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Scientific-Technical Analyses of Impacts, Adaptations, and Mitigation of Climate Change Human health, terrestrial and aquatic ecological systems, and Options for adapting to change or mitigating change that can socioeconomic systems (e.g., agriculture, forestry, fisheries, be justified for other reasons today (e.g., abatement of air and water resources) are all vital to human development and and water pollution) and make society more flexible or well-being and are all sensitive to changes in climate. Whereas resilient to anticipated adverse effects of climate change many regions are likely to experience the adverse effects of cli- appear particularly desirable. male change--some of which are potentially irreversiblesome effects of climate change are likely to be beneficial. Hence, different segments of society can expect to confront a 3. Vulnerability to Climate Change variety of changes and the need to adapt to them.
Article 2 of the UNFCCC explicitly acknowledges the imporPolicymakers are faced with responding to the risks posed by cance of natural ecosystems, food production, and sustainable anthropogenic emissions of greenhouse gases in the face of economic development. This report addresses the potential significant scientific uncertainties. It is appropriate to con- sensitivity, adaptability, and vulnerability of ecological and sider these uncertainties in the context of information indi- socioeconomic systems—including hydrology and water cating that climate-induced environmental changes cannot resources management, human infrastructure, and human be reversed quickly, if at all, due to the long time scales health-to changes in climate (see Box 3). associated with the climate system (see Box 2). Decisions taken during the next few years may limit the range of pos. Human-induced climate change adds an important new sible policy options in the future because high near-term stress. Human-induced climate change represents an important emissions would require deeper reductions in the future to additional stress, particularly to the many ecological and meet any given target concentration. Delaying action might socioeconomic systems already affected by pollution, increasreduce the overall costs of mitigation because of potential ing resource demands, and nonsustainable management practechnological advances but could increase both the rate and tices. The most vulnerable systems are those with the greatest the eventual magnitude of climate change, hence the adapta- sensitivity to climate changes and the least adaptability. tion and damage costs.
Most systems are sensitive to climate change. Natural ecoPolicymakers will have to decide to what degree they want logical systems, socioeconomic systems, and human health are to take precautionary measures by mitigating greenhouse gas all sensitive to both the magnitude and the rate of climate emissions and enhancing the resilience of vulnerable sys- change. tems by means of adaptation. Uncertainty does not mean that a nation or the world community cannot position itself better Impacts are difficult to quantify, and existing studies are to cope with the broad range of possible climate changes or limited in scope. Although our knowledge has increased sig. protect against potentially costly future outcomes. Delaying nificantly during the last decade, and qualitative estimates can such measures may leave a nation or the world poorly pre- be developed, quantitative projections of the impacts of clipared to deal with adverse changes and may increase the mate change on any particular system at any particular location possibility of irreversible or very costly consequences. are difficult because regional-scale climate change predictions
are uncertain; our current understanding of many critical
processes is limited; and systems are subject to multiple cliBox 2. Time Scales of Processes
matic and non-climatic stresses, the interactions of which are Influencing the Climate System
not always linear or additive. Most impact studies have
assessed how systems would respond to climate change resultTumover of the capital stock responsible for emis- ing from an arbitrary doubling of equivalent atmospheric car. sions of greenhouse gases: Years to decades
bon dioxide (CO2) concentrations. Furthermore, very few (without premature retirement)
studies have considered dynamic responses to steadily increasStabilization of atmospheric concentrations of long- ing concentrations of greenhouse gases; fewer still have examlived greenhouse gases given a stable level of
ined the consequences of increases beyond a doubling of greenhouse gas emissions: Decades to millennia equivalent atmospheric CO, concentrations or assessed the Equilibration of the climate system given a stable implications of multiple stress factors. level of greenhouse gas concentrations: Decades to centuries
Successful adaptation depends upon technological advances, Equilibration of sea level given a stable climate: institutional arrangements, availability of financing, and Centuries
information exchange. Technological advances generally have Restoration/rehabilitation of damaged or disturbed increased adaptation options for managed systems such as agriecological systems: Decades to centuries
culture and water supply. However, many regions of the world (some changes, such as species extinction, are currently have limited access to these technologies and approirreversible, and it may be impossible to recon- priate information. The efficacy and cost-effective use of adapstruct and reestablish some disturbed ecosystems) tation strategies will depend upon the availability of sinanScientific-Technical Analyses of Impacts, Adaptations, and Mitigation of Climate Change
changes in most ecological and social systems will prove Box 3. Sensitivity, Adaptability, and Vulnerability extremely difficult in the coming decades. This is because of
the complexity of these systems, their many non-linear feedSensitivity is the degree to which a system will respond backs, and their sensitivity to a large number of climatic and to a change in climatic conditions (e.g., the extent of non-climatic factors, all of which are expected to continue to change in ecosystem composition, structure, and func- change simultaneously. The development of a baseline projecttioning, including primary productivity, resulting from ing future conditions without climate change is crucial, for it is a given change in temperature or precipitation).
this baseline against which all projected impacts are measured.
As future climate extends beyond the boundaries of empirical Adaptability refers to the degree to which adjustments knowledge (i.c., the documented impacts of climate variation are possible in practices, processes, or structures of in the past), it becomes more likely that actual outcomes will systems to projected or actual changes of climate.
include surprises and unanticipated rapid changes. Adaptation can be spontaneous or planned, and can be carried out in response to or in anticipation of changes Further research and monitoring are essential. Enhanced in conditions.
support for research and monitoring, including cooperative
efforts from national, international, and multi-lateral instituVulnerability defines the extent to which climate
tions, is essential in order to improve significantly regionalchange may damage or harm a system. It depends not scale climate projections; understand the responses of human only on a system's sensitivity but also on its ability to health, ecological, and socioeconomic systems to changes in adapt to new climatic conditions.
climate and other stress factors; and improve our understanding
of the efficacy and cost-effectiveness of adaptation strategies. Both the magnitude and the rate of climate change are important in determining the sensitivity, adaptability, and vulnerability of a system.
3.1. Terrestrial and Aquatic Ecosystems
Ecosystems contain the Earth's entire reservoir of genetic and managerial, institutional, legal, and regulatory practices, both species diversity and provide many goods and services critical domestic and international in scope. Incorporating climate- to individuals and societies. These goods and services include change concerns into resource-use and development decisions (i) providing food, fiber, medicines, and energy; (ii) processing and plans for regularly scheduled investments in infrastructure and storing carbon and other nutrients; (iii) assimilating will facilitate adaptation.
wastes, purifying water, regulating water runoff, and control
ling floods, soil degradation, and beach erosion; and (iv) proVulnerability increases as adaptive capacity decreases. The viding opportunities for recreation and tourism. These systems vulnerability of human health and socioeconomic systems and the functions they provide are sensitive to the rate and and, to a lesser extent, ecological systems--depends upon eco- extent of changes in climate. Figure 1 illustrates that mean nomic circumstances and institutional infrastructure. This annual temperature and mean annual precipitation can be corimplies that systems typically are more vulnerable in develop related with the distribution of the world's major biomes. ing countries where economic and institutional circumstances are less favorable. People who live on arid or semi-arid lands, The composition and geographic distribution of many ecosysin low-lying coastal areas, in water-limited or flood-prone tems will shift as individual species respond to changes in cliareas, or on small islands are particularly vulnerable to climate mate; there will likely be reductions in biological diversity and change. Some regions have become more vulnerable to haz- in the goods and services that ecosystems provide society. ards such as storms, floods, and droughts as a result of increas. Some ecological systems may not reach a new equilibrium for ing population density in sensitive areas such as river basins several centuries after the climate achieves a new balance. and coastal plains. Human activities, which fragment many landscapes, have increased the vulnerability of lightly man. Forests. Models project that a sustained increase of 1°C in glob aged and unmanaged ecosystems. Fragmentation limits natural al mean temperature is sufficient to cause changes in regional adaptation potential and the potential effectiveness of measures climates that will affect the growth and regeneration capacity of to assist adaptation in these systems, such as the provision of forests in many regions. In several instances this will alter the migration corridors. A changing climate's near-term effects on function and composition of forests significantly. As a conseecological and socioeconomic systems most likely will result quence of possible changes in temperature and water availabilifrom changes in the intensity and seasonal and geographic dis- ty under doubled equivalent-CO, equilibrium conditions, a subtribution of common weather hazards such as storms, floods, stantial fraction (a global average of one-third, varying by region and droughts. In most of these examples, vulnerability can be from one-seventh to two-thirds) of the existing forested area of reduced by strengthening adaptive capacity.
the world will undergo major changes in broad vegetation
types—with the greatest changes occurring in high latitudes and Detection will be difficult, and unexpected changes cannot the least in the tropics. Climate change is expected to occur at a be ruled out. Unambiguous detection of climate-induced rapid rate relative to the speed at which forest species grow.
Scrubland Thorn Scrub
Mean Annual Temperature (°C)
Scientific-Technical Analyses of Impacts, Adoplations, and Mitigation of Climate Change
species composition, but altered rainfall amount and seasonal
ity and increased evapotranspiration will. Increases in atmos25
pheric CO, concentration may raise the carbon-to-nitrogen
ratio of forage for herbivores, thus reducing its food value.
Shifts in temperature and precipitation in temperate rangelands
may result in altered growing seasons and boundary shifts 15181
between grasslands, forests, and shrublands. TEMPERATE
Deserts and Desertification. Deserts are likely to become 10
more extreme—in thal, with few exceptions, they are project
ed 10 become hotter but not significantly wetter. Temperature S
increases could be a threat to organisms that exist near their heat-tolerance limits. The impacts on water balance, hydrology, and vegetation are uncertain. Desertification, as defined by
the UN Convention to Combat Desertification, is land degraBOREAL
dation in arid, semi-arid, and dry sub-humid areas resulting Scientific-Technical Analyses of Impacts, Adaptations, and Mitigation of Climate Change of larger and deeper temperate zone lakes would increase their of runoff and the intensity of floods and droughts; however, productivity; although in some shallow lakes and in streams, at present, specific regional effects are uncertain. Relatively warming could increase the likelihood of anoxic conditions. small changes in temperature and precipitation, together with Increases in flow variability, particularly the frequency and the non-linear effects on evapotranspiration and soil moisduration of large Noods and droughts, would tend to reduce ture, can result in relatively large changes in runoff, especialwater quality and biological productivity and habitat in ly in arid and semi-arid regions. High-latitude regions may streams. Water-level declines will be most severe in lakes and experience increased runoff due to increased precipitation, streams in dry evaporative drainages and in basins with small whereas runoff may decrease at lower latitudes due to the catchments. The geographical distribution of wetlands is likely combined effects of increased evapotranspiration and to shift with changes in temperature and precipitation. There decreased precipitation. More intense rainfall would tend to will be an impact of climate change on greenhouse gas release increase runoff and the risk of Mooding, although this would from non-tidal wetlands, but there is uncertainty regarding the depend not only on the change in rainfall but also on catchexact effects from site to site.
from various factors, including climatic variations and human Tundra
activities. Desertification is more likely to become irreversible -10
if the environment becomes drier and the soil becomes further degraded through erosion and compaction. Adaptation 10
drought and desertification may rely on the development of 500 1000 1500 2000 2500 3000 3500 4000
diversified production systems. Mean Annual Precipitation (mm)
Cryosphere. Models project that between one-third and oneFigure 1: This figure illustrates that mean annual temperature half of existing mountain glacier mass could disappear over the and mean annual precipitation can be correlated with the distribu- next 100 years. The reduced extent of glaciers and depth of tion of the world's major biomes. While the role of these annual snow cover also would affect the seasonal distribution of river means in affecting this distribution is important, it should be noted flow and water supply for hydroelectric generation and agriculthat the distribution of biomes may also strongly depend on sea- ture. Anticipated hydrological changes and reductions in the sonal factors such as the length of the dry season or the lowest areal extent and depth of permafrost could lead to large-scale absolute minimum temperature, on soil properties such as water- damage to infrastructure, an additional flux of CO, into the holding capacity, on land-use history such as agriculture or graz- atmosphere, and changes in processes that contribute to the flux ing, and on disturbance regimes such as the frequency of fire. of methane (CH.) into the atmosphere. Reduced sea-ice extent
and thickness would increase the seasonal duration of naviga
Lion on rivers and in coastal areas that are presently affected by reproduce, and reestablish themselves. For mid-latitude regions, seasonal ice cover, and may increase navigability in the Arctic a global average warming of 1-3.5°C over the next 100 years Ocean. Little change in the extent of the Greenland and would be equivalent to a poleward shift of the present isotherms Antarctic ice sheets is expected over the next 50-100 years. by approximately 150-550 km or an altitude shift of about 150-550 m; in low latitudes, temperatures would generally be Mountain Regions. The projected decrease in the extent of increased to higher levels than now exist. This compares to past mountain glaciers, permafrost, and snow cover caused by a tree species migration rates that are believed to be on the order warmer climate will affect hydrologic systems, soil stability, of 4–200 km per century. Therefore, the species composition of and related socioeconomic systems. The altitudinal distribution forests is likely to change; entire forest types may disappear, of vegetation is projected to shift to higher elevation; some while new assemblages of species, hence new ecosystems, may species with climatic ranges limited to mountain tops could be established. Figure 2 depicts potential distribution of biomes become extinct because of disappearance of habitat or reduced under current and a doubled equivalent-CO, climate. Although migration potential. Mountain resources such as food and fuel net primary productivity could increase, the standing biomass of for indigenous populations may be disrupted in many developforests may not because of more frequent outbreaks and extend- ing countries. Recreational industries-of increasing economed ranges of pests and pathogens, and increasing frequency and ic importance to many regions-also are likely to be disrupled. intensity of fires. Large amounts of carbon could be released into the atmosphere during transitions from one forest type to anoth- Lakes, Streams, and Wetlands. Inland aquatic ecosystems a because the rate at which carbon can be lost during times of will be influenced by climate change through altered water high forest mortality is greater than the rate at which it can be temperatures, Now regimes, and water levels. In lakes and gained through growth to maturity.
streams, warming would have the greatest biological effects at
high latitudes, where biological productivity would increase, Rangelands. In tropical rangelands, mean temperature increas- and at the low-latitude boundaries of cold- and cool-water es should not lead to major alterations in productivity and species ranges, where extinctions would be greatest. Warning
ment physical and biological characteristics. A warmer cli
mate could decrease the proportion of precipitation falling as Coastal Systems. Coastal systems are economically and eco snow, leading to reductions in spring runoff and increases in logically important and are expected to vary widely in their winter runoff. response to changes in climate and sea level. Climate change and a rise in sea level or changes in storms or storm surges could The quantity and quality of water supplies already are serious result in the crosion of shores and associated habitat, increased problems today in many regions, including some low-lying salinity of estuaries and freshwater aquifers, altered tidal ranges coastal areas, deltas, and small islands, making countries in in rivers and bays, changes in sediment and nutrient transport, a these regions particularly vulnerable to any additional reducchange in the pattern of chemical and microbiological contami- tion in indigenous water supplies. Water availability currently nation in coastal areas, and increased coastal flooding. Some falls below 1,000 mper person per year--a common benchcoastal ecosystems are particularly at risk, including saltwater mark for water scarcity—in a number of countries (e.g., marshes, mangrove ecosystems, coastal wetlands, coral reefs, Kuwait, Jordan, Israel, Rwanda, Somalia, Algeria, Kenya) or is coral atolls, and river deltas. Changes in these ecosystems would expected to fall below this benchmark in the next 2 to 3 have major negative effects on tourism, freshyater supplies, decades (e.g., Libya, Egypt, South Africa, Iran, Ethiopia). In fisheries, and biodiversity. Such impacts would add to modifica- addition, a number of countries in conflict-prone areas are tions in the functioning of coastal oceans and inland waters that highly dependent on water originating outside their borders already have resulted from pollution, physical modification, and (e.g., Cambodia, Syria, Sudan, Egypt, Iraq). material inputs due to human activities.
The impacts of climate change will depend on the baseline Oceans. Climate change will lead to changes in sea level, condition of the water supply system and the ability of water increasing it on average, and also could lead to altered ocean resource managers to respond not only to climate change but circulation, vertical mixing, wave climatc, and reductions in also to population growth and changes in demands, technolosea-ice cover. As a result, nutrient availability, biological pro- gy, and economic, social, and legislative conditions. In some ductivity, the structure and functions of marine ecosystems, cases-particularly in wealthier countries with integrated and heat and carbon storage capacity may be affected, with water-management systems--improved management may important feedbacks to the climate system. These changes protect water users from climate change at minimal cost; in would have implications for coastal regions, fisheries, lourism many others, however, there could be substantial economic, and recreation, transport, off-shore structures, and communica- social, and environmental costs, particularly in regions that tion. Paleoclimatic data and model experiments suggest that already are water-limited and where there is a considerable abrupt climatic changes can occur if freshwater influx from the competition among users. Experts disagree over whether movement and melting of sea ice or ice sheets significantly water supply systems will evolve substantially enough in the weakens global thermohaline circulation.
future to compensate for the anticipated negative impacts of climate change on water resources and for potential increases
in demand. 3.2 Hydrology and Water Resources Management
Options for dealing with the possible impacts of a changed Climate change will lead to an intensification of the global climate and increased uncertainty about future supply and hydrological cycle and can have major impacts on regional demand for freshwater include more efficient management of water resources. A change in the volume and distribution of existing supplies and infrastructure; institutional arrangewater will affect both ground and surface water supply for ments to limit future demands/promote conservation: domestic and industrial uses, irrigation, hydropower generation, improved monitoring and forecasting systems for navigation, instream ecosystems, and water-based recreation. floods/droughts; rehabilitation of watersheds, especially in
the tropics, and construction of new reservoir capacity to capChanges in the total amount of precipitation and in its fre- ture and store excess flows produced by altered patterns of quency and intensity directly affect the magnitude and timing snowmelt and storms.