Urbanization and Global Environmental Change

7/28/2019 Urbanization and Global Environmental Change

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Cities and global environmental change 83

2007 The Authors. Journal compilation 2007 The Royal Geographical Society

Urbanization and global environmental change:local effects of urban warming

SUE GRIMMONDEnvironmental Monitoring and Modelling Group, Department of Geography,

Kings College London, Strand, London WC2R 2LS

E-mail: [email protected]

This paper was accepted for publication in January 2007

Introduction

The extent and rate of global environmentalchanges, whether greenhouse gas-inducedwarming, deforestation, desertification, or loss

in biodiversity, are driven largely by the rapidgrowth of the Earths human population. Given thelarge and ever-increasing fraction of the worldspopulation living in cities, and the disproportionateshare of resources used by these urban residents,especially in the global North, cities and theirinhabitants are key drivers of global environmentalchange. Here attention is directed to the impact ofcities on climate. The focus is not on the effects of

cities on global-scale climate, rather the effectsglobally of cities at regional and local scales.Distinct urban climates at these scales have longbeen recognized (dating back to Howard 1833).Locally, they are of greater magnitude thanprojected global-scale climate change and enhancethe vulnerability of urban residents to future globalenvironmental change. Moreover, interventions atthese scales have the potential to mitigate broaderenvironmental change both directly and indirectly.

The links between urbanization and global climatechange are complex (Snchez-Rodrguez et al. 2005;Simon this issue). In the context of enhanced global

warming, cities affect greenhouse gas sources andsinks both directly and indirectly. For instance,urban areas are the major sources of anthropogeniccarbon dioxide emissions from the burning of fossilfuels for heating and cooling; from industrialprocesses; transportation of people and goods, andso forth. Svirejeva-Hopkins et al. (2004) suggestthat more than 90% of anthropogenic carbonemissions are generated in cities. The clearing ofland for cities and roads, and the demand for goodsand resources by urban residents, both historicallyand today, are the major drivers of regional landuse change, such as deforestation, which has

reduced the magnitude of global carbon sinks.

While predicting climate change and its impacts

at a global scale is still highly uncertain, localeffects of urbanization on the climate have longbeen documented (see descriptions in Landsberg1981). Surface and atmospheric changes associatedwith the construction and functioning of cities areprofound. New surface materials, associated withbuildings, roads, and other infrastructure, alongwith changes to the morphology of the surface, alterenergy and water exchanges and airflow. Combinedwith direct anthropogenic emissions of heat,carbon dioxide and pollutants, these result indistinct urban climates (Landsberg 1981; Oke 1997).

One of the best-known urban effects of such

development is urban warming; globally cities arealmost always warmer than the surrounding ruralarea (Oke 1973). The magnitude of urban warming ishighly variable over both time and space. On average,urban temperatures may be 13C warmer, but underappropriate meteorological conditions (calm, cloud-less nights in winter) air temperatures can be morethan 10C warmer than surrounding rural environ-ments (Oke 1981). However, in some regional settings,for example in arid environments, cities with largeamounts of irrigated greenspace (parks, suburbanvegetation) may actually be cooler than the surround-ing dry areas (see the results of Grimmond et al. 1993,

for example, for Sacramento, California, USA).The underlying physical causes of the urban heatisland are complex (Table 1). For any neighbour-hood in any city, the relative balance of controlsdepends on the nature of the urban environment,human activity, and meteorological conditions.

Urban climatologists commonly characterize themorphology of a city in terms of the height, widthand density of buildings (see Figure 3, in which skyview factor is defined). These properties, in combi-nation, affect the loss of long wave radiation atnight and therefore cooling rates, the solar accessduring the day and thus the diurnal pattern of

heating, and airflow and wind speed at street level.


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84 Cities and global environmental change


Table 1 Causes of urban warming and examples of mitigation strategies

Urban heat island causes Mitigation strategy

Increased surface areaLarge vertical facesReduced sky view factor

Increased absorption of shortwave (solar) radiationDecreased longwave (terrestrial) radiation lossDecreased total turbulent heat transportReduced wind speeds

High reflection building and road materials, highreflection paints for vehiclesSpacing of buildingsVariability of building heights

Surface materials

Thermal characteristics

Higher heat capacities Reduce surface temperatures (changing albedo andemissivity)Improved roof insulation

Higher conductivitiesIncreased surface heat storage

Moisture characteristicsUrban areas have larger areas that are impervious Porous pavementShed water more rapidly changes the hydrographIncreased runoff with a more rapid peakDecreased evapotranspiration (latent heat flux, QE)

Neighbourhood detention ponds and wetlands whichcollect stormwaterIncrease greenspace fractionGreenroofs, greenwalls

Additional supply of energy anthropogenic heat flux QFElectricity and combustion of fossil fuels: heating and coolingsystems, machinery, vehicles.3-D geometry of buildings canyon geometry

Reduced solar loading internally, reduce need foractive cooling (shades on windows, change materials)District heating and cooling systemsCombined heat and power systemsHigh reflection paint on vehicles to reduce temperature

Air pollutionHuman activities lead to ejection of pollutants and dust intothe atmosphere

District heating and cooling systemsCombined heat and power or cogeneration systems

Increased longwave radiation from the skyGreater absorption and re-emission (greenhouse effect)

Figure 3 The sky view factor a commonly used measure by urban climatologists to quantify the openness of a site within anurban setting that has important implications for incoming and outgoing radiation (solar and terrestrial) and thus heating and

cooling patterns


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The radiative, thermal and hydraulic propertiesof construction materials differ markedly fromthose of bare rock, soil, vegetation and water, theirpre-existing counterparts. In many, though not all,cities the area covered by vegetation decreases.Thus the fraction of solar energy driving evapotran-spiration (the latent heat flux), rather than warmingthe urban fabric and air (the sensible heat fluxes),decreases. However, the properties of buildingmaterials differ widely (Plate 1). For example, roofingmaterials asphalt tiles, ceramic tiles, thatch, slate,

and corrugated steel/iron have very different radia-tive properties (albedo, emissivity) and conductiveproperties (thermal admittance, conductivity) whichgreatly affect energy uptake and release and thusheating/cooling patterns. Other facets of buildings,for example, walls, are constructed of materials withequally different properties, which have profoundeffects on heating and cooling patterns and theresulting building and air temperatures.

Spatial and temporal dynamics

Within a city, urbanrural temperature differences

show significant spatial and temporal variability.

Temperatures from one side of a street to the other,from a park to an industrial neighbourhood, or onesuburb to another may be significantly different,and the nature of these differences changes throughtime. Generally, the greatest intra-urban tempera-ture differences are associated with clear skies andlow wind speeds. The clear skies allow maximumsolar radiation receipt during the day, thusenhanced heating of vertical surfaces and roofs.Under cloudy and windy conditions there is likelyto be less solar gain and greater mixing, so that

differences in air temperatures are reduced.Typically the greatest urbanrural temperature differ-ence is observed 23 h after sunset. Given that therate of radiative cooling is influenced by the skyview factor, narrower streets (smaller sky viewfactors) result in reduced longwave radiative lossand remain warmer than more open (high sky viewfactor) areas. Cities have higher building densitiesin the centre, so warmer temperatures tend to befound in these locations. Changes in wind direction,especially under low wind speed conditions, candisplace these maxima downwind. The locations ofparks (vegetated) or other wide open areas can be

influential in creating complex patterns.

Plate 1 Photographs by the author of Melbourne, Australia (top); Oklahoma City, USA (lower left); Ouagadougou, Burkina Faso(lower middle); and Marseille, France (lower right) to illustrate variations in materials and morphology of urban landscapes both

within and between cities. These result in distinct spatial and temporal patterns of urban warming


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The magnitude of the temperature range across acity can be very large. The range typically is greaterfor surface temperatures (e.g. as seen by a satellite

such as ASTER) than for air temperatures, given thatair temperatures respond to mixing. When thesespatial patterns are studied over time (seasonally,diurnally), the location of the maximum tempera-ture varies (e.g. Potchter et al. 2006; Figure 4).

Urban areas are dynamic, thus urban tempera-ture patterns change over longer periods. Somecities develop over time through processes of verydeliberate planning, others in a more ad hocway.The form of urban development also varies. Forexample, in China, where there is very rapid growthin urban populations, new construction tends to beof tall buildings (Whitehand and Gu 2006); in

India, by contrast, this is not the case. This has

implications not only for the nature of the citiesand the environmental conditions within them, butalso the spatial extent of new development (regional

land use change) and commuting distances of cityresidents. Also important are the ways in which oldbuildings are refurbished and brownfields deve-loped. The former may involve reroofing, refittinginterior heating and cooling systems, painting theexterior or covering a building with new materials.All these changes have impacts on the micro andlocal thermal environment.

Impacts

Urban warming has important implications for humancomfort, health and well-being. Many examples

exist of the vulnerability of urban populations,

Figure 4 Spatial and temporal dynamics of urban warming: Beer Sheva, Israel. The impacts of urban development andmeteorological conditions are evident. Top left dawn in winter; top right dawn in summer;

lower left midday in winter; lower right midday in summerSource: Adapted from Potchter et al. (2006)


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most often the elderly and the poor, associatedwith heat waves; for example in India in 1998 andFrance and Spain in 2003 (Souch and Grimmond2004). Future climate scenarios, which predict an

increase in summertime maximum temperaturesand also in the frequency and magnitude of extremeconditions, suggest greater risks in the future.Warmer conditions in cities will also increasedemand for air conditioning. More air conditionersgenerate more heat and have significant effects onthe local-scale external climate, with implicationsfor human comfort and the demand for cooling. Ata larger scale, greater use of air conditioningresults in more greenhouse gases through increasedelectricity generation. Significant growth in the useof air conditioning in North America, Europe andAsia has been documented and recent simulations

indicate the resultant increase in energy demandwill more than offset reductions in energy demandfor heating under cold conditions (Hadley et al.2006).

Mitigation

Understanding the causes of the urban heat islandeffect allows insight into strategies for mitigation.This has broader implications in terms of themanagement of energy resources. Peak energydemand for many regions of the world is now inthe summer rather than winter. On occasions,

utility companies are now unable to meet demandunder these conditions and blackouts or rolling-blackouts result. For example, during some of thewarmer periods of the summer of 2006, energysupply in London and Los Angeles was not able tomeet demand and power cuts resulted. Mitigatingenhanced urban temperatures, and thus reducingenergy demand, has significant implications.

A wide range of strategies is being considered tomitigate urban warming (Table 1). The scales at whichthese can be applied vary; for example, individualbuilding versus a neighbourhood, new developmentversus redevelopment/retrofitting. Some mitigation

strategies involve changing the material propertiesof individual buildings (e.g. Akabari et al. 2001), othersto the spatial arrangement (separation of individualbuildings). Changes in materials have the advantagethat they can be used on current buildings, so donot require the costs or time of new development.Moreover, significant developments in new buildingmaterials mean that many that have high reflectivityand modified emissivity no longer need to be white.Thus individual preferences in terms of colour canbe retained (Cool Roof Rating Council 2006). Inmany cases, new materials may cost no more thanthe traditional alternatives. Hence cost is not a

barrier to integration of cool building materials.

Many strategies benefit multiple aspects of urbanenvironmental change. For example, the addition ofwater detention ponds and wetlands reduces peakurban runoff, which has the advantages of reducing

the need to engineer larger systems to deal with flashfloods and/or manage the release of untreated waterdownstream. With careful design of a wetland area,the quality of the stormwater can also be enhancedas well as providing the open areas of parks (highersky view factors) and enhanced evaporation. Additionalsocial, cultural, and psychological benefits fromnatural space can accrue too. Also, new residentialdevelopments (e.g. Lynbrooks in Melbourne, Australia)employ water-sensitive urban design that involvesthe use of grey water to irrigate residential vegetation(Mitchell et al. 2002). This reduces the demand forwater to be diverted into a city for irrigation purposes.

Other strategies involve developing district heatingand cooling (DHC) using combined heat and power(CHP) or co-generation systems. These aim to reducethe emissions of carbon dioxide and other airpollutants (IEA 2006) and have been developed forbuilding to (small) citywide scales (they can gener-ate several kilowatts to hundreds of megawatts ofelectricity). Technological changes are increasingthe viability of these systems; notably reducingenergy losses in the transmission process, and byrecapturing waste heat and energy to avoid warmingthe air unnecessarily. The captured heat can be usedto meet heating requirements, provide cooling using

advanced absorption cooling technology, and alsoto generate more electricity with a steam turbine.

Final comments

Clearly the direct contributions of urban warmingto global climates are small. Urban areas coveronly a small fraction on the Earths surface andtheir moisture, thermal and kinematic effects extenddownwind only a few kilometres. However, thegreenhouse gas emissions from the constructionand operation of cities are large and increasing; thegases from urban areas are the dominant anthropo-

genic sources. Moreover, the warmer conditions inmany cities result in greater energy and resourceconsumption by the inhabitants to offset the effectand also make urban populations more vulnerableto heat waves and other extreme conditions. Thusit is critical that cities and the drivers of urbaniza-tion are central to global environmental research.Urban areas and urban populations will continueto grow in size and number. Existing urban areaswill experience redevelopment and refurbishment.The decisions made about how this will occur willimpact upon the people living within the buildings,neighbourhoods and cities. In combination, they

will have global implications and consequences.


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March20071731OriginalArticleCitiesand globalenvironmentalchangeCitiesand globalenvironmentalchange

Climatic perturbation and urbanization in SenegalCHEIKH GUYE*, ABDOU SALAM FALL AND SERIGNE MANSOUR TALL

*Environnement et Dveloppement du Tiers Monde (Enda-tm), 4 & 5 rue Jacques Bugnicourt ex Kleber,

BP 3370 Dakar, Senegal

E-mail: [email protected] / [email protected]

UN Habitat Programme Manager, UNDP, 19 rue Parchappe, BP 154, Dakar, Senegal

E-mail: [email protected]

Institut Fondamental dAfrique Noire (IFAN), BP 206, Dakar, Senegal

E-mail: [email protected] paper was accepted for publication in January 2007

Introduction

Senegal benefits from a Sudano-Guineanclimate and thus from more favourableecological conditions in the southern part ofits territory. Furthermore, the temperate climateof the Atlantic seaboard has played an importantrole in the establishment of most of its major

cities. The resulting imbalances between North and

South, East and West, both eco-geographically andwith respect to economic potential, significantlyinfluence the internal mobility of the population.Reflecting these factors, Senegals current popu-lation of almost 11 million is distributed veryunevenly across the countrys total area of196 722 km2.

Changing climatic conditions have stimulated

this mobility and contributed to reinforcing the role

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Urbanization and Global Environmental Change