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anthropogenic interference with the climate system", as referred to in Article 2, and in evaluating adaptation options. However, it is not yet possible to link particular impacts with specific atmospheric concentrations of greenhouse gases.

3.2 Human health, terrestrial and aquatic ecological systems, and socioeconomic systems (e.g., agriculture, forestry, fisheries, and water resources) are all vital to human development and well-being and are all sensitive to both the magnitude and the rate of climate change. Whereas many regions are likely to experience the adverse effects of climate change - some of which are potentially irreversible - some effects of climate change are likely to be beneficial. Hence, different segments of society can expect to confront a variety of changes and the need to adapt to them.

3.3

Human-induced climate change represents an important additional stress, particularly to the many ecological and socioeconomic systems already affected by pollution, increasing resource demands, and non-sustainable management practices. The vulnerability of human bealth and socioeconomic systems - and, to a lesser extent, ecological systems - depends upon economic circumstances and institutional infrastructure. This implies that systems typically are more vulnerable in developing countries where economic and institutional circumstances are less favourable.

3.4 Although our knowledge has increased significantly during the last decade and qualitative estimates can be developed, quantitative projections of the impacts of climate change on any particular system at any particular location are difficult because regional-scale climate change projections are uncertain; our current understanding of many critical processes is limited; systems are subject to multiple climatic and non-climatic stresses, the interactions of which are not always linear or additive; and very few studies have considered dynamic responses to steadily increasing concentrations of greenhouse gases or the consequences of increases beyond a doubling of equivalent atmospheric CO, concentrations.

- 3.5 Unambiguous detection of climate-induced changes in most ecological and social systems will prove extremely difficult in the coming decades. This is because of the complexity of these systems, their many non-linear feedbacks, and their sensitivity to a large number of climatic and non-climatic factors, all of which are expected to continue to change simultaneously. As future climate extends beyond the boundaries of empirical lowledge (i.e., the documented impacts of climate variation in the past), it becomes more likely that actual outcomes will include surprises and unanticipated rapid changes.

Sensitivity of Systems

Terrestrial and Aquaric Ecosystems

3.6

Ecosystems contain the Earth's entire reservoir of genetic and species diversity and provide many goods and services including: (i) providing food, fibre, medicines, and energy; (ii) processing and storing carbon and other nutrients; (iii) assimilating wastes, purifying water, regulating water runoff, and controlling floods, soil degradation and beach crosion; and (iv) providing opportunities for recreation and tourism. The composition and geographic distribution

of many ecosystems (e.g., forests, rangelands, deserts, mountain systems, lakes, wetlands, and oceans) will shift as individual species respond to changes in climate; there will likely be reductions in biological diversity and in the goods and services that ecosystems provide society. Some ecological systems may not reach a new equilibrium for several centuries after the climate achieves a new balance. This section illustrates the impact of climate change on a number of selected ecological systems.

3.7 Forests: Models project that as a consequence of possible changes in temperature and water availability under doubled equivalent CO2 equilibrium conditions, a substantial fraction (a global average of one-third, varying by region from one-seventh to two-thirds) of the existing forested area of the world will undergo major changes in broad vegetation types - with the greatest changes occurring in high latitudes and the least in the tropics. Climate change is expected to occur at a rapid rate relative to the speed at which forest species grow, reproduce, and reestablish themselves. Therefore, the species composition of forests is likely to change; entire forest types may disappear, while new assemblages of species and hence new ecosystems may be established. Large amounts of carbon could be released into the atmosphere during transitions from one forest type to another because the rate at which carbon can be lost during times of high forest mortality is greater than the rate at which it can be gained through growth to maturity.

3.8 Deserts and desertification: Deserts are likely to become more extreme - in that, with few exceptions, they are projected to become hotter but not significantly wetter. Temperature increases could be a threat to organisms that exist near their heat tolerance limits. Desertification - land degradation in arid, semi-arid and dry sub-humid areas resulting from various factors, including climatic variations and human activities - is more likely to become irreversible if the environment becomes drier and the soil becomes further degraded through erosion and compaction.

3.9 Mountain ecosystems: The altitudinal distribution of vegetation is projected to shift to higher elevation; some species with climatic ranges limited to mountain tops could become extinct because of disappearance of habitat or reduced migration potential.

3.10 Aquatic and coastal ecosystems: In lakes and streams, warming would have the greatest biological effects at high latitudes, where biological productivity would increase, and at the low-latitude boundaries of cold- and coolwater species ranges, where extinctions would be greatest. The geographical distribution of wetlands is likely to shift with changes in temperature and precipitation. Coastal systems are economically and ecologically important and are expected to vary widely in their response to changes in climate and sea level. Some coastal ecosystems are particularly at risk, including saltwater marshes, mangrove ecosystems, coastal wetlands, sandy beaches, coral reefs, coral atolls, and river deltas. Changes in these ecosystems would have major negative effects on tourism, freshwater supplies, fisheries, and biodiversity.

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Hydrology and Water Resources Management

3.11 Models project that between one-third and one-half of existing mountain glacier mass could disappear over the next hundred years. The reduced extent of glaciers and depth of snow cover also would affect the seasonal distribution of river flow and water supply for hydroelectric generation and agriculture. Anticipated hydrological changes and reductions in the areal extent and depth of permafrost could lead to large-scale damage to infrastructure, an additional flux of carbon dioxide into the atmosphere, and changes in processes that contribute to the flux of methane into the atmosphere.

3.12 Climate change will lead to an intensification of the global hydrological cycle and can have major impacts on regional water resources. Changes in the total amount of precipitation and in its frequency and intensity directly affect the magnitude and timing of runoff and the intensity of floods and droughts; however, at present, specific regional effects are uncertain. Relatively small changes in temperature and precipitation, together with the non-linear effects on evapotranspiration and soil moisture, can result in relatively large changes in runoff, especially in arid and semi-arid regions. The quantity and quality of water supplies already are serious problems today in many regions, including some low-lying coastal areas, deltas, and small islands, making countries in these regions particularly vulnerable to any additional reduction in indigenous water supplies.

Agriculture and Forestry 3.13 Crop yields and changes in productivity due to climate change will vary considerably across regions and among localities, thus changing the patterns of production. Productivity is projected to increase in some areas and decrease in others, especially the tropics and subtropics. Existing studies show that on the whole, global agricultural production could be maintained relative to baseline production in the face of climate change projected under doubled equivalent CO, equilibrium conditions. This conclusion takes into account the beneficial effects of CQ - fertilization but does not allow for changes in agricultural pests and the possible effects of changing climatic variability. However, focusing on global agricultural production does not address the potentially serious consequences of large differences at local and regional scales, even at mid-latitudes. There may be increased risk of hunger and famine in some locations, many of the world's poorest people - particularly those living in subtropical and tropical areas and dependent on isolated agricultural systems in semi-arid and arid regions - are most at risk of increased hunger. Global wood supplies during the next century may become increasingly inadequate to meet projected consumption due to both climatic and non-climatic factors.

Human Infrastructure

3.14 Climate change clearly will increase the vulnerability of some coastal populations to flooding and erosional land loss. Estimates put about 46 million people per year currently at risk of flooding due to storm surges. In the absence of adaptation measures, and not taking into account anticipated population growth, 50-cm sea-level rise would increase this number to about 92 million; a 1-meter sea-level rise would raise it to about 118 million. Studies using a 1-meter projection show a panicular risk for small islands and deltas. This increase is at the top range of IPCC Working Group I estimates for 2100; it should be noted, however, that sea level is actually projected to continue to rise in future centuries beyond 2100. Estimated land losses range from 0.05% in Uruguay, 1.0% for Egypt, 6% for the Netherlands, and 17.5% for Bangladesh to about 80% for the Majuro Atoll in the Marshall Islands, given the present state of protection systems. Some small island nations and other countries will confront greater vulnerability because their existing sea and coastal defense systems are less well-established. Countries with higher population densities would be more vulnerable. Storm-surges and flooding could threaten entire cultures. For these countries, sea-level rise could force intemal or international migration of populations.

Human Health

3.15 Climate change is likely to bave wide-ranging and mostly adverse impacts on human health, with significant loss of life. Direct health effects include increases in (predominantly cardio-respiratory) mortality and illness due to an anticipated increase in the intensity and duration of heat waves. Temperature increases in colder regions should result in fewer coldrelated deaths. Indirect effects of climate change, which are expected to predominate, include increases in the potential transmission of vector-bome infectious diseases (e.g., malaria, dengue, yellow fever, and some viral encephalitis) resulting from extensions of the geographical range and season for vector organisms. Models (that entail necessary simplifying assumptions) project that temperature increases of 3-5° C (compared to the IPCC projection of 1-3.5° C by 2100) could lead to potential increases in malaria incidence (of the order of 50–80 million additional annual cases, relative to an assumed global background total of 500 million cases), primarily in tropical, subtropical, and less well-protected temperate-zone populations. Some increases in nonvector-bome infectious diseases - such as salmonellosis, cholera, and giardiasis - also could occur as a result of elevated temperatures and increased flooding. Limitations on freshwater supplies and on nutritious food, as well as the aggravation of air pollution, will also bave human health consequences.

3.16 Quantifying the projected impacts is difficult because the extent of climate-induced health disorders depends on numerous coexistent and interacting factors that characterize the vulnerability of the particular population, including environmental and socioeconomic circumstances, nutritional and immune status, population density, and access to quality health care services. Hence, populations with different levels of natural, technical, and social resources would differ in their vulnerability to climate-induced health impacts.

Technology and Policy Options for Adaptation

3.17 Technological advances generally have increased adaptation options for managed systems. Adaptation options for freshwater resources include more efficient management of existing supplies and infrastructure; institutional arrangements to limit future demands/promote conservation; improved monitoring and forecasting systems for floods/droughts; rehabilitation of watersheds, especially in the tropics; and construction of new reservoir capacity. Adaptation options for agriculture - such as changes in types and varieties of crops, improved watermanagement and irrigation systems, and changes in planting schedules and tillage practices - will be important in limiting negative effects and taking advantage of beneficial changes in climate. Effective coastal-zone management and land-use planning can help direct population shifts away from vulnerable locations such as flood plains, steep hillsides, and low-lying coastlines. Adaptive options to reduce health impacts include protective technology (e.g., housing, air conditioning, water purification, and vaco tion), disaster preparedness, and appropriate health care.

3.18 However, many regions of the world currently have limited access to these technologies and appropriate information. For some island nations, the high cost of providing adequate protection would make it essentially infeasible, especially given the limited availability of capital for investment. The efficacy and cost-effective use of adaptation strategies will depend upon the availability of financial resources, technology transfer, and cultural, educational, managerial, institutional, legal, and regulatory practices, both domestic and international in scope. Incorporating climate change concerns into resource-use and development decisions and plans for regularly scheduled investments in infrastructure will facilitate adaptation.

4.

ANALYTICAL APPROACH TO STABILIZATION OF ATMOSPHERIC
CONCENTRATIONS OF GREENHOUSE GASES

4.1 Article 2 of the UN Framework Convention on Climate Change refers explicitly to "stabilization of greenhouse gas concentrations”. This section provides information on the relative importance of various greenhouse gases to climate forcing and discusses how greenhouse gas emissions might be varied to achieve stabilization at selected atmospheric concentration levels.

4.2 Carbon dioxide, methane and nitrous oxide bave natural as well as anthropogenic origins. The anthropogenic emissions of these gases have contributed about 80% of the additional climate forcing due to greenhouse gases since pre-industrial times (i.e. since about 1750 A.D). The contribution of CO2 is about 60% of this forcing, about four times that from CH..

4.3 Other greenhouse gases include tropospheric ozone (whose chemical precursors include · nitrogea oxides, non-methane hydrocarbons and carbon monoxide), halocarbons (including HCFCsand HFCs) and SFg. Tropospheric aerosols and tropospheric ozone are inhomogeneously distributed in time and space and their atmospheric lifetimes are short (days to weeks). Sulphate aerosols are amenable to abatement measures and such measures are presumed in the PCC scenarios.

4.4 Most emission scenarios indicate that, in the absence of mitigation policies, greenhouse gas emissions will continue to rise during the next century and lead to greenhouse gas concentrations that by the year 2100 are projected to change climate more than that projected for twice the pre-industrial concentrations of carbon dioxide.

Most halocarbons, but neither HFCs nor PFCs, are controlled by the Montreal Protocol and its

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