The world community faces many risks from climate change. Clearly, it is important to understand the nature of those risks, where natural and human systems are likely to be most vulnerable, and what may be achieved by adaptive responses. To understand better the potential impacts and associated dangers of global climate change, Working Group II of the Intergovernmental Panel on Climate Change (IPCC) offers this Third Assessment Report (TAR) on the state of knowledge concerning the sensitivity, adaptability, and vulnerability of physical, ecological, and social systems to climate change. Building on the Second Assessment Report (SAR), this new report reexamines key findings of the earlier assessment and emphasizes new information and implications on the basis of more recent studies.
Human activities-primarily burning of fossil fuels and changes in land cover-are modifying the concentration of atmospheric constituents or properties of the Earth's surface that absorb or scatter radiant energy. In particular, increases in the concentrations of greenhouse gases (GHGs) and aerosols are strongly implicated as contributors to climatic changes observed during the 20th century and are expected to contribute to further changes in climate in the 21st century and beyond. These changes in atmospheric composition are likely to alter temperatures, precipitation patterns, sea level, extreme events, and other aspects of climate on which the natural environment and human systems depend.
One of several primary issues this report has been organized to address is a key question before the United Nations Framework Convention on Climate Change (UNFCCC): What are the potential impacts for societies and ecosystems of different atmospheric concentrations of GHGs and aerosols that absorb and scatter sunlight (United Nations, 1992)? Answering this question is a necessary step in assessing what constitutes "dangerous anthropogenic interference in the climate system." This report does not make any judgments about what level of concentrations is "dangerous" because that is not a question of science per se but a value judgment about relative risks and tradeoffs. The task is to make the evidence about relative risks as clear as possible. This report therefore describes what is known about the distribution of impacts; how, why, and to what extent they differ from region to region or place to place; and how this relates to the distribution of vulnerability and capacity to adapt. However, it critically assesses the literature to help inform policymakers about effects associated with different concentration levels, so they may judge what levels of risk are acceptable. Assessment of what constitutes dangerous interference in the climate systems will require analysis of the interactions of climate change and social and economic conditions, which are inextricably linked. Understanding the role of socioeconomic factors, particularly adaptive responses and capacity, is critical.
Although this report builds on previous assessments, including the SAR and the IPCC's Special Report on Regional Impacts of Climate Change (IPCC, 1998), the TAR departs from them in important respects. In comparison to previous assessments, greater attention is given to climate change adaptation; multiple pressures on systems; links between climate change, sustainable development, and equity; and characterization of the state-of-the-science and confidence levels associated with key conclusions of the assessment (see Box 1-1). This overview chapter does not attempt to provide a comprehensive summary of the principal findings of the TAR, but it helps to illustrate basic concepts by selectively reporting on a few key conclusions, as well as providing a more comprehensive road map to the materials presented later in the report:
Box 1-1 Uncertainties and Confidence Scale
The many conclusions presented in this report are subject to varying degrees of uncertainty. The degree of uncertainty attached to conclusions in this report are assessed and reported in two different ways. One is to assess and report a confidence level for a conclusion, using a Bayesian probability framework. (Bayesian assessments of probability distributions would lead to the following interpretation of probability statements: The probability of an event is the degree of belief that exists among lead authors and reviewers that the event will occur, given observations, modeling results, and theory currently available.) The second is to assess and report the quality or level of scientific understanding that supports a conclusion.
The 5-point confidence scale below is used to assign confidence levels to selected conclusions. The confidence levels are stated as Bayesian probabilities, meaning that they represent the degree of belief among the authors of the report in the validity of a conclusion, based on their collective expert judgment of all observational evidence, modeling results, and theory currently available to them.
5-Point Quantitative Scale for Confidence Levels
For some conclusions, the 5-point quantitative scale is not appropriate as a characterization of associated uncertainty. In these instances, authors qualitatively evaluate the level of scientific understanding in support of a conclusion, based on the amount of supporting evidence and the level of agreement among experts about the interpretation of the evidence. The matrix below has been used to characterize the level of scientific understanding.
Key to Qualitative "State of Knowledge" Descriptors
Areas of important new findings include detection of impacts of observed climatic changes on environmental systems, transient scenarios, vulnerability to changes in climate variability, and vulnerability to strongly nonlinear, complex, and discontinuous responses to climate change. Another distinction from previous assessments is the recognition in the TAR that the many complexities of analysis logically lead to a focus on ranges of outcomes and characterizations, using subjective probabilities of events, rather than primary emphasis on "best guesses," point estimates, single "optimum," or aggregate conclusions. Finally, the TAR has circulated cross-cutting "guidance papers" to all three working groups to try to achieve more consistency in dealing with four areas: development, sustainability, and equity; uncertainties; costing methodologies; and decision analytic frameworks (see Box 1-2).
The IPCC's charge to Working Group II for the TAR implies that consideration of the impacts of climate change in the SAR is insufficient per se as a basis for decisionmaking. In general, the SAR was able to address the implications of climate change only for single economic sectors or environmental components. With this in mind, Chapters 4-19 consider the various decision analysis framework tools to improve upon the responses to the impacts of climate change provided in the SAR. In the current exercise, not only are possible implications of climate change for the various economic sectors or environmental components assessed, options to alleviate identified impacts are investigated. In addition, direct and indirect costs of adaptation options are explored, and an extensive assessment of direct and indirect benefits is provided. Where monetary values can be assigned, CBA is employed to determine the optimal value of adaptation measures, including sensitivity analysis to critical parameters, and CEA is adopted to identify the least-cost solution to targeted mitigation objectives.
Box 1-2. Cross-Cutting Issues Guidance Papers
Four cross-cutting guidance papers (Pachauri et al., 2000) were available to all Lead Authors of all three IPCC working groups. Many of the concepts in these papers were previously unfamiliar to a large number of the Lead Authors. Significant efforts were made to incorporate the uncertainties guidance and at least consider the guidance in other papers by each working group. Future assessments will be increasingly able to benefit from the suggestions and frameworks in these four cross-cutting guidance papers.
Development, Sustainability, and Equity (DSE) (Munasinghe, 2000)
DSE is relevant to Working Group II with respect to three questions: How do development paths and equity conditions influence vulnerability to climate change and the capacity to adapt to or cope with climate change? How might climate change impacts and adaptation responses affect prospects for attaining sustainable development and equity goals? What types of policies are capable of reducing climate change vulnerability and promoting sustainable development and equity objectives?
DSE is a response to principles contained in UNFCCC Article 3.4 (to promote sustainable development). Article 3.2 takes into account the special needs and circumstances of developing countries. Article 4 deals, for example, with the responsibility of developed nations, competing priorities for developing nations, and "common, but differentiated responsibilities," and Article 2 says to avoid "dangerous" interference with the climate system (United Nations, 1992). DSE is closely tied to sustainable development with respect to three underlying dimensions: economic, social, and environmental.
Development has been characterized as "qualitative improvement" (Ishida, 1998), including economic growth and social dimensions. Sustainability of a system refers to its durability or its capacity to withstand and recover from disturbances (WCED, 1987)-in other words, its resilience. Equity refers to procedural as well as distributional issues. Procedural issues relate to how decisions are made (e.g., internal equity and governance structures within nations could have significant effects on adaptive capacity). Distributional equity, on the other hand, relates to how the costs of impacts, mitigation, and adaptation are shared. Equity considerations are important in addressing global climate change for several reasons, including moral and ethical concerns; facilitating cooperation because equitable decisions carry greater legitimacy; the social dimension of sustainable development; and the UNFCCC itself, which considers equity as one of its basis principles (in Article 3.1).
Climate change could undermine social welfare, equity, and the sustainability of future development. In particular, it is generally believed that developing countries and disadvantaged groups within all countries are more vulnerable to the impacts of climate change (e.g., Chapter 18) as a result of limited resources and low adaptive capacity.
Uncertainty (Moss and Schneider, 2000)
Anticipating the imperfect nature of available information, UNFCCC Article 3.3 provides guidance to the effect that "where there are threats of serious or irreversible damage, lack of full scientific certainty should not be used as a reason for postponing measures to anticipate, prevent, or minimize the causes of climate change and mitigate its adverse effects" (United Nations, 1992). The uncertainties guidance paper develops a unified approach for assessing, characterizing, and reporting uncertainties in the TAR. The most important contribution of the uncertainties guidance paper is the construction of confidence schemes and qualitative terms to describe the state of science, which are reproduced in Box 1-1. The goal is to promote consistency in evaluating the judgments of scientific experts, to facilitate communication of these judgments to nonspecialists, and to provide peer-reviewed guidance for policymakers. Thus, a great deal of importance is attached to assessing the scientific merit of information in the literature and "explicitly distinguishing and communicating which findings are well understood, which are somewhat understood, and which are speculative" (Moss and Schneider, 1997). Section 2.6 discusses in more detail the differences between the well-calibrated ranges in the literature and the much larger, full range of uncertainty as well as the cascade of uncertainty that occurs when ranges in climate scenarios are cascaded with uncertainties in each successive step of assessment. In Chapters 4-19, selected findings are assigned levels of confidence, using either the scale for assessing confidence level quantitatively or the matrix for assessing the state of knowledge qualitatively.
Especially in the regional chapters, uncertainty about future climate is the dominant cause of uncertainty about the character and magnitude of impacts. In such cases, confidence estimates are evaluated conditionally on a specific climate change scenarios to avoid "cascades" (see Figure 2-2) in which confidence in the occurrence of an event does not include compounded uncertainties in each factor that contributes to the final outcome. Instead, the assessment evaluates each step in the cascade separately-what is called a "traceable account"-and is particularly appropriate for any aggregate conclusions.
The concluding section of this report (Chapters 18 and 19) examines global issues and offers a synthesis. A significant addition to previous assessments is Chapter 18, which is devoted to assessment of opportunities for and barriers to adaptation. This chapter considers determinants of adaptive capacity; lessons from adaptation to present-day climate variability and extremes; the potential effectiveness of adaptation measures; and global-, national-, and local-scale options for strengthening adaptive capacity, especially for vulnerable populations, countries, or zones. Chapter 19, a synthesis, also is new to the TAR. It draws on the analyses of other chapters, synthesizing information that is important for interpretation of Article 2 of the UNFCCC and key provisions of international agreements to address climate change. Potential global impacts of different stabilization levels of atmospheric concentrations of GHGs are assessed. Chapter 19 assesses vulnerability within the framework of sustainable development and equity, acknowledging common but differentiated responsibilities.
The issue of what constitutes sustainable development was advanced in 1987 by the World Commission on Environment and Development (the so-called Brundtland Commission; WCED, 1987). The commission defined sustainable development as "development that meets the needs of the present without compromising the ability of future generations to meet their own needs" and notes that "even the narrow notion of physical sustainability implies a concern for social equity between generations, a concern that must logically be extended to equity within each generation." The goal of sustainable development is a stable human environmental system in which available resources are sufficient to meet the needs of society in perpetuity. Questions have been asked about whether "needs," as conceived in the Brundtland Commission report, should be limited to environment-dependent basic necessities of food, clothing, shelter, and health or should include more qualitative aspects such as comfort, convenience, or other "quality of life" measures. There is no consensus in the literature regarding what constitutes the limits of "needs" in this context.
Because available studies have not employed a common set of climate scenarios and methods and because of uncertainties regarding the sensitivities and adaptability of natural and human-dominated systems, assessment of regional vulnerabilities is necessarily qualitative. Whenever possible, quantitative estimates of the impacts of climate change are cited in this report. Field or experimental data often provide quantitative underpinnings for specific circumstances, but rarely are complex systems sufficiently described by the limited number of cases in which a large quantity of "hard data" is available. Thus, most quantitative estimates are still dependent on the specific assumptions employed regarding future changes in climate, as well as the particular methods and models applied in the analyses. On the other hand, issues in which there is a great deal of relevant field or lab data are likely to carry higher confidence in any such quantitative estimate. To interpret these estimates, it is important to bear in mind uncertainties regarding the character, magnitude, and rates of future climate change that will affect society's degree of exposure. Of comparable importance are uncertainties associated with future states of the human condition-for example, the extent and quality of economic development throughout the world and the evolution of traditions and institutions in societies-that will affect profoundly the capacity for coping and adaptation, hence level of vulnerability. These uncertainties impose limitations on the ability of the research community to project the impacts of climate change, particularly at regional and smaller scales.
This introductory chapter is organized to address a series of questions: What is potentially at stake as a result of changes in climate (Section 1.2)? How has society responded to the risks and potential opportunities (Section 1.3)? How are impacts, adaptation, and vulnerability assessed in the report (Section 1.4)? How do the complexities of analysis affect the assessment (Section 1.5)? How can this assessment be used to address policy-relevant questions (Section 1.6)?
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