Restricted distribution MARlNF/88 Paris, April 1992 English only

UNITED NATIONS EDUCATIONAL, SCIENTIFIC AIW CULTURAL ORGANIZATION

UNEP/UNESCO Task Team on the

Impact of Expected Climatic Change on Mangroves

REPORT OF THE PREPARATORY MEETING

Bangkok, 19-21 November 1991

Restricted distribution

UNITED NATIONS EDUCATIONAL, SClENTIFX AND CULTURAL ORGANIZATION

MARINF/88 Paris, April 1992 English only

UNEWUNESCO Task Team on the

Impact of Expected Climatic Change on Mangroves

REPORT OF THIE PREPARATORY MEETING

Bangkok, 19-21 November 1991

PREFACE

The United Nations proclaimed at its General Assembly (28 October 1982) that nature shall be respected, genetic viability on earth shall not be compromised, conservation shall be practiced, sustainable management shall be utilized by man and nature shall be secured against degradation. A major international effort is now under way to understand the interacting physical, chemical and biological processes that govern the global envi- ronment. Relative sea-level rise is among those recognized negative consequences, which may effect socioeconomic structures and activities in future. The environmental problems associated with the potential impact of expected climatic changes may prove to be among the major problems facing the marine environment and adjacent coastal areas.

In this context, UNEP, through its Oceans and Coastal Areas Programme Activity Center (OCA/PAC) launched a number of activities designed to assess the potential impact of climate change and to assist governments in the identification and implementation of suitable policy options and response measures which may mitigate the negative conse- quences of the impact.

To this end, and in order to study in particular the possible effects of the expected cli- mate change and sea-level rise on the mangrove ecosystems (common for large coastal areas, especially in tropical zones), UNJZP invited UNESCO, through its Major Inter- regional project on research and Training Leading to the Integrated Management of the Coastal Marine Systems (COMAR), to co-operate in the establishment and co-sponsor- ship of a Global Task Team on the impact of climate change on Mangroves.

A preparatory meeting was convened by UNESCO on behalf of the two Organizations, to discuss the feasibility of establishing a Joint Task Team on the potential impact of climate changes on mangroves. The International Society for Mangrove Ecosystems (ISME) was invited to be associated with the study. The meeting took place in Bangkok from 18-22 November 1991. This document presents the results of the discussions, as well as the recommendations and summary of background papers which were specially prepared for the meeting.

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TABLE OF CONTENTS

PAGES

Summary 3

Introduction 4

Objectives of Preparatory Meeting 5

Feasibility 5

UNEP/UNESCO Task Team 6

Scope of the Study 7

Workplan and Timetable 11

Conclusion 11

Appendix 1 - Delegates to Preparatory Meeting

Appendix 2 - Observers to Preparatory Meeting

Appendix 3 - Agenda

Appendix 4 - Abstracts of Presentation

3

SuMhlARY

Recommendations of the Preparatory Meeting of the Joint UNEP/UNESCO Task Team on the potential impact of expected climatic changes on mangroves.

1. A study of the potential impact of expected climatic changes on mangroves based on the best available information is feasible.

2. The study should consist of three phases: general overview, specific case studies and the design of a long term study and monitoring programme.

3. Three distinct major types of mangrove ecosystem were identified for the study: deltaic, low island and arid coast.

4. The long term study and monitoring programme should be comparative and designed to detect changes produced by expected effects of climatic change.

5. The Task Team should consist of six to ten individuals in specified areas of expertise.

6. The study should last for one year.

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INTRODUCTION

A preparatory meeting of a joint UNEP/UNESCO Task Team on the potehtial impact of climatic change on mangroves was held in Bangkok on the 21st and 22nd November 1991. The meeting was attended by the ten delegates listed in Appendix 1. The meeting was also attended by a number of observers, listed in Appendix 2, who were attending a International Society or Mangrove Ecosystems P

arallel workshop on man f rove ecosystems organised by the ISME). The meeting was opened

by Dr Marc Steyaert (UNESCO) who brlefed the participants on the background and detailed objectives of the preparatory meeting. The meeting then discussed and adopted an agenda (Appendix 3). Professor Colin Field (Hong Kong) was asked to take the chair for the meeting.

The meeting heard four presentations from people with experience that was relevant to the consideration of effects of climatic change on mangroves. Abstracts of these presentations are given Appendix 4. The participants then broke into two groups to give detailed consideration to different aspects of the effects of climatic change on mangrove ecosystems.

The full group reconvened after lunch and were addressed by Professor Sanga Sabhasri, the Minister of Science, Technology and Energy in the Government of Thailand. The Minister expressed strong support for the work of the meeting and he indicated Thailand’s keen interest in problems of climatic change and the future of mangrove ecosystems. The Minister stayed to participate in the subsequent discussions as the meeting continued by receiving reports from the conveners of the separate discussions groups that had been held in the morning. The group then turned its attention to formulating an initial response to the matters that had been raised in the specific objectives that it had been given. The meeting adjourned at 1600 hrs and the conveners and rapporteurs, together with the chairman and Dr Marc Steyaert, convened to commence the task of drafting an initial response.

The full group reconvened the following day to consider the draft report and the final report was agreed at the completion of the session.

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OBJECTIVES OF THE PREPARATORY MEETING

(a) To examine the feasibility of reparing a study on the potential impact of expected climatic c K anges on mangroves based on the best available existing knowledge and insight into the problem and, subject to the meetings conclusion that such a study is feasible;

(b) To constitute a joint UNEP/UNESCO Task Team on the potential impact of expected climatic changes on mangroves;

(c) To elaborate the scope of the study consonant with the objectives of and assumptions used by regional Task Teams;

w To prepare a workplan and timetable for the completion of the study.

FEASIBILITY

Mangroves ecosystems exist at the interface between land and sea in tropical and subtropical regions. They are therefore likely to be particularily sensitive to rising sea level and climatic change. Indeed, the distribution and location of mangroves may make changes to them a useful indicator of the first manifestations of climatic change. In particular, mangrove ecosystems could show changes in species composition and structure given small variations in the global pattern of temperature, sea level, tidal amplitude and the frequency of episodic events such as sudden floods and tropical storms.

Given this and considering the importance of mangrove ecosystems to the well- being of coastal communities, particularily in developing countries, it is imperative that the potential impact of expected changes in climate on mangroves ecosystems be studied and monitored.

The monitoring and study of mangrove ecosystems would require the measurement of a wide range of abiotic and biotic parameters over short and long time scales, requiring a multidisciplinary team to formulate, develop and coordmate an appropriate programme. An extensive pool of technologies and experienced manpower exists to carry out such studies. As well, a substantial body of information and data already exists from sites around the world that could be used as a basis for the study of impacts of ossible climatic changes on mangrove ecosystems. Such knowledge could a so be used to carry out an early assessment of Y the possible potential impact of climatic changes on the coastal environment and its associated socio-economrc structures. Short term studies should be able to identify possible response measures to likely negative consequences of climatic change in coastal regions.

The Group resolved that it was necessary and highly feasible for the preparation of a study on the potential impact of expected climatic changes on mangrove ecosystems to be undertaken.

UNEP/UNESCO TASK TEAM

Terms of Reference of the Task Team

With due consideration given to the deliberations, conclusions and recommendations of the present preparatory meeting the following terms of reference are proposed:

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69

W (9 (ii)

(iii)

(iv)

(v)

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(vii)

To prepare an overview based on the best available knowledge of the potential impact of expected climatic change on mangrove ecosystem;

To prepare selected case studies using best available knowledge for specific regions;

To prepare on the basis of (a) and (b) a statement of possible policy options and response measures which may mitigate the negative consequences of the impact of climatic change on mangrove ecosystems and their associated socio-economic structures;

To design a detailed and specific plan for the implementation and execution of a global long-term mangrove ecosystem monitoring programme;

To include in the plan referred to in (d):

selection of critical parameters to be measured in long term monitoring.

guidelines and procedures for analysis and synthesis of the collected data.

design of experiments to be carried out in conjunction with the monitoring programmes.

selection and justification of three suitable primary mangrove sites for comparative study in the programme; and consideration of associated secondary sites.

evidence of appropriate government and local support for sites identified in (iv).

identification of existing or planned monitoring programmes that would be pertinent to the present task; establishment of links with other relevant International programmes.

a provisional budget for the monitoring and study programme.

Comuosition of Task Team

The composition of the Task Team should reflect the range of disciplines required in the study. It should ideally consist of between six and ten individuals with diverse experience in research and management. Such a team needs to include at least one ;ix~; with extensive experience of mangrove ecosystems from each of the following . .

Coastal dynamic geomorphology (including palynology)

Coastal water dynamics (hydrology)

Geochemistry

Plant biology

Animal biology

Socio-economics

Microclimatology

UNEP and UNESCO (COMAR) are responsible for appointing the members of the Task Team and selecting the Chairman. It is expected that UNEP and UNESCO (COMAR) will each nominate one ex-officio member to work with the team.

SCOPE OF THE STUDY

The Task Team decided that the first phase of the study should be to analyse the extensive existing literature with the arm of producing an overview of the potential impact of expected climatic changes on mangrove ecosystems and their associate socio-economic structures. As man oves are characteristic littoral plant formations of sheltered tro ical and subtropi

K CaY coastlines with very site specific ecological

structures and nctions an overview could be only very general in its conclusions. However, it was felt that the quality of the literature was such that some very useful indicators could be derived to guide poli both in terms of the coastal systems and 31

options and suitable res P

onse measures e communities that dwel in coastal

regions.

A second phase of the study would be to prepare specific case studies from the best existing data for specific regions. There are several very well studied mangrove ecosystems and it is considered possible to use this data to derive some very positive recommendations on the likely impact of expected climatic change in these selected sites.

There was general agreement that the effect of changes in climate might be most pronounced in those places where the mangrove ecosystem was close to the extremes of its occurrence and that it might be easier to detect change in systems that are relatively stable. Mangrove ecosystems are inherently dynamic and if the frequency of natural change is too rapid it might be difficult to detect trends that are occurring as a result of global changes in climate. In order to narrow the scope of the study to reasonable proportions it is recommended that three prime types of mangrove ecosystem be examined:

A well developed deltaic site. This assumes an estuarine dominated environment with large freshwater inputs, high biological diversity and biomass, complex forest structure, high humidity and, if possible, some anthropogenic influences. Such sites probably represent the majority of mangrove forests worldwide.

A low island site. This should be a marine dominated environment of preferably oceanic type, with low tidal variation and small influence from elevated areas. Such a site, although covering a relatively small area, may be very significant in terms of biological diversity. It may also be a good indicator of the effects of climatic change as it is a relatively simple system.

An arid coast site. This should experience extremes of temperature and precipitation to evapotranspiration ratios,

The k eneral overview and case studies produced by the first two parts of the study

shou d provide short term results in predicting the likely effects of climatic change on mangrove ecosystems and the populations that they support and possibly suggest what steps can be taken to minirruse the impact.

A third phase of the study is considered to be essential and that is to design a long term monitoring and study programme that can be implemented following the completion of the initial work by the Task Team. To develop the long term study there is agreement that a small number of sites should be chosen for intensive studies and data generation; and that “secondary sites” should be indicated which may provide specific insights.

The prime sites for intensive studies should ail present well documented measurements of major local variables to be used in the study (eg. meteorological record, species composition; geomorphology) and of sea level change, to avoid repetition of data acquisition and to achieve optimization of resources. They should also be representative of typical environmental settings where mangroves occur. The three prime types of mangrove site defined above (deltaic, low island and arid coast) should be used. In each chosen prime site a training

! rogramme should be

instituted. This will encourage the long term acquisition of ield data. The training

R rogramme should provide the site based technical personnel with the necessary

owled e to carry on the study but also to take right decisions when unpredictable events a B feet the data acquisition.

Secondary sites are desirable to contemplate the existin diversity of mangrove habitats. These should be chosen for the study of a sing e or a few typical f parameters of a given site which will complement results generated in the prime sites. These sites will not necessarily be a part of the study and may be selected from the many mangrove studies being carried on worldwide.

It is important to have a reliable monitoring system for the collection of data over a long term. However, it is equally important to design associated experiments at the prime sites and to analyse optimally the data collected from the monitoring. It should always be remembered that modifications to mangrove ecosystems due to climatic change may be very difficult to discern from modifications due to natural driving forces at a given site. Experimental design should be evaluated with consideration of this complication.

As a prerequisite to selecting a mangrove site and measuring parameters at the site it is necessary to:

(a) Select an appropriate geomorphological unit with a well developed mangrove system to serve as a study site;

(b) Select a study site, where the evolution of the site has already been well studied with the existence and availability of extensive data sets, aerial photographs, satellite images and meteorological information;

(c) Map the aerial extent of the mangrove geomorphological unit in a geographical information system (GIS) as a time series, repeated every few years to ascertain the change and extent of biomass;

(d) Ascertain the existence of an adjacent GLOSS tide gauge network site and the availability of high-quality water level data.

The parameters of importance to define changes in the mangrove ecosystem are judged to be:

Rate of relative sea-level change to assess trends and variability (on a variety of time scales) of local relative sea level change, and the analysis of these data to identify global and local signals;

Rate of sedimentation within the mangrove system (which in the case of a negative rate indicates erosion);

Topography of the mangrove geomorphological unit to assess elevation, bathymetry, relief, and microtopographic features to an absolute bench mark (ideally geo-referenced using GPS satellites and altimetry);

Quality of sediments, including quantification of materials from mangrove, marine, terrestrial, organic and inorganic sources, using a variety of approaches, including isotope ratios, geochemical markers, and microfossils;

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(g)

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Hydrological trends and variability to assess patterns and trends in precipitation, evapotranspiration, air and water temperatures, salinity, and other meteorologrcal parameters;

Fluvial and marine influence on the mangrove system, considering both water and dissolved and particulate constituents;

Groundwater salinity and salt intrusion into the aquifer underlying the mangrove system and adjacent upland margins;

Temperature, precipitation and radiation patterns;

Species composition of plants and animals; reproductive patterns; species migration;

Physiological characteristics of plants and animals;

Anthropogenic use of a mangrove system and the extent to which this activity is sustainable.

As an integral and necess “r

component of the programme, besides monitoring of the above parameters, a we 1 defined, process-oriented research programme needs to be implemented at each site, the sites joined in a scientific network, and comparisons between the sites carried out on a regular basis.

The Task Team considered that the three phases of the proposed study (general overview, specific case studies and design of long term study and monitoring programme) would produce a very significant contribution to the present and future assessment of the impact of expected climatic changes on mangrove ecosystems.

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WORKPLAN AND TIMETABLE

The pro osed study represents a rather complex undertaking given the envisaged scope o P the tasks. It is therefore proposed that the study extend over one full ear from the time that the study is comrmssioned. The study would commence wit h a three da meeting of the Task Team to decide on the detailed objectives of each phase o 2 the study and to assign individual responsibilities.

Individual members of the Task Team would be asked to return their corn leted assignments to the Task Team leader by a specified deadline approximate y four P months after the initial meeting, so that an edited version of the material could be prepared. A second three day meetin the edited material, to establish a dra B

would be held two months later to consider t report on the study and to identify where

additional information was required. This meeting would also give initial consideration to site selection for the long term study and monitoring programme.

Selected members of the Task Team would be asked to carry out site visits and to report on their suitability for the long term study and monitoring programme. There may also be a need for selected Task Team members to visit or contact other agencies, research institutions and countries in order to be fully briefed on how the proposed long term study and monitoring programme could cooperate with other proposed programmes or existing studies.

Members of the Task Team would again submit reports of their activities to the Task Team leader, who would edit a draft version of the final report. A third and final three day meetin of the Task Team would be convened eleven months after the commencement o B the study to finalise the report for submission.

CONCLiJSION

The Group concluded that it was necessary and feasible to carry out a study on the potential impact of expected climatic than concluded that such a study should extend f

es on mangrove ecosystems. They also or one year and consist of three phases.

The three phases to be: a general ovetiew, specific case studies and the design of a long term study and monitoring programme. The Group identified three mam types of mangrove ecosystems that warranted study and comparison: deltaic, low island and arid coast.

Finally, the Group recommended on the terms of reference, composition and workplan for a Task Team to carry out the study.

Appendix 1

Delegates to the preparatory meeting of the UNEP/UNESCO Task Team on the potential impact of expected climatic change on mangroves.

Prof. Sanit Aksornkoe Thailand

Dr C. Caratini India

Dr H. T. Chan Malaysia

Dr B. F. Clough Australia

Dr Salif Diop

Ms. J. Ellison

Prof. C.D. Field, Chairman

Mr. Abeed Ullan Jan

Prof. Y. Khoda

Prof. B. Kjerfve

Mr. M. Kogo

Prof. L. D. de Lacerda

Prof. Sanga Sabhasri

Dr M. Steyaert, Moderator

Dr I. G. M. Tantra

Dr M. Vannucci

Senegal

USA

Hong Kong

Pakistan

Japan

USA

Japan

Brazil

Thailand

UNESCO

Indonesia

India

Appendix 2

Observers to the preparatory meeting of the UNEP/UNESCO Task Team on the potential impact of

expected climatic change on mangroves

Arvind G. Aruga John A. Amarasirghe Mala Baba Shigeyuki Chansang Hansa Holdaway Jim Jaffar Mohammed Jeatman Christopher Kashio Masakazu Kwanairabra Daniel Minato-Mirai 1.1. Oshiro Nozomi Patanaponpalboon Pipat Peng, Lin Tissot C. Wattayakorn Gulaya

India Papua New Guinea Sri Lanka Japan Thailand New Sealand Fiji Canada Thailand Solomon Islands Japan Japan Thailand China India Thailand

Appendix 3

UNEP/DNESCO TASK TEAM

ON

IMPACT OF EXPECTED CLIMATE CHANGES ON MANGROVES

PREPARATORY MEETING

Bamgkok, 20-21 November 1991

Garden Ballroom

PROVISIONAL AGENDA

20 November

09.00 - Opening

- Adoption of the Agenda

- Conduct of the meeting: Moderator, Dr. Marc Steyaert Chairman, Dr. Colin Field

09.15 - Mangrove response to climate change: Background papers

B. KJERFVE: Global climatic warming does not imply sea level rise everywhere: the need for local measurement programs.

J. ELLISON: Impacts of climate change on mangrove ecosystems and species.

B. CLOUGH: Mangrove physiological response to climate change.

C. CARATINI: Mangrove and climate: a geological point of view.

10.30 - coffee break

10.45 - Task Team separates into two groups to give detailed consideration to the following issues:

Group A Group B

Convenor: Kjerfve Barry Cough Rapporteur: Salif Diop Sanit Aksornkoae

*Parameters of climate *vulnerability of mangrove change of significance ecosystemtoclimate change importance for the and impact on habitat and mangrove ecosystem resources;itssignificance (sea level variation, comparedtomandegradation water temperature, of habitat and resources etc.)

*Mode and magnitude of *consequences for the mangrove ecosystem ad for the economy response; an indicator

*criteria for site selection

*criteria for selection

site

12.30 Lunch

14.00 Report by conveners of groups A & B, and general discussion

15.00 (Convenor: Dr. C. Field, Rapporteur: Dr. L.D. de Lacerda) General discussion of feasibility of a UNEP/UNESCO (COMAR) Task Team to prepare a study/monitoring programme on potential impact of expected climatic changes on mangroves

*scope of a study/monitoring programme on changes and impacts on basics of selected cases studies and overviews

*identification of suitable policy options and measures to mitigate negative impact on environment and society and economy

*content of a publication

*elements of a study/monitoring programme

*workplan and timetable for completing study, implementing monitoring programme and issuing policy guidances

“terms of reference of a UNEP/UNESCO(COMAR) Task Team to prepare a study/monitoring programme on potential impact of expected climatic changes on mangroves

16.00 Coffee break

16.15 Drafting committee conveners: Moderator, Convenors and Rapporteurs

Preparation of draft documentation

2 1 November

09.00 Drafting committee continues

11.00 Presentation, discussion and approval of drafting committee's work

12.00 Recommendation and conclusion.

Appendix 4

Impacts of climate change on mangrove ecosystems and species

Joanna C. ELLISON Department of Geography,

University of California at Berkeley and Bermuda Biological Station for Research

Mangroves are a taxonomically diverse group of mainly arboreal angiosperms that grow in the upper intertidal zone of sheltered coastlines of the tropics. They show high fidelity to particular habitats, because of special physiological and morphological adaptations to environmental stresses of high salinity, low oxygen, poor nutrient availability and substrate mobility, and this results in zonation of species according to elevation of the substrate. Mangrove species mostly occur between mean sea level and the level of mean high water spring tides. The importance of mangrove ecosystems is well established, as sediment traps promoting aggradation and maintaining the quality of coastal waters, natural breakwaters protection coastlines from erosion during storms, a natural resource base for silviculture and a large range of economic products, habitats for rare fauna, and nurseries for commercially valuable fish and crustacean species, as well as for educational and tourism uses.

The main impacts of climate change that can be expected to affect mangrove ecosystems and species are sea level rise, climate warming, changes in precipitation and changes in frequency or intensity of hurricanes as well as changes in productivity caused by higher levels of atmospheric carbon dioxide. To date, there has been very little research that directly addresses these issues. These changes will occur in combination with each other as well as with stresses on mangrove communities consequent from sharing the tropical coastal zone with the majority of the world's human population.

It is indicated from past analogues that their close relationship with sea-level position renders mangrove swamps particularly vulnerable to disruption by sea-level rise consequent from the enhanced greenhouse effect. In the earlier Holocene, when rates of sea-level rise were rapid, expansive mangrove ecosystems did not exist. Rather, mangroves occurred as disorganized, patchy individuals with dominant flushing of organic material away from their roots, as seen today on more exposed shorelines. As sea-levelstabilized in the mid-Holocene, the first expansive mangrove ecosystems established in sheltered locations. These are indicated by peat stratigraphy, large ecosystems trapping organic and allochthonous debris to build up substrate, allowing mangroves to keep up with sea-level rise to a degree.

These stratigraphic records indicate that low island mangroves with low sedimentation can keep up with a sea-level rise of up to 8-9cm/lOO years, but at rates of over 12 cm/100 years could not persist. Mangroves of high islands and continental margins can keep up with higher rates of sea-level

rise, owing to fluvial allochtonous sediment input. Mangroves of low islands can be considered to be already under stress from sea-level rise, mean global rates this century being determined to be lo-12 cm/100 years.

Of low oceanic island locations, Bermuda and the Gulf of Mexico have experienced the most rapid rates of sea-level rise this last century, in excess of 20cm/lOO years. We have been monitoring sites in Bermuda undergoing recent mangrove die-back. Erosion is the greatest problem, and subtidal redeposition indicates that the Bruun model of beach erosion with rising sea- level is also appropriate for mangrove swamps. The swamp surface is of low elevation relative to the tidal spectrum, with rapid ebb currents removing all leaf litter and eroding the peat surface, leading to weakening of trees and windthrows. Litter loss has caused apparent reductions in fauna such as snails and crabs. Effective management would be sediment stabilization and enhancement of leaf litter retention. In mangrove areas not yet affected by sea-level rise monitoring should be established, of tidal patterns, mangrove substrate elevations with respect to the tidal prism, productivity and sedimentation rates, and changes in spatial patterns.

With respect to climate change, correlation has been shown betweenmangrove distributions and areas where meanmonthlywater temperature exceeds 24OC, excepting locations too distant to be colonized by mangrove propagules. While most GCM'S predict limited warming around the equator relative to high latitudes, the predicted sea-surface temperature increase of 1.5OC by 2025 can be expected to affect sub-tropical mangroves, causing increased diversity of higher latitude marginal mangroves, and some expansion of the ranges of mangroves into salt marsh environments. Thermal stress affecting root structures and seedling establishment has been shown in water temperatures above 35oc. Photosynthesis typically has an optimum in air temperatures below 35OC, above which rates sharply decline. Diversity of mangroves is greater in areas of higher rainfall, owing to availability of fluvially derived nutrients and sediments. Hence changes in diversity, extent and biomass of mangroves can be expected to show positive correlation with changes in rainfall.

As well as changing climate, increased CO2 directly affects plant growth and development. Mangroves have a C3 pathway of carbon fixation in photosynthesis, where metabolic response to increased atmospheric CO2 has been shown to be increased productivity, and more efficient water use due to reduced stomata1 conductance. It is possible that reduced transpiration will cause changes in salt regulation, particularly in salt secreting species. There is an evident inverse correlation between yield and latitude inmangrove litter production, broadly between 5-10 t/ha/a across the range of mangroves. This implies a marginal increase in litter production with warmer temperatures and higher CO2 levels, though litter removal by tides controls the degree of accumulation, and this is likely to be efficient with rising sea-level.

Modelling studies suggest an increase in the destructive energy and frequency of tropical hurricanes, as a result of increasing sea-surface temperatures, and changes in spatial frequency. From combinations of other factors of sea-level rise, and changes in ecophysiology and community composition relative to climate change, mangroves may be prone to damage in lesser magnitude storms than has been shown previously. This concept may extend to all natural and anthropogenic stresses : mangroves will become far more fragile as ecosystems, hence justifying stronger conservation measures and increased research and management activity.

Manifestations of global warming : the need for coastal measurements

Bjijrn Kjerfve Belle W.Baruch Institute for Marine Biology

and Coastal Research, University of South Carolina, Columbia, SC 29208, USA

There is currently much focus on global warming. During the past century, the average global temperature increased approximately 0.5OC in response to either greenhouse warming or a rebound from the Little Ice Age or both. Although global warming during the next century has been embraced as nearly a reality, it should be borne in mind that many uncertainties remain regarding the magnitudes and effects of climate change.

The main reason for predicting future climate warming is that reradiated energy from the earth surface will be trapped more efficiently by increases in the greenhouse gases of the earth atmosphere. These include carbon dioxide, methane, nitrous oxides, and chloro-fluorocarbons. Carbon dioxide concentration has been monitored intensely and is known to have increased from 280 ppm in 1880 to 350 ppm in 1990, primarily in response to anthropogenic burning of fossil fuels. By most estimates, the efficiency of the greenhouse gases to trap reradiated energy will double within the next 50-100 years and most likely bring about an average global temperature increase from 1.5-3.0°C. However, large spatial temperature differences are likely to persist between different regions, which illustrates the obstacle in appraising climate change in terms of an average global temperature increase.

How climate change will manifest itself in coastal areas is still very speculative. However, the main coastal impacts are likely to be due to (1) sea level rise as a result of eustatic sea level rise and local-regional processes responsible for sea level change between sites; (2) changing hydrology regimes (rainfall, evapotranspiration, runoff, salinity, etc.); (3) changes in coastal ocean-atmosphere climate (waves, winds, currents, etc.); and (4) increase in tropical storm frequency, magnitude, and occurrence. Each of these effects will display great local variability and will require local measurement programs to document and understand current global change. Only then would it be possible to predict coastal impacts associated with future climate change with confidence. Whereas there would be no general global trend with respect to hydrology and ocean climate changes, local sea level is likely to rise in most coastal locations.

Climatic warming does certainly not imply rising sea level everywhere. Although rising sea level will result in the melting of small and large glaciers, only minor effects on the ice caps are expected to occur. During the past century, eustatic sea level rose 0.15 m. Whereas doomsday predictions of global sea level rise as high as 4.5 m during the next century have been threatened, the best estimate of global sea level rise is from 0.6 to 1.0 m during the next 100 years. Whereas the absolute

vertical rise in global water level is related to climatic warming, it is the change in sea level relative to land level locally that is measured and must be analysed. This is referred to as relative sea level change. Relative sea level change is the combined effect of eustatic sea level rise and local-regional effects due to (1) large scale tectonic processes (shift in center of gravity of earth, slight deepening of ocean basins, warping of continental margins, isostatic rebound, and other effects); (2) anthropogenic activities (pumping of ground water or hydrocarbons, or changes in sedimentation patterns as a result of dam construction and other coastal development projects); and (3) changes in the patterns of ocean-atmosphere dynamics along different coasts.

A warmer climate is likely to result in more favorable conditions for the formation of tropical storms. Tropical storms form where the sea surface temperature exceeds 26OC, which is likely to be the case over larger ocean areas and for longer periods each year as compared to the present situation. As a result, we can expect to experience tropical storms more frequently and possibly more intense storms. In fact, the synergistic effect between sea level rise and storms is likely to be the catastrophic manner in which rising sea level locally will have the greatest impact in coastal areas. Areas not previously affected may suddenly become inundated by sea water during storms, water becomes trapped in depressions, irreversibly causing soil and vegetation changes. It is in this manner that relative sea level rise episodically will impact and invade coastal areas in a catastrophic fashion as the marine system encroaches on adjacent terrestrial ecosystems.

As global climate change occurs, most coastal sites, including mangrove coasts, are likely to experience relative sea level rise, changing regimes with respect to hydrology and ocean atmosphere effects, and frequent and more destructive impacts by storms. Since processes and impacts vary greatly from coast to coast, any monitoring program to be implemented must necessarily be conducted at many and diverse sites.

Physiological responses of mangroves to possible climatic change

Barry CLOUGH Australian Institute of Marine Sciences,

Townsville, Australia

In addition to a possible rise in sea level, changes in global climate could also involve changes in UV and visible radiation levels, changing rainfall/evaporation patterns, a rise in temperature, and a rise in greenhouse gases, especially CO?. Change in one or more of these climatic variables, quite independently of any rise in sea level, is likely to have some impact on geographic distribution and growth rate.

The most likely scenario predicted is a rise in global temperature, atmospheric CO, concentration and UV radiation, coupled with changing spatial and temporal patterns of rainfall such that some areas will become drier, others wetter. Existing geographic and zonal distributions suggest that individual mangrove species may respond differently to these changes. However, as a broad generalisatiorl, most evidence suggests that mangroves have adopted a general strategy of minimising water loss for maximum carbon gain, with the emphasis being on conserving water. In other words, mangrove have a high water use efflciency.They have adopted this strategy presumably in response to saiine conditions, which in the case of mangroves have a similar effect to that of drought in terrestrial plants. In addition, there is some evidence that salt uptake by mangroves increases both with salinity and with the rate of water loss. Therefore, minimisation of water loss not only reduces the likelihood of severe plant water deficits, but also probably assists in avoidance of potentially harmful internal salt concentrations. Accepting this as a general strategy, and on the basis of other observed ecophysiological information, the following responses of mangroves to climate change seem likely:

l.Changingrainfall patterns.

The most luxuriant and best developed mangrove forest occurs in the wet, humid tropics. FIoristic diversity and growth rate both decrease with increasing aridity (i.e. in climates with low precipitation/evapotranspiration ratios). Furthermore, floristic diversity and growth rate are also usually somewhat less in monsoonal climates with pronounced seasonal asymetry in rainfall. The better development and faster growth of mangroves in wet, humid tropical climates compared with those in arid or seasonally monsoonal climates has more to do with persistently overcast conditions and high atmospheric humidity in wet, humid tropical climates than with rainfall per se. Under such conditions, direct solar radiation levels are lower, leaf temperature is lower, and the trees lose less water, thereby maintaining a more favourable water balance and minimising salt loading. High rainfall may also be beneficial in reducing soil salinity, but this a secondary rather than primary benefit. It is therefore reasonable to conclude that floristic diversity and growth will both be enhanced in areas where climatic change brings about an increase in rainfall, and particularly in areas where the rainfall becomes more evenly distributed throughout the year. On the other hand floristic diversity and growth rates will both decrease in areas where climatic changes bring about an increase in aridity or greater seasonal asymmetry in the rainfa.lI pattern.

There are also other physiological costs associated with arid climates. Exposure to persistently high solar radiation levels typical of arid climates and those with st.rongIy seasonal monsoons often results in the absorption by leaves of visible radiation far in excess of that which can be used for photosynthesis. Dissipation of this excess visible energy may lead to reversible or. in some cases, irreversible damage. This appears to be species dependent. In some species, notably B. parviflora, this excess light energy is dissipated relatively harmlessly through the

xanthophyll cycle. In others, where this cycle seems to be less effective, excess visible light may lead to the breakdown of metabolic processes, leading to what is commonly called photoinhibition.

High W levels in arid climates may also pose a problem for mangroves. Leaf tannin has been shown to provide an effective barrier against high levels of UV-B radiation. The tannin content of mangrove leaves increases with increasing W-B radiation levels. Synthesis of tannin to protect against high levels of UV-B radiation presumably constitutes an energy cost, leaving less carbon available for growth.

2. Ambient CO, concentration

A rise in ambient CO, concentration is unlikely to result in a significant rise in mangrove canopy photosynthesis. This view is based on the following evidence:

a) Although there is some evidence to suggest that photosynthesis may be co-limited by the rate of supply of CO?, most evidence indicates that photosynthetic capacity is regulated mainly by endogenous (internal) parameters such as substrate availability or enzyme activity.

b) Accepting the general strategy of minimisation of water loss, it is likely that any gain in photosynthesis as a result of increased CO, supply will be offset by greater stomatal closure to minimise water loss. This could give rise to an increase in water use efficiency without a concommitant gain in net carbon fixation.

3. Temperature Given the existing geographical distribution of different species, and accepting the common view that these are correlated with either or both air temperature and sea temperature, it is likely that geographical distribution patterns will extend polewards with rising temperature. This would be expected to lead to greater floristic diversity and higher rates of growth at higher latitudes, particularly where accompanied by an increase in cloud cover, rainfall and atmospheric humidity.

On the other hand, temperatures much above 35oC are unfavourable for mangroves. In some areas, global warming could therefore lead to a decrease in floristic diversity and growth rate, and in extreme cases to disappearance of mangroves.

It is also possible that changes in temperature will bring about changes in phenology, leading in some cases to increase in reproductive capacity, in others to a decrease in reproductive capacity. These effects, however, are little known.

In general, the extremes of temperature appear to be more critical for mangrove survival and growth than the average temperature. This generalisation, however, has not been experimentally confirmed.

Conclusion

Notwithstanding some uncertainty concerning the physiological response of mangroves to climatic variables, it appears likely that predicted increases in temperature and possible increases in rainfall, at least in some areas of the world, could lead to an increase in floristic diversity and an enhancement of productivity of mangrove ecosystems.

Impact of climate and sea level changes on mangrove ecosystems. A geological approach.

CARATINI C. French institute, BP 33, Pondicherry, INDIA

In the tropics, the impact of climate and sea level changes on the mangrove ecosystem during the Late Quatemary is evaluated through modifications in the development of mangrove forests, monitored by palynological analyses of marine sediments. The climatic evolution is deduced from isotopic measurements (180/160 on planktonic foraminifera, a 13C on organic matter), as well as from continental palynological markers (evergreen forest vs savanna). Example I: a core taken from the inner shelf off-shore the West coast of India and deposited during the Late Holocene, i.e. during the long high sea level period corresponding to the current sea level. In this core, the well known decrease in the humid conditions at c. 3500 years BP in India (and also in Africa) induced a steep fall in the percentage of mangrove pollen. Example 2: a long sequence covering the last 250,000 years BP, off-shore Ivory Coast. The maximum development of mangroves always occurs during the warmer and more humid periods when the sea level is high, while the colder and dryer stages when the sea level is low, are marked by very feeble values of mangrove pollen.

From these two examples (confirmed by many other records) it is obvious that the optimum development of mangrove forests takes place when climatic conditions are warmer and more humid; hence a close relationship exists between the climate and mangrove development, the response of mangroves to climatic events being well established.

Meanwhile, the influence of sea level changes on the extension of mangrove may not be very clear for at least one important reason: the eustatic sea level changes (which during the Quatemary depend on the climate) are contemporaneous of climatic stages, the low sea levels occurring during colder periods, the high sea levels during the warmer periods. Because of the overlapping of these two variables, sea level and climate, it may seem questionable to decipher the role each of them plays on mangrove development. Nevertheless, the influence of climate is more global and often more pronounced than that of the sea level whose influence tends to elicit a local response mostly due to modifications in the coastal geomorphology.

The time lag between a climatic (or a sea level) change and the reaction of the mangrove ecosystem recorded in sediments is short, in the order of one or two decades, since the curves representing the physical data (0 and C isotopes) are quite similar to those of the mangrove pollen. This observation is of great value for the UNEP/UNESCO project ” Impact of expected climate changes on mangroves”.

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