The stakes surrounding anthropogenic climate change can be very high in terms of the vulnerabilities of some sectors and regions and in terms of the distributional consequences of actions taken to deal with these possibilities. The context is that humankind already is challenged today to provide the opportunity for this and future generations to achieve a more sustainable and equitable standard of living. Billions of people today live without adequate nourishment, access to clean water, modern energy services, and other basic human needs (see, e.g., UNDP, 1999). Providing for the increasing well-being of humans, especially the poor, in the context of sustainable and equitable development is one of the great challenges of the 21st century. Unabated climate change is likely to make meeting this challenge significantly more difficult. On the other hand, it also is argued (e.g., Grossman and Krueger, 1995) that increasing economic growth may lead to reductions in population growth and environmental degradation. Throughout the past century, however, per capita carbon dioxide (CO₂) emissions from combustion of fossil fuels have been driven primarily by growth in gross domestic product (GDP) per capita (although the growth rate in CO₂ emissions generally has not been as fast as the growth in GDP, owing to improvements in energy and carbon intensities of industrial economies (e.g., Hoffert et al., 1998).

The impact (I) of a given population on the environment can be decomposed into the product of three factors: population size (P), affluence per capita (A), and unit impact per unit of affluence, which is related to technologies used (T) (Ehrlich and Holdren, 1971; Ehrlich and Ehrlich, 1990). Rising per capita consumption and a growing world population have resulted in unprecedented human resource use, which is altering global systems, including climate (Bartiaux and van Ypersele, 1993; Yang and Schneider, 1998). According to all of the scenarios considered in the IPCC�s Special Report on Emissions Scenarios (SRES) (IPCC, 2000), the human population will continue to grow until at least 2050, reaching a population that is 60�100% larger than it was in 1990. The SRES scenarios describe futures that generally are more affluent than today; many of the SRES scenarios assume a narrowing of income differences (in relative but not absolute terms) among world regions. This implies that the third factor in the �I=PAT� identity, the unit impact per unit of affluence, will have a very important role in assessment of the global impact of human activities. Increasing population and affluence, if not accompanied by significant decreases in unit impact per unit of affluence, will make the challenge of promoting sustainable development even more difficult�particularly in developing countries, where most of the increase in population is projected to take place.

We have reached the point that the cumulative interaction of several factors related to human activities (e.g., land-use changes and emissions of GHGs, ozone-depleting substances, and local air pollutants) increases the risk of causing or aggravating potentially irreversible events, such as loss of species, forests, human settlements, glaciers, or heritage sites near coastlines and, in the long term, altered oceanic circulation regimes.

Although some regions may experience beneficial effects of climate change (e.g., increasing agricultural productivity at high latitudes), previous IPCC assessments have concluded that net negative climate impacts are more likely in most parts of the world (assessment of potential positive and negative impacts is one of the main purposes of this report; see Sections 2.5.6 and 2.6.4 and subsequent sections in this chapter for a discussion of uses of and problems with net monetary aggregation of impacts, and see Chapter 19 for a synthesis). These impacts will affect human welfare directly and indirectly, in many cases undercutting efforts to promote sustainable development that, in turn, serve as driving forces of environmental change.

Moreover, the time scales of change vary tremendously. For environmental systems, these time scales range from decades (for restoration of slightly disturbed ecosystems) to many centuries (for equilibration of the climate system and sea level), even with a stable level of atmospheric GHG concentrations. These environmental time scales imply that human activities in the short term will set in motion a chain of events with long-term consequences for the climate that cannot be reversed quickly, if at all.

For most human institutions, the time scales range from years for very short electoral cycles that determine the tenure of a government to a half-century or more for the useful lifetimes of buildings and major infrastructure such as irrigation projects, transportation networks, or energy supply systems. It may take a generation or more to effect significant changes to institutions. Because of these time scales, some decisions taken during the next few decades may limit the range of possible options in the future with respect to emissions reduction and adaptation, whereas other decisions may expand this range of options. During this period, many more insights into the effects and impacts of climatic changes will emerge. However, it is well established that uncertainties will remain, and efforts to manage risks in the face of considerable uncertainty will be a characteristic of climatic change assessments for decades more.

Working Group II�s contribution to the TAR focuses principally on the time horizon reaching from the present to the year 2100�which reflects the preponderance of studies on this time period in the literature and the high degree of uncertainty about the state of socioenvironmental systems beyond the 21st century. By 2100, most projections of human-induced climate change fall into ranges of about 1.4 to almost 5.8�C increase in annual global mean surface temperature (see Figure 5d in the TAR WGI Summary for Policymakers) compared to 1990 (although estimates that are outliers to both ends of even this large range can be found in the literature; Morgan and Keith, 1995) and about 10- to 90-cm rise in mean sea level (Figure 5e, TAR WGI Summary for Policymakers). By the time of doubling of CO₂-equivalent concentration, the global mean precipitation is projected to be about 1�5% higher than in 1990. These global numbers hide complex spatial patterns of changes (especially for temperature and precipitation), which are summarized in Chapter 3. In some regions, temperature increases are projected to be three times the global mean. In addition, high confidence is attached to �projected changes in extreme weather and climatic events�. Such changes, particularly at the higher ends of the ranges given, represent significant deviations from the climatic conditions of recent centuries. As noted above, warming of the climate and sea-level rise would continue for centuries beyond 2100, even if atmospheric concentrations of GHGs stabilize during the 21st century. For perspective, it should be noted that since the early Miocene (about 24 million years ago), atmospheric CO₂ concentrations appear to have remained below 500 ppmv (Pearson and Palmer, 2000). If human emissions of GHG until 2100 remain at or�as in many scenarios in the literature�increase well beyond current levels, CO₂ concentrations will be significantly above this value. It can therefore be remarked that climate changes in the 22nd century could exceed any experienced in more than 1 million years (see, e.g., Crowley, 1990; Crowley and North, 1991). Indeed, these authors estimate that global temperature was never significantly warmer than the present during the past 2 million years and that one would need to return to the early Pliocene (3�5 million years ago) or even the Miocene (5�25 million years ago) to find a climate that is warmer than today by more than 2�C. The potential impacts of these very large projected changes cannot be disregarded, even though it is difficult to imagine what human societies would look like in the 22nd century (see, e.g., Cline, 1992). However�reflecting the scarcity of studies of climatic impacts beyond 2100, despite their potential relevance to Article 2 of the UNFCCC�these impacts are not a major focus in the TAR (although Chapter 19 does focus on the possibilities of abrupt, nonlinear, and/or irreversible climatic changes in the centuries ahead).

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