (1977) lsquoPastoral development in Somaliaherding cooperatives as a strategy againstdesertification and faminersquo in MHGlantz (ed)Desertification Environmental Degradation in andaround Arid Lands Boulder Colorado WestviewPress

Taylor NK (1992) lsquoThe role of the ocean in the globalcarbon cyclersquo Weather 47146ndash50 (Part 1) and47237ndash41 (Part 2)

Tegart WJ Sheldon GW and Griffiths DC (1990)Climate Change The IPCC Impacts AssessmentCanberra Australian Government PublishingService

Teller E (1984) lsquoWidespread after-effects of nuclearwarrsquo Nature 310621ndash4

Thackrey TO (1971) lsquoPittsburgh how one city diditrsquo in RRevelle AKhosla and MVinovskis (eds)The Survival Equation Boston Houghton Mifflin

Thomas G and Henderson-Sellers A (1987)lsquoEvaluation of satellite derived land covercharacteristics for global climate warmingrsquoClimatic Change 11313ndash47

Thompson SL and Schneider SH (1986) lsquoNuclearwinter reappraisedrsquo Foreign Affairs 64981ndash1005

Thornthwaite CW (1931) lsquoThe climate of NorthAmerica according to a new classificationrsquoGeographical Review 21 633ndash55

mdashmdash(1947) lsquoClimate and moisture conservationrsquoAnnals of the Association of AmericanGeographers 3787ndash100

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BIBLIOGRAPHY 215

classification of climatersquo Geographical Review 3855ndash94

Thornton FC Schaedle M and Raynal DJ (1986)lsquoEffect of aluminum on the growth of sugar maplein solution culturersquo Canadian Journal of ForestResearch 16892ndash6

Tickell O (1992) lsquoFire-fighters find gas thatrsquos easyon ozonersquo New Scientist 134 (1818)19

Titus JG (1986) lsquoGreenhouse effect sea level riseand coastal zone managementrsquo Coastal ZoneManagement Journal 14147ndash72

Tomlinson GH (1985) lsquoForest vulnerability and thecumulative effects of acid depositionrsquo in DDAdams and WPPage (eds) Acid Deposition-Environmental Economic and Political Issues NewYork Plenum Press

Toon OB and Pollack JB (1981) lsquoAtmosphericaerosols and climatersquo in BJSkinner (ed) ClimatesPast and Present Los Altos CaliforniaKauffmann

Toro T (1992) lsquoGerman industry freezes out greenfridgersquo New Scientist 135 (1835)16

Trewartha GT and Horn LH (1980) AnIntroduction to Climate New York McGraw-Hill

Tucker A (1987) lsquoOzone hole culprit pin-pointedrsquoManchester Guardian Weekly 137 (8)4

Tullett MT (1984) lsquoSaharan dust-fall in NorthernIrelandrsquo Weather 39151ndash2

Turco RP Toon OB Ackerman TP Pollack JBand Sagan C (1983) lsquoNuclear winter globalconsequences of multiple nuclear explosionsrsquoScience 2221283ndash92

Turco RP Toon OB Ackerman TP Pollack JBand Sagan C (1990) lsquoClimate and smoke anappraisal of nuclear winterrsquo Science 247166ndash76

Turk J (1980) Introduction to Environmental StudiesPhiladelphia Saunders

Turk J and Turk A (1988) Environmental SciencePhiladelphia Saunders

Ulrich B (1983) lsquoA concept of forest ecosystemstability and of acid deposition as driving force fordestabilizationrsquo in BUlrich and JPankrath (eds)Effects of Accumulation of Air Pollutants in ForestEcosystems Dordrecht Netherlands Reidel

mdashmdash(1989) lsquoEffects of acidic precipitation on forestecosystems in Europersquo in DCAdriano andAHJohnson (eds) Acidic Precipitation Volume 2Biological and Ecological Effects New YorkSpringer-Verlag

Ulrich B Mayer R and Khanna TK (1980)lsquoChemical changes due to acid precipitation in aloess-derived soil in central Europersquo Soil Science130193ndash9

Valerie K Delers A Bruck C Thiriart CRosenberg H Debouck C and Rosenberg M(1988) lsquoActivation of human immunodeficiency

virus type 1 by DNA damage in human cellsrsquoNature 33378ndash81

Van Cleve K Oliver L Schleuther R Viereck LAand Dyrness CT (1983) lsquoProductivity and nutrientcycling in taiga forest eco-systemsrsquo CanadianJournal of Forest Resources 13747ndash66

Van Kooten GC Arthur L and Wilson WR (1992)lsquoPotential to sequester carbon in Canadian forestssome economic considerationsrsquo Canadian PublicPolicy 18127ndash38

Van Royen W (1937) lsquoPrehistoric droughts in thecentral Great Plainsrsquo Geographical Review 27637ndash50

van Ypersele JP and Verstraete MM (1986)lsquoClimate and desertificationmdasheditorialrsquo ClimaticChange 91ndash4

Verstraete MM (1986) lsquoDefining desertification areviewrsquo Climatic Change 95ndash18

Viereck LA and Van Cleve K (1984) lsquoSame aspectsof vegetation and temperature relationships in theAlaska Taigarsquo in JHMcBeath (ed) The PotentialEffects of Carbon Dioxide-Induced ClimaticChange in Alaska Conference ProceedingsFairbanks School of Agriculture and LandResources Management University of Alaska

Ware H (1977) lsquoDesertification and Population Sub-Saharan Africarsquo in MHGlantz (ed)Desertification Environmental Degradation in andaround Arid Lands Boulder Colorado WestviewPress

Warren A and Agnew C (1988) An Assessment ofDesertification and Land Degradation in Arid andSemi-arid Areas London Ecology andConservation Unit University College

Warrick RA (1993) lsquoSlowing global warming andsealevel rise the rough road from RiorsquoTransactions of the Institute of British GeographersNS 18140ndash8

Warrick RA and Oerlemans H (1990) lsquoSea levelrisersquo in JTHoughton GJJenkins and JJEphraums (eds) Climate Change The IPCCScientific Assessment Cambridge CambridgeUniversity Press

Washington WM and Parkinson CL (1986) AnIntroduction to Three Dimensional ClimateModeling Mill Valley California UniversityScience Books

Waterstone M (1993) lsquoAdrift in a sea of platitudesWhy we will not resolve the greenhouse issuersquoEnvironmental Management17141ndash52

Watson JW (1963) North America Its Countries andRegions London Longman

Watson RT Rodhe H Oeschger H andSiegenthaler U (1990) lsquoGreenhouse gases andaerosolsrsquo in JTHoughton GJJenkins and JJEphraums (eds) Climate Change The IPCC

BIBLIOGRAPHY216

Scientific Assessment Cambridge CambridgeUniversity Press

Webb RS and Overpeck JT (1993) lsquoCarbon reservesreleasedrsquo Nature 361497ndash8

Webster M (1988) lsquoQueries and quandriesrsquoHarrowsmith 80115ndash16

Webster PJ (1984) lsquoThe carbon dioxideclimatecontroversy some personal comments on tworecent publicationsrsquo Climatic Change 6377ndash90

Weiss H Courty MA Wetterstrom W GuichardF Senior L Meadow R and Curnow A (1993)lsquoThe genesis and collapse of Third Millenium northMesopotamian civilizationrsquo Science 261995ndash1004

Wheaton EE Singh T Dempster RHigginbotham KO Thorpe JP Van KootenGC and Taylor JS (1989) An Exploration andAssessment of the Implications of Climate Changefor the Boreal Forest and Forestry Economics ofthe Prairie Provinces and Northwest Territoriesphase one CCD 89ndash02 Ottawa AtmosphericEnvironment Service

Wigley TML and Barnett TP (1990) lsquoDetection ofthe greenhouse effect in the observationsrsquo inJTHoughton GJJenkins and JJEphraums (eds)Climate Change The IPCC Scientific Assessment Cambridge Cambridge University Press

Wigley TML Jones PD and Kelly PM (1986)lsquoEmpirical climate studiesrsquo in BBolin BRDoosJJager and RAWarrick (eds) The Greenhouse

Effect Climatic Change and Ecosystems SCOPE29 New York Wiley

Williams GDV Fantley RA Jones KH StewartRB and Wheaton EE (1988) Estimating Effectsof Climatic Change on Agriculture in SaskatchewanCCD 88ndash06 Ottawa Atmospheric EnvironmentService

Williamson SJ (1973) Fundamentals of AirPollution Reading Mass Addison-Wesley

Wilson AT (1978) lsquoPioneer agricultural explosionof CO

2 levels in the atmospherersquo Nature 273

40ndash1Wilson CA and Mitchell JFB (1987) lsquoSimulated

climate and CO2-induced climate change over

western Europersquo Climatic Change 1011ndash42Wittwer S (1984) lsquoThe rising level of atmospheric

carbon dioxide an agricultural perspectiversquo in JHMcBeath (ed) The Potential Effects of CarbonDioxide-Induced Climatic Change in AlaskaConference Proceedings Fairbanks School ofAgriculture and Land Resources ManagementUniversity of Alaska

Wofsy SC McElroy MB and Sze ND (1975)lsquoFreon consumption implications for atmosphericozonersquo Science 187535ndash7

World Resources Institute (1992) World Resources1992ndash93 A Guide to the Global Environment NewYork Oxford University Press

acid gases 16ndash17 control technology 92ndash4 mainexporters 96ndash9 main producers 71 74ndash8 95ndash6reduction at source 92 SO

x v NO

acid lakes 79ndash84 aluminium toxicity 81ndash2 pHlevels 81 treatment with lime 91ndash2

acid precipitation see acid rainacid rain 1 10 16ndash17 70ndash99 176 aquatic

environment 79ndash84 Britain 96ndash9 builtenvironment 88ndash9 Canada 96 contribution ofheavy metals 73 damage to cultural properties88ndash9 dry deposition 70 89 economics 95ndash6extremes 73 geography 74ndash8 geology 78ndash9human health 89ndash90 natural sources 70 natureand development 70ndash4 politics 95ndash9 reductioncosts 98 Scandinavia 96ndash9 solutions 90ndash5terrestrial environment 84ndash8 Third World 76United States 96 wet deposition 70 89

acid sensitive diatom species 79acid soils 84ndash6actual evapotranspiration 47actuarial forecasts 65Adirondack Mountains USA 87Advisory Group on Greenhouse Gases (AGGG) 151aerosols 19ndash20 100ndash5 114 115 anthropogenic

sources 110ndash14 classification 101ndash3 coolingability 118ndash19 distribution 100 globalproduction 100 natural sources 100ndash1 opticalproperties 103ndash4 organic sources 101 primary103 radiation effects 103ndash5 residence time 101secondary 103 stratospheric 100 104 108tropospheric 101 118 types 100 warmingability 119ndash20

Africa 40 42 48 65 67 68 desertification 59 6162ndash5 drought and famine 48ndash54 populationpressures 9 rural depopulation 65

Agenda 213agricultural drought 41air pollution 11 12ndash13 19 70ndash1 73 100 113ndash14

potential for global cooling 118 173monitoring 120

airstream tracers 78Alaska 118 162 164albedo 22 118

Alberta Canada 118Alliance of Small Island States (AOSIS) 167Alps 111alternative energy sources 181Anaconda Montana USA 71Anglo-French Concorde 128 129anions 73 86Antarctic 9 circumpolar vortex 114 135 ozone

hole 133ndash6 turbidity 113 114anticyclonic subsidence 48 67aquatic ecosystems 17Arctic 9 105 151 acidic pollution 78 human

impact 9 ozone depletion 135ndash6 turbidity122ndash3 volcanic activity 106

Arctic Haze 19 78 components 122ndash3 sources 78Arctic jet 30Argentina 134ArrheniusS 150 160aridity 41ndash2 relationship to drought 41Atlantic Ocean 50atmosphere 14ndash39 aerosols 19ndash20 background

pollution levels 12 circulation patterns 26ndash32composition 14 function 14 gases 14ndash17human impact 9ndash10 minor gases 16 nitrogen14 oxides of nitrogen 16ndash17 oxides of sulphur16ndash17 oxygen 15ndash16 water 17ndash19 verticalstructure 20ndash1

atmospheric circulation 26ndash32 energy transfer 25Hadley model 26ndash8 influence of land and sea30ndash2 jet streams 29ndash30 seasonal variations30ndash2 three-cell model 28

atmospheric effect 146atmospheric models 32ndash9 weather forecasting

models 33ndash4atmospheric moisture 17ndash19atmospheric turbidity 19 100ndash20 role in

atmospheric cooling 19 118ndash20 role inatmospheric warming 20 118ndash20

atmospheric window 22atomic oxygen 15 122atomic tests 127ndash8Australia 25 32 34 105 107 deforestation 149

desertification 59 drought 43 48 59 68 dust

INDEX218

storms 101 global warming 159 164 165 167ozone depletion 138 139 177 turbidity 101

autovariations 32Azobacter 14 Bach W 1Baden-Wurttemburg Germany 87Bangladesh 68 166Bavaria 87Bay of Bengal 9Beijing China 42 dust storms 101Belgium 166Bezymianny (Kamchatka) Russia 105Biodiversity Convention 3biogeophysical feedback mechanism 67Black Forest Germany 87Blue Mountains Virginia USA 101Boeing SST 128 129boreal forest 163 impact of global warming 163ndash4

insect infestation 164Bormann H 79Brazil 68 149Britain acid rain 70 71 72 73 74 75 90 92

96ndash9 air pollution 12 74 Clean Air Acts 7489 drought 44 46 flooding 166 167 globalwarming 150 167

British Antarctic Survey 133bromofluorocarbons 129Brundtland Commission 3BrysonR 42 118buffering of acidity artificial 90ndash2 natural 78ndash9Burkina Faso 48lsquobusiness-as-usualrsquo scenario 156 178 calcium carbonate 88 91calcium hydroxide 91calcium sulphate 88California USA 12Cameroon 48Canada 30 54 56 acid rain 71 72 73 74 78 79

83 86ndash7 89 96 boreal forest 163 drought 56global warming 158 163 164 165 166 168ozone depletion 132 134 135 136 141 sea-level change 166 urban air pollution 12

Canadian Atmospheric Environment Service 136178

Canadian Climate Centre 158Canterbury Cathedral England 89carbon cycle 6 146ndash50 carbon sinks and sources

147 greenhouse effect 146ndash50 models 36 roleof oceans 26 150

carbon dioxide (CO2) 16 146 149 agriculture 162

atmospheric levels 16 153ndash6 Cretaceousconcentration 153 human contribution 6 149171 Quaternary concentration 150 role in

photosynthesis 16 149 temperature change153ndash9

carbon tax 180carbon tetrachloride 136 141Caribbean 9 101Carpenter R 44carrying capacity of land 63catalyst 73 122ndash4 in formation of acid rain 73catalytic chain reactions 122 132cataracts 139cations 73 86Caucasus Mountains 19 111 112Central African Republic 48central Asian republics (ex-USSR) 59Central Electricity Generating Board (CEGB) 72

74 97CFCs see chlorofluorocarbonsChad 48 61Chamberlin TC 150Chapman S 122Charney J 67Chartres Cathedral France 89chemical models see computerized modellingchemical weathering 88ndash9Chernobyl Ukraine 30 92Chile 134China 30 118 176 acid rain 75 atmospheric

turbidity 101 desertification 59 drought 42 43dust storms 101 global warming 159 160 162165 166 methane production 159 160 sea-level change 166

chlorine 127 132 134chlorine monoxide 127 132chlorine oxides 127chlorofluorocarbons (CFCs) 129ndash30 159

alternatives 141ndash2 bans 141ndash2 conferences 141177 destruction of ozone layer 129ndash33 asgreenhouse gases 159 161ndash2 photochemicaldegradation 137 uses 129ndash30 world production132

circumpolar vortex 114 134ndash5civilizations and drought 44Clean Air legislation 12 74 89 95 96climate forcing 23 119Climate Impact Assessment Program 128climate models see computerized modellingClimate Institute 178climatic change 10 42 56ndash7 atmospheric turbidity

118ndash20 global warming 153ndash9 168ndash71international conferences 10 nuclear winter115ndash17 ozone depletion 140ndash1 volcanic activity108ndash10

Climatic Optimum 144 176Clostridium 14coal gasification 93coal liquefaction 93

INDEX 219

Cold War 1 10 173Cologne Cathedral Germany 89Colorado River USA 57Commission of European Communities (CEC) 151computerized modelling 32ndash9 116 168ndash70 1-D

2-D and 3-D models 35 116 Australian nestedmodel 34 GCMs 35ndash9 151 chemical models36 climate models 34ndash9 coupled models 35ndash9equilibrium models 38ndash9 grid point models 3335 interactive models 36 radiative-convectivemodels 35 sea-ice models 36 lsquoslabrsquo models 36spectral models 35 transient models 39 weatherforecasting models 33ndash4

condensation nuclei 101 118conferences 1 2 10 177 acid rain 95ndash6 climate

change 10 global warming 151 153 177 ozonelayer 141 SCEP 10 SMIC 10 UNCED 1UNCHE 1 UNCOD 61

continental tropical air mass 50contingent drought 46 48 on Great Plains 54contour ploughing 62 64convection 22 28convective circulation 27ndash8Copenhagen Denmark 141Coriolis effect 28coupled models see computerized modelling

CrutzenP 128 129Czech Republic 87 Darwin Australia 25ndash6Darwin C 8Davos Switzerland 19 111lsquodead lakesrsquo 83ndash4deforestation 60ndash1 64 impact on CO

2 levels

149ndash50degradation induced drought 67Denmark 164denitrifying bacteria 136Depression 56desertification 59ndash65 175ndash6 areas at risk 59

drought initiated 59ndash60 at Earth Summit 62extent 59 failure of preventative measures 61ndash2initiated by human activity 60ndash2 initiated byland-use change 60 prevention and reversal62ndash5 shifting sands 59

Desertification Convention 62diatom species and aquatic acidity 79 80diatomic oxygen 15 122dieback 86ndash8 in coniferous forests 87 in maple

groves 87dimethyl sulphide (DMS) 70Dorset Ontario Canada 73drought 1 9 13 actuarial forecasting 65

adaptation of plants and animals 45 Africa48ndash54 aridity 41ndash2 causes 66ndash9 definitions 41ENSO 68 famine 40 53ndash4 global warming

164ndash5 Great Plains 54ndash9 human response 42ndash5images 42 modern views 44ndash5 prediction 65ndash9societal response 45 types 45ndash6

dry deposition 70 89dry farming 42 57 64DuPont 132 142 CFC production 132Dust Veil Index (DVI) 106ndash8 perceived

inadequacies 107ndash8Dustbowl 42 56 60 earthatmosphere system 6ndash8 energy flow 6

instability 8 role of linkages 13earthrsquos energy budget 21ndash4 latitudinal imbalance

23ndash4EarthFirst 2Earth Summit (Rio) 1 3 62 Agenda 213 financial

commitments 3 future impact 4 Rio Declaration3

East Africa 42 50 61easterly (tropical) jet 30 51Eastern Europe 74 75ECELRTAP convention 95ndash6economic drought 41El Chichoacuten Mexico 105 106 107 109 110El Nintildeo 25ndash6 68 109energy 4ndash5 179ndash81 consumption 4 179 demand

5 efficiency 180 sources and alternatives 92ENSO 25ndash6 68environment 4ndash13 human impact 4ndash5 population

pressures 9 response to human interference 6ndash9scale and complexity 6 stability 8 time scales181ndash2

Environment Canada 151 178lsquoenvironmental brinksmanshiprsquo 8Environmental Consequences of Nuclear War

(ENUWAR) 116ndash17environmental deterioration 1 impact of

demography and technology 4ndash5 9 impact ofhunting and gathering groups 4

environmental equilibrium 8environmental groups 11environmental impacts acid rain 79ndash90 energy use

6 global warming 162ndash8 nuclear winter116ndash17 ozone depletion 137ndash41

environmental issues 1 atmospheric component9ndash10 development of concern 10ndash11 economicand political components 1ndash2 global nature 1high profile topics 13 information gathering anddissemination 10ndash11 political and businessresponses 11 public interest 1 public perception173

environmental organizations 1 11 changingmembership 1

Environmental Protection Agency (EPA) 138 141142

environmental strategies 4 international

INDEX220

cooperation 182ndash3 planning and policyproblems 181

equilibrium models see computerized modellingEritrea Ethiopia 54Ethiopia 13 42 48 53 54 64 176European Economic Community 135 141

Environment Ministers Statement (1988) 98European Arctic Stratospheric Ozone Experiment

(EASOE) 135evapotranspiration 46ndash7 famine 40 48ndash54 cultural political and socio-

economic aspects 40feedback mechanisms 32 35 116 169ndash70fertilizers ozone depletion 136ndash7field capacity (water supply) 47Finland 164fish population decline 81ndash3flue gas desulphurization (FGD) 93ndash4 98ndash9fluidized bed combustion (FBC) 93forests 3 forest decline 84 86ndash8 international

monitoring and development 4fossil fuels 10 17 145 148 171 179 180 acid

rain 71 92ndash4 98 alternatives 181 globalwarming 145 148

Framework Convention on Climate Change 2 3153 167

Freon 129Friends of the Earth 11fuel desulphurization 93fuel switching 92fuelwood 60ndashl 64 Gaia hypothesis 8ndash9 biocentric nature 8 human

inputs 8 species extinction 9gas phase reaction 73General Circulation Models (GCMs) 35ndash9 116

168ndash70 183Geneva Switzerland 10Germany 84 87 139 151Ghana 48Glaciological Volcanic Index (GVI) 108global cooling 109 115 118ndash19Global Forum 3global warming 11 119 151ndash3 156ndash9 177ndash8

alternative points of view 168ndash71 lsquobusiness asusualrsquo scenario 156 178 causes 144ndash5 CO

atmospheric turbidity 151 controversy overestimates 168 estimates 156ndash9 impact onagriculture 164ndash5 impact on moisture patterns159 165 in the 1980s and 1990s 144storminess 166ndash7

Gobi Desert China 59 101Goddard Institute of Space Studies 156Gorham E 79Great American Desert 56

Great Lakes 151 165Great Plains 54ndash9 60 European agricultural

settlement 56 historical drought 56ndash7 irrigation57ndash9 precipitation variability 54

Greece 59 87 164Green parties 1lsquogreenrsquo refrigerator 142greenhouse effect 1 16 144ndash72 177 179 carbon

cycle 146ndash50 changes 150ndash3 creation 146during the Quaternary 153 impacts 162ndash71

greenhouse gases 16 changing concentrations 145Greenland 134Greenpeace 2 11grid point models see computerized modellingGulf Stream 24 150Gulf War 114ndash15 Hadley G 26Hadley cells 28 32 67 68 119Hadley Centre for Prediction and Research 68Halley Bay Antarctica 133halons 129 130 142 177 in fire-fighting

equipment 130Hamburg Germany 10Hansen J 156Harrapan civilization 44Harmattan 50heavy metals 73 80 impact on forests 80Helsinki Finland 141heterogeneous chemical reactions 134Holocene climate simulations 39Hudson Bay 57humus 60 86hydrochloric acid 127 132hydrochlorofluorocarbon (HCFC) 130 141 142hydrogen ion concentration see pHhydrogen oxides 123ndash4hydrologic cycle 17ndash18hydroxyl radical (OH) 123 129 Ice Ages 8 24 119 170 173Iceland 106 108Idso S 168inadvertent climate modification 9ndash10index cycle see zonal indexIndia 45 48 68 89 175 desertification 59

drought 48 68 methane production 160Indian Ocean 50Indonesia 68 105Industrial Revolution 5 environmental implications

150 153infrared radiation 16 102 104 118 141 146insolation 25 124interactive models see computerized modellingIntergovernmental Panel on Climate Change (IPCC)

10 152ndash3 lsquobusiness as usualrsquo scenario 155ndash6

INDEX 221

IPCC Supplementary Report 153International Council of Scientific Unions (ICSU)

151International Institute for Applied Systems Analysis

(IIASA) 76 151International Nickle Company (INCO) 74Intertropical Convergence Zone (ITCZ) 31ndash2

48ndash52 66 precipitation patterns 51 role in thedevelopment of drought 52ndash3 seasonalmigration 52ndash3

invisible drought 46ions 73irrigation 57 on Great Plains 57ndash8 using

groundwater 64isocyanate-based insulating foam 142Israel 45 175 Japan 75 114 135 FGD 93jet streams 29ndash30 78 100 influence on weather

30influence on the spread of pollution 30 78 Kansas USA 55 56 57Kazakhstan 74Kellogg W 10Kenya 48 54 61 65 agroforestry 65Krakatoa Indonesia 19 104 105 106 108 109

110 sunsets 104 spread of volcanic dust 105Kuwait oil fires 114ndash15 173 Lake District England 79 83Lake Oxsjon Sweden 90Lamb HH 106 108La Nintildea 26 68latent heat 18Likens G 79lime injection multi-stage burning (LIMB) 93 99lime as a buffer 78 90ndash2liquid phase reactions 73Little Ice Age 108 120Live Aid concerts 13 41London England 11 15 smog (1952) 89Long Range Transportation of Atmospheric

Pollution (LRTAP) 74 78 95ndash6Los Angeles California USA 76Lovelock J 8ndash9long-wave radiation 16 22 104 146 Maldive Islands 166Mali 48 64maritime tropical air mass 50Marshall Islands 166Mauna Loa Observatory Hawaii 19 112 113

148 149 153Mauretania 48melanoma 138

Melbourne Australia 101mesopause 21Mesopotamia 44mesosphere 21meteorological drought 41methane (CH

4) 123 132 171 sources 159ndash60

methane sulphonic acid (MSA) 70methyl bromide 136 142methyl chloroform 136Mid-Atlantic states 96Middle Ages warm spell 145 147Middle East 59 180 nuclear capabilities 175moisture deficit 47ndash8moisture index 41moisture surplus 47molecular oxygen 15 122Molina M 132Montreal Canada 10Montreal Protocol 141ndash2 177 182Mozambique 48Mount Agung Bali 105 106 107 109 112Mount Hudson Chile 105 106 114 134Mount Pinatubo Philippines 12 105 106 107

109 110 114 134 185Mount St Helens USA 105 109Mount St Katherines Sinai Egypt 112Munich Germany 96Mycenaean civilization 44 Nairobi Kenya 61natural recycling processes 6 carbon dioxide 146

nitrogen 14 oxygen 14Nebraska USA 56 57negative feedback mechanisms 32 169ndash70Negev Desert Israel 45Netherlands 79 151 166New England USA 78New Hampshire USA 79New South Wales Australia 101New York State USA 81New Zealand 135 139 164Niger 48Nigeria 48 68Nimbus-7 satellite 133nitric acid 70 73 124nitrogen 14ndash15 70 fertilizers 136ndash7nitrogen-fixing bacteria 14non-governmental organizations (NGOs) 3nomadic lifestyle 5North American Water and Power Alliance

(NAWAPA) 57North Atlantic Ocean 24Norway 79 81 83 87Norwegian School of Meteorology 33Nova Scotia Canada 80 81 83nuclear autumn (fall) 116

INDEX222

nuclear capability 175nuclear war 115 127ndash8 175 environmental and

societal consequences 116ndash17 impact on ozonelayer 127ndash8

nuclear weapons tests 127ndash8nuclear winter 10 115ndash17 current status 175numerical weather prediction 33Nyamuragira Zaire 106 oceanic circulation 24ndash6 development 24 North

Atlantic currents 24 lsquoslabrsquo models 36 169Ohio valley USA 96oil crisis 11Okies 56Ontario Canada acid rain 71 72 73 74 78 79

81 83 87 89 90 92 164 165 global warming164 167

open system see systemOrganization of Petroleum Exporting Countries

(OPEC) 180Ottawa Canada 89 96overgrazing of pasture 61 63oxides of nitrogen (NOx) 14 16ndash17 127 129 acid

rain 70 71 73 74 75 79 90 92 93 94 99changing emission levels 75 nitric oxide (NO)124 128 136 nitrous oxide (N2O) 124 136nuclear war 127 ozone destruction 124 SSTs128ndash9

oxides of sulphur (SOx) 16ndash17 acid rain 70 71 7375 78 79 90 91 92 93 94 95 96 97 98anthropogenic sources 71 changing emissionlevels 74 75 emissions abatement 90ndash5industrial sources 84 sulphur dioxide (SO2) 7071 73 74 75 90 92ndash9 lsquo30 per cent clubrsquo 9698

oxygen 14ndash16 atomic 15 122 diatomic(molecular) 15 122 triatomic (ozone) 15 122

ozone 15 21 as greenhouse gas 147 variations invertical distribution 140

ozone layer 1 15 121ndash43 Antarctic ozone hole133ndash6 bans on destructive gases 141ndash2biological effects of depletion 137ndash40 CFCs129ndash33 chemistry 122 climatological effects ofdepletion 140ndash1 conferences 141 human impact127ndash37 Montreal Protocol 141ndash2 nuclear war127ndash8 ozone destroying catalysts 122ndash7 SSTs128ndash9

Ozone Protection Act (Australia) 177 Pacific Ocean 25 26 66 68Pakistan 59 114 175parameterization 34 39 169particle coagulation 19permanent drought 45Peru 25pH (potential hydrogen) 71ndash2 acid rain 72ndash3

aquatic environment 79ndash82 critical levels foraquatic organisms 81ndash2 Arctic Haze 78 normalrain 71 72

photochemical smog 12photo-oxidants 73photosynthesis 16 149 165 impact of rising CO2

levels 162ndash3Pitlochry Scotland 73Pittsburgh US A 12Poland 87polar front jet stream 29ndash30polar stratospheric clouds 134 135pollution 11 12ndash13 19 70 73ndash4 100 112ndash13

119 acid rain 70ndash4 local and regional problems11 natural cleansing processes 11 impact onweather and climate 100

Pollution Probe 11polymer foams 130population growth and the environment 5positive feedback mechanisms 32 67 169potential evapotranspiration 47pre-Cambrian Shield 79precipitation 18 41 42 44 46 47 51ndash2 55 57

120 159 165 acid precipitation 70 72ndash3precipitation variability 42 44 46 54preservation of natural biological diversity 3Pueblo drought USA 56 Quaternary era 150 153Quebec Canada 78 84 87 RAINS 76radiation 21 22 23 34 103ndash5 diffuse 21 107

direct 21 107 infrared 16 102 104 118 141146 long-wave 16 22 104 146 short-wave 1621 104 140 146 solar 19 32 67 103 104107 114 115 terrestrial 20 22 104 ultraviolet102 121 122 132 137ndash9

radiation absorption coefficient 104radiation blindness 137radiative convective models see computerized

modellingradiative forcing agents 23 156 role of greenhouse

gases 23radioactive fallout 30rainfall variability 42lsquoredrsquo rain 19remote sensing (of desertification) 61Republic of Georgia 113Richardson LF 33Rio Declaration 3Rio de Janeiro Brazil (1992) 1Rossby waves 28ndash9Rowland S 132 141Royal Meteorological Society 135Ruhr Germany 78

INDEX 223

Russia 74 110 135 global warming 164 165 Sahara Desert 19 48 50 101 advance of desert

59 shifting sands 60Sahel 42 45 48 53 54 101 175 desertification

59 60 64 66 67 drought and society 51ndash54drought prediction 66 67 forms of agriculture53ndash4 impact of global warming 159 164

salinization 57Scandinavia 79 80 83 108 acid rain 96ndash9Scientific Committee on Problems of the

Environment (SCOPE) 116Scotland 73 79 84 108 melanoma growth rates

138scrubbers wet and dry 93ndash4 98sea-ice models see computerized modellingsea-level change 166ndash8sea surface temperatures (SSTs) 67ndash8 use in

drought prediction 67 predicting Saheliandrought 68

seasonal drought 45ndash6 in sub-tropics 48selective catalytic reduction (SCR) 94Senegal 48Sheffield England 71short-wave radiation 16 21 104 140 146shifting sands 60Sierra Club 1 11skin cancer 16 121 129 132 137 in Australia

and New Zealand 138ndash9 in Scotland 138 inUnited States 129 137

lsquoslabrsquo models see computerized modellingSlovak Republic 87Smith R 79smoke in the atmosphere 89 111 114 115 116

from Kuwait oil fires 114soil acidity 85 impact on forest regeneration 88soil colloids 86soil moisture deficit (SMD) 47soil moisture storage 47solar variability 171Solomon S 135Somalia 48 54 176soot particles 104 impact on temperature 114South Africa 149South America 26 59 149Southern Oscillation 25Spain 59specific absorption coefficient 104Sphagnum moss 84spring flush 82ndash3Statement of Forest Principles 3steady state systems 7ndash8Steinbeck J 42Stockholm Sweden 1 96stratopause 21stratosphere 21 aerosols 100 102 105 106 111

114 118 119 impact of aircraft 111 isothermalconditions 21 ozone 121 122 124 127 129131 132 136 140 141 soot loading 111

Study of Critical Environmental Problems (SCEP)10

Study of Manrsquos Impact on Climate (SMIC) 10 119sub-Saharan Africa 40 48 53 54 61 atmospheric

circulation 48ndash51Subir Mesopotamia 44Sudbury Ontario Canada 74 79 91Sudan 48 54 60 61 64sulphate particles 78 103 107 ozone depletion

134sulphur dioxide (SO

2) (see oxides of sulphur)

sunspots 65ndash6 127ndash8 135 impact on ozone layer135 links with drought 65ndash6

supersonic transports (SSTs) 111 128ndash9Surtsey Iceland 106sustainable development 3Sustainable Development Commission 3Sweden 81 83 87 90 91 97Switzerland 19 111Syria 44 lsquo30 per cent clubrsquo 96 98 182Tahiti 25ndash6Taj Mahal India 89tall stacks policy 73ndash4 89Tambora Indonesia 105 106 108teleconnections 67ndash9temperature inversion 21 in stratosphere 119terrestrial ecosystems 17terpenes 101thermosphere 21The Grapes of Wrath 42 56lsquothe year without a summerrsquo 108Third World 2 76 136 141 180 182 acid rain

76 drought famine and desertification 40economic growth 3 environmental strategies 2

Thornthwaite CW 45ndash7 48Tigray Ethiopia 54Tolstoy L 84Toronto Canada 10 78Tupolev-144 129transient models see computerized modellingtracer elements (acid rain) 78Trail British Columbia Canada 71triatomic oxygen 15 122tropical rain-forest destruction 149tropopause 21 105 106troposphere 21ndash2 101 104 114 lapse rate 20TTAPS 35 115ndash17 baseline scenario 116 critical

assessment 116ndash17turbidity 100ndash20 anthropogenic contribution

110ndash17 climatic effects 108ndash10 119ndash20volcanic contribution 105ndash10

INDEX224

Tyndall J 150 Uganda 54UK Meteorological Office 33 68 151 drought

prediction methods 68Ukraine 74 165Ulrich B 87 88ultraviolet radiation 16 102 121 122 132 137ndash9

impact on human health 137ndash9UN Conference on Environment and Development

(UNCED) 1 3 Rio Declaration 3 Agenda 21 3UN Conference on Desertification (UNCOD) 61 62UN Conference on the Human Environment

(UNCHE) 1UN Economic Commission for Europe (UNECE) 95UN World Food Program 54UN Environment Program (UNEP) 59 141United States 55 73 79 93 95 96 114 128 132

135 182 acid rain 93 95 96 drought 54ndash7ozone depletion 128ndash32 global warming 151165

University of WisconsinMadison USA 44urban air pollution 12upper westerlies and spread of dust 106US Agency for International Development (USAID)

59US Department of Energy 151UV-A 121UV-B 121 137 138 impact on AIDS virus 139UV-C 121 Vermont USA 84 87vegetation groups C

3 and C

Victoria Australia 101Vienna Convention for the Protection of the Ozone

Layer 141 177Villach Austria 151

volcanic activity climatic impact 108ndash10 role inLittle Ice Age 108 turbidity 105ndash8 see alsoBezymianny El Chichon Krakatoa MountAgung Mount Hudson Mount St HelensMount Pinatubo Nyamuragira SurtseyTambora

Volcanic Explosivity Index (VEI) 108 Waldsterben 87Wales 83Walker Sir Gilbert 25Walker Circulation 25water 17ndash19 22 irrigation 57ndash9weather forecasting models 32ndash4 spatial resolution

problems 34West Africa 50 52(West) Germany 84West Virginia US A 72wet deposition 70Wilderness Society 1World Commission on Environment and

Development (Brundtland Commission) 3World Conference on the Changing Atmosphere

Toronto (1988) 177World Conference on Climate and Development

Hamburg (1988) 177World Meteorological Organization (WMO) 120

151 152world population growth 5 136 xerophytic plants 45 Yosnaya Polyana Russia 84 Zaire 106Zimbabwe 54zonal index 28ndash9

Book Cover

Contents

List of figures

List of tables

Preface

Acknowledgements

List of acronyms

SETTING THE SCENE

THE ATMOSPHERE

DROUGHT FAMINE AND DESERTIFICATION

ACID RAIN

ATMOSPHERIC TURBIDITY

THE THREAT TO THE OZONE LAYER

THE GREENHOUSE EFFECT AND GLOBAL WARMING

PRESENT PROBLEMS FUTURE PROSPECTS

Glossary

Bibliography

Global environmental issues This book provides a balanced account of the global environmental issues which threaten our societyand which we neglect at our peril Analysing both societal and environmental components of theissuesmdashglobal warming ozone depletion acid rain and droughtmdashthe book offers a valuableintegrative approach At a time when the technical level of publications on environmental issues isrising this introductory text expresses sophisticated scientific ideas in a clear non-technical manner

Emphasising the climatological dimension common to all environmental issues GlobalEnvironmental Issues recognises the multi-faceted nature of the issues their common causes andthe possibility of common solutions Assessment of socio-economic cultural and political factorsprovides a balanced introduction to both the dangers and advantages of human interference withthe environment What have we done to deserve our current environmental crisis Can we solve ourcurrent environmental problems or is it too late

This new edition of a best-selling text is completely updated and expanded to include greaterdetail and new material such as a new section on atmospheric modelling A glossary has been addedtogether with a bibliography for further reading at the end of each chapter allowing readers todevelop their interest in specific topics This interdisciplinary text will prove invaluable to studentsin geography environmental studies and other courses in which the environmental approach isemphasised

David DKemp is Professor of Geography at Lakehead University Canada and has over twenty-fiveyears of experience in teaching climatology and environmental studies

SECOND EDITION

DAVID DKEMP

London and New York

Contents

2 THE ATMOSPHERE 14

4 ACID RAIN 70

Figures

FIGURES vii

FIGURESviii

Tables

Preface

PREFACExii

Acknowledgements

ACKNOWLEDGEMENTSxiv

Acronyms

ACRONYMSxvi

Setting the scene

SETTING THE SCENE 3

SETTING THE SCENE 5

SETTING THE SCENE 7

SETTING THE SCENE 9

Oxygen and nitrogen

The atmosphere

THE ATMOSPHERE 15

THE ATMOSPHERE 17

THE ATMOSPHERE 19

THE ATMOSPHERE 21

THE ATMOSPHERE 23

THE ATMOSPHERE 25

THE ATMOSPHERE 27

THE ATMOSPHERE 29

The jet streams

THE ATMOSPHERE 31

THE ATMOSPHERE 33

THE ATMOSPHERE 35

THE ATMOSPHERE 37

Table 24 continued

THE ATMOSPHERE 39

SUMMARY

DROUGHT

Aridity and drought

Types of drought

Historical drought

DESERTIFICATION

DROUGHT PREDICTION

SUMMARY

Acid rain

ACID RAIN 71

ACID RAIN 73

ACID RAIN 75

ACID RAIN 77

ACID RAIN 79

ACID RAIN 81

ACID RAIN 83

ACID RAIN 85

ACID RAIN 87

ACID RAIN 89

ACID RAIN 91

ACID RAIN 93

ACID RAIN 95

2 from

2 emissions in

ACID RAIN 97

ACID RAIN 99

SUMMARY

The dust veil index

NUCLEAR WINTER

COOLING OR WARMING

SUMMARY

Hydrogen oxides

Nitrogen oxides

Chlorine oxides

Biological effects

SUMMARY

share by region

2 which is being

2concentrations

2 levels

SUMMARY

Nuclear winter

Acid rain

SOLUTIONS

Glossary

GLOSSARY 187

GLOSSARY188

GLOSSARY 189

GLOSSARY190

GLOSSARY 191

GLOSSARY192

GLOSSARY 193

GLOSSARY194

GLOSSARY 195

GLOSSARY196

GLOSSARY 197

GLOSSARY198

GLOSSARY 199

GLOSSARY200

GLOSSARY 201

GLOSSARY202

Bibliography

BIBLIOGRAPHY204

BIBLIOGRAPHY 205

2 20000

BIBLIOGRAPHY206

2 Emission 1975

BIBLIOGRAPHY 207

BIBLIOGRAPHY208

BIBLIOGRAPHY 209

BIBLIOGRAPHY210

2 Content

BIBLIOGRAPHY 211

BIBLIOGRAPHY212

2-warmed earthrsquo

BIBLIOGRAPHY 213

BIBLIOGRAPHY214

BIBLIOGRAPHY 215

BIBLIOGRAPHY216

x v NO

INDEX218

INDEX 219

2 levels

INDEX220

INDEX 221

4) 123 132 171 sources 159ndash60

INDEX222

81 83 87 89 90 92 164 165 global warming164 167

INDEX 223

INDEX224

3 and C

Book Cover

Contents

List of figures

List of tables

Preface

Acknowledgements

List of acronyms

SETTING THE SCENE

THE ATMOSPHERE

ACID RAIN

Glossary

Bibliography

SECOND EDITION

DAVID DKEMP

London and New York

Contents

2 THE ATMOSPHERE 14

4 ACID RAIN 70

Figures

FIGURES vii

FIGURESviii

Tables

Preface

PREFACExii

Acknowledgements

ACKNOWLEDGEMENTSxiv

Acronyms

ACRONYMSxvi

Setting the scene

SETTING THE SCENE 3

SETTING THE SCENE 5

SETTING THE SCENE 7

SETTING THE SCENE 9

Oxygen and nitrogen

The atmosphere

THE ATMOSPHERE 15

THE ATMOSPHERE 17

THE ATMOSPHERE 19

THE ATMOSPHERE 21

THE ATMOSPHERE 23

THE ATMOSPHERE 25

THE ATMOSPHERE 27

THE ATMOSPHERE 29

The jet streams

THE ATMOSPHERE 31

THE ATMOSPHERE 33

THE ATMOSPHERE 35

THE ATMOSPHERE 37

Table 24 continued

THE ATMOSPHERE 39

SUMMARY

DROUGHT

Aridity and drought

Types of drought

Historical drought

DESERTIFICATION

DROUGHT PREDICTION

SUMMARY

Acid rain

ACID RAIN 71

ACID RAIN 73

ACID RAIN 75

ACID RAIN 77

ACID RAIN 79

ACID RAIN 81

ACID RAIN 83

ACID RAIN 85

ACID RAIN 87

ACID RAIN 89

ACID RAIN 91

ACID RAIN 93

ACID RAIN 95

2 from

2 emissions in

ACID RAIN 97

ACID RAIN 99

SUMMARY

The dust veil index

NUCLEAR WINTER

COOLING OR WARMING

SUMMARY

Hydrogen oxides

Nitrogen oxides

Chlorine oxides

Biological effects

SUMMARY

share by region

2 which is being

2concentrations

2 levels

SUMMARY

Nuclear winter

Acid rain

SOLUTIONS

Glossary

GLOSSARY 187

GLOSSARY188

GLOSSARY 189

GLOSSARY190

GLOSSARY 191

GLOSSARY192

GLOSSARY 193

GLOSSARY194

GLOSSARY 195

GLOSSARY196

GLOSSARY 197

GLOSSARY198

GLOSSARY 199

GLOSSARY200

GLOSSARY 201

GLOSSARY202

Bibliography

BIBLIOGRAPHY204

BIBLIOGRAPHY 205

2 20000

BIBLIOGRAPHY206

2 Emission 1975

BIBLIOGRAPHY 207

BIBLIOGRAPHY208

BIBLIOGRAPHY 209

BIBLIOGRAPHY210

2 Content

BIBLIOGRAPHY 211

BIBLIOGRAPHY212

2-warmed earthrsquo

BIBLIOGRAPHY 213

BIBLIOGRAPHY214

BIBLIOGRAPHY 215

BIBLIOGRAPHY216

x v NO

INDEX218

INDEX 219

2 levels

INDEX220

INDEX 221

4) 123 132 171 sources 159ndash60

INDEX222

81 83 87 89 90 92 164 165 global warming164 167

INDEX 223

INDEX224

3 and C

Book Cover

Contents

List of figures

List of tables

Preface

Acknowledgements

List of acronyms

SETTING THE SCENE

THE ATMOSPHERE

ACID RAIN

Glossary

Bibliography

Documents

Global environmental issues - پایگاه مهندسی بهداشت محیط ایران · 2012. 12. 5. · Global environmental issues This book provides a balanced account of the