climate change and the least adaptability, (iii) most systems are sensitive to both the magnitude and rate of climate change; (iv) many of the impacts are difficult to quantify because existing studies are limited in scope; and (v) successful adaptation depends upon technological advances, institutional arrangements, availability of financing and information exchange, and that vulnerability increases as adaptation capacity decreases. Therefore, developing countries are more vulnerable to climate change than developed countries.

The range of adaptation options for managed systems such as agriculture and water supply is generally increasing because of technological advances. However, some regions of the world, i.e., developing countries, have limited access to these technologies and appropriate information. The efficacy and cost-effectiveness of adaptation strategies will depend upon cultural, educational, managerial, institutional, legal and regulatory practices that are both domestic and international in scope. Incorporation of climate change concerns into resource-use and development decisions and plans for regularly scheduled investments in infrastructure will facilitate adaptation.

Let me now briefly discuss the implications of climate change for a representative number of systems: natural ecosystems (forests and coral reefs), food security, water resources, sea level rise, and human health.

Natural Ecosystems-Forests

The composition and geographic distribution of many ecosystems will shift as individual species respond to changes in climate, and there will likely be reductions in biological diversity (particularly species diversity) and in the goods and services ecosystems provide society, e.g., sources of food, fiber, medicines, recreation and tourism, and ecological services such as controlling nutrient cycling, waste quality, water run-off, and soil erosion. Models project that as a consequence of possible changes in temperature and water availability under doubled carbon dioxide equilibrium conditions, a substantial fraction (a global average of one-third, varying by region from one-seventh in tropical forests to two-thirds in Boreal forests) of the existing forested area of the world will undergo major changes in broad vegetation types. Climate change is expected to occur at a rapid rate relative to the speed at which forest species grow, reproduce and re-establish themselves. For mid-latitude regions a global average warming of 1-3.5°C over the next 100 years would be equivalent to a poleward shift of isotherms of approximately 150-550 km or an altitude shift of 150-550 meters. This compares to past tree species migration rates that are believed to be on the order of 4 200 km per century. Therefore, species composition of impacted forests is likely to change, entire forest types may disappear, while new assemblages of species and hence new forest ecosystems may be established. Large amounts of carbon could be released into the atmosphere during times of high forest mortality prior to regrowth of a mature forest.

Ecological models suggest significant changes in vegetation cover within the U.S. Potential changes include a northward shift in forest and other vegetation species, e.g., there would be an almost complete loss of sugar maple and beach trees in the Eastern US.

Coral reefs, the most biologically diverse marine ecosystems, are important for fisheries, tourism, coastal protection, and erosion control. Coral reef systems, which are already being threatened by pollution, unsustainable tourism and fishing practices, are very vulnerable to changes in climate. While these systems may be able to adapt to the projected increases in sea level, sustained increases in water temperatures of 3-4°C above long-term average seasonal maxima over a 6-month period can cause significant coral mortality; short-term increases on the order of only 1-2°C can cause “bleaching", leading to reef destruction. Indications are that the full restoration of coral communities could require several centuries.

Food Security

Currently, 800 million people are malnourished; as the world's population increases and incomes in some countries rise, food consumption is expected to double over the next three to four decades. Studies show that on the whole, global agricultural production could be maintained relative to baseline production in the face of climate change under doubled carbon dioxide equilibrium conditions. However, crop yields and changes in productivity due to climate change will vary considerably across regions and among localities, thus changing the patterns of production. In general, productivity is projected to increase in middle to high latitudes, depending on crop type, growing season, changes in temperature regime, and seasonally of precipitation, where-as in the tropics and subtropics, where some crops are near their maximum temperature tolerance and where dryland, non-irrigated agriculture dominates, yields are likely to decrease, especially in Africa and Latin America, where decreases in overall agricultural productivity of 30% are projected under doubled carbon dioxide conditions. Therefore there may be increased risk of hunger in some locations in the tropics and subtropics where many of the world's poorest people live.

While the productivity of agriculture in North America is moderately to highly sensitive to climate change, the vulnerability is thought to be low at the continental scale, although subregional variation losses or gains are likely. For example, a warming of 4-5 degrees Centigrade is projected to lead to negative impacts in Eastern, Southeastern and corn belt regions, but positive effects in northern plains and Western regions. These model calculations have considered the positive effects of higher atmospheric levels of carbon dioxide, but have not fully considered the effects of potential changes in climate variability, water availability, stresses from pests, diseases, and interactions with other existing stresses.

Water Resources

Currently 1.3 billion people do not have access to adequate supplies of safe water, and 2 billion people do not have access to adequate sanitation. Today, some nineteen countries, primarily in the Middle East and Africa, are classified as water-scarce or water-stressed. Even in the absence of climate change, this number is expected to double by 2025, in large part because of increases in demand from economic and population growth. Climate change will further exacerbate the frequency and magnitude of droughts in some places, in particular Africa where droughts are already a recurrent feature. Developing countries are highly vulnerable to climate

The Great Plains and prairie regions of the US are particularly vulnerable, with projected increases in run-off in winter and spring and decreases in soil moisture and run-off in summer.

Sea Level Rise

Sea-level rise can have negative impacts on tourism, freshwater supplies, fisheries, exposed infrastructure, agricultural and dry lands, and wetlands. It is currently estimated that about half of the world's population lives in coastal zones, although there is a large variation among countries. Changes in climate will affect coastal systems through sea-level rise and an increase in stormsurge hazards, and possible changes in the frequency and/or intensity of extreme events. Impacts may vary across regions, and societal costs will greatly depend upon the vulnerability of the coastal system and the economic situation of the country. Sea-level rise will increase the vulnerability of coastal populations to flooding. An average of about 46 million people per year currently experience flooding due to storm surges; a 50 cm sea-level rise would increase this number to about 92 million; a 1 meter sea-level rise would increase this number to 118 million. The estimates will be substantially higher if one incorporates population growth projections. A number of studies have shown that small islands and deltaic areas are particularly vulnerable to a one-meter sea-level rise. In the absence of mitigation actions (e.g., building sea walls), land losses are projected to range from 1.0% for Egypt, 6% for Netherlands, 17.5% for Bangladesh, to about 80% of the Marshall Islands, displacing tens of millions of people, and in the case of low-lying Small Island States, the possible loss of whole cultures. Many nations face lost capital value in excess of 10% of GDP. While annual adaptation/protection costs for most of these nations are relatively modest (about 0.1% GDP), average annual costs to many small island states are much higher, several percent of GDP, assuming adaptation is possible.

Sea level rise is not only an issue for Small Island States and deltaic areas in developing countries, but poses a threat to many low-lying areas in the USA, in particular, the East coast and Gulf coast. IPCC noted that sea level has been rising relative to the land along most of the coast of North America, and falling in a few areas, for thousands of years. During the next century, a 50-cm rise in sea level from climate change alone could inundate 8,500 to 19,000 square kms of dry land, expand the 100 year flood plain by more than 23,000 square kms, and eliminate as much as 50% of North America's coastal wetlands. The projected changes in sea level due to climate change alone would under-estimate the total change in sea level from all causes along the eastern seaboard and Gulf coast of North America.

Human Health

Human health is sensitive to changes in climate because of changes in food security, water supply and quality, and the distribution of ecological systems. These impacts would be mostly adverse, and in many cases would cause some loss of life. Direct health effects would include increases in heat-related mortality and illness resulting from an anticipated increase in heatwaves. Indirect effects would include extensions of the range and season for vector organisms, thus increasing the transmission of vector-borne infectious diseases (e.g., malaria, dengue, yellow fever and encephalitis). Projected changes in climate under doubled carbon dioxide equilibrium conditions could lead to potential increases in malaria incidence of the order of 50-80 million additional cases annually, primarily in tropical, subtropical, and less well-protected temperate

zone populations. Some increases in non-vector-borne infectious diseases such as salmonellosis, cholera and other food- and water-related infections could also occur, particularly in tropical and subtropical regions, because of climatic impacts on water distribution and temperature, and on micro-organism proliferation.

Social Costs of Climate Change

The range of estimates of economic damages caused by changes in climate are quite uncertain. Taking into account both market and non-market costs, IPCC reported a reduction in world GDP of 1.5-2.0% for a doubled carbon dioxide environment. This value was obtained by summing widely varying estimates of damages by sector, including socio-economic sectors (e.g., agriculture, forestry, fisheries), ecological systems, and human health. Nordhaus, conducted an "expert" survey which resulted in a range from 0 to 21% for loss of world GDP, with a mean value of 3.6% and a median value of 1.9%.

Losses in developing countries are estimated to be much higher than the world average, ranging from 5% to 9%. Alternate assumptions about the value of a statistical life could increase the estimate of economic damages in developing countries.

IPCC reported values for the marginal damage of one extra ton of carbon emitted ranging from $5 to $125. A value of $5 to $12 per ton of carbon is obtained using a 5% social rate of time preference (discount rate). Lower discount rates increase this estimate, e.g. a 2% discount rate would increase this estimate by an order of magnitude.

Approaches to Reduce Emissions and Enhance Sinks

Significant reductions in net greenhouse gas emissions are technically, and often economically, feasible and can be achieved by utilizing an extensive array of technologies and policy measures that accelerate technology diffusion in the energy supply (more efficient conversion of fossil fuels, switching from high to low carbon fossil fuels; decarbonization of flue gases and fuels, coupled with carbon dioxide storage, increasing the use of nuclear energy; and increased use of modern renewable sources of energy (e.g., plantation biomass, micro-hydro, and solar), energy demand (industry, transportation, and residential/commercial buildings) and agricultural/forestry sectors (altered management of agricultural soils and rangelands, restoration of degraded agricultural lands and rangelands, slowing deforestation, natural forest generation, establishment of tree plantations, promoting agroforestry, and improving the quality of the diet of ruminants). By the year 2100, the world's commercial energy system will be replaced at least twice offering opportunities to change the energy system without premature retirement of capital stock. However, full technical potential is rarely achieved because of a lack of information and cultural, institutional, legal and economic barriers.

Policy instruments can be used to facilitate the penetration of lower carbon intensive technologies and modified consumption patterns. These policies include: energy pricing strategies (e.g., carbon taxes and reduced energy subsidies); reducing or removing other subsidies that

increase greenhouse gas emissions (e.g., agricultural and transport subsidies); incentives such as provisions for accelerated depreciation and reduced costs for the consumer; tradable emissions permits (and joint implementation); voluntary programs and negotiated agreements with industry; utility demand-side management programs; regulatory programs including minimum energy efficiency standards; market pull and demonstration programs that stimulate the development and application of advanced technologies; and product labeling. The optimum mix of policies will vary from country to country; policies need to be tailored for local situations and developed through consultation with stakeholders.

Estimates of the costs of mitigating climate change should take into account secondary benefits of switching from a fossil fuel based economy to a lower-carbon intensity energy system. Secondary benefits include lower levels of local and regional pollution, including particulates, ozone and acid rain.

The Challenge of Stabilization

It is important to recognize what emissions limitations are required in order to stabilize the atmospheric concentrations of carbon dioxide at different levels. Figure 1 shows time-dependent emissions profiles for stabilization at different levels (450 to 1000 ppmv). It is instructive to note that: (i) global emissions will have to depart from IPCC IS 92a (classically referred to as businessas-usual) within the next few decades to achieve any of these stabilization levels, and (ii) independent of the eventual stabilization level, global emissions in the long-term will have to be well below today's level. Figure 1 illustrates two pathways for each stabilization level--this recognizes that the final stabilization level is sensitive to cumulative emissions over time, not instantaneous emissions in any given year, i.e., higher emissions in the near-term would have to be compensated for lower emissions later. Examination of the 550ppmv profile shows that even if we take the "delayed action" scenario (i.e., the one which some economists believe to be more economically efficient), global emissions will have to depart from business-as-usual between 2010 and 2015, and peak between 2030 and 2035. In order to deviate from business-as-usual by 2010 to 2015 would require actions to be taken now.

Summary

Policymakers are faced with responding to the risks posed by anthropogenic emissions of greenhouse gases in the face of significant scientific uncertainties. They should consider these uncertainties in the context that climate-induced environmental changes cannot be reversed quickly, if at all, due to the long time scales (decades to millennia) associated with the climate system. Decisions taken during the next few years may limit the range of possible policy options in the future because high near-term emissions would require deeper reductions in the future to meet any given target concentration. Delaying action might reduce the overall costs of mitigation because of potential technological advances but could increase both the rate and the eventual magnitude of climate change, and hence the adaptation and damage costs.