Climate Change 2001:
Working Group II: Impacts, Adaptation and Vulnerability
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The purpose of this chapter is to address several overarching methodological issues that transcend individual sectoral and regional concerns. In so doing, this chapter focuses on five related questions: How can current effects of climate change be detected? How can future effects of climate change be anticipated, estimated, and integrated? How can impacts and adaptations be valued and costed? How can uncertainties be expressed and characterized? What frameworks are available for decisionmaking? In addressing these questions, each section of Chapter 2 seeks to identify methodological developments since the Second Assessment Report (SAR) and to identify gaps and needs for further development of methods and tools.

Detection of Response to Climate Change by Using Indicator Species or Systems
Assessment of the impacts on human and natural systems that already have occurred as a result of recent climate change is an important complement to model projections of future impacts. Such detection is impeded by multiple, often inter-correlated, nonclimatic forces that are concurrently affecting those systems. Attempts to overcome this problem have involved the use of indicator species to detect responses to climate change and to infer more general impacts of climate change on natural systems. An important component of the detection process is the search for systematic patterns of change across many studies that are consistent with expectations, based on observed or predicted changes in climate. Confidence in attribution of these observed changes to climate change increases as studies are replicated across diverse systems and geographic regions.

Since the SAR, approaches to analysing and synthesizing existing data sets from abiotic and biotic systems have been developed and applied to detection of present impacts of 20th-century climate change. Even though studies now number in the hundreds, some regions and systems are underrepresented. However, there is a substantial amount of existing data that could fill these gaps. Organized efforts are needed to identify, analyze, and synthesize those data sets.

Anticipating the Effects of Climate Change
A wide range of methods and tools are now used and available for studies of local, regional, and global impacts. Since the SAR, improvements have included greater emphasis on the use of process-oriented models and transient climate change scenarios, refined socioeconomic baselines, and higher resolution assessments. Country studies and regional assessments in every continent have tested models and tools in a variety of contexts. First-order impact models have been linked to global systems models. Adaptation has been included in many assessments, often for the first time.

Methodological gaps remain concerning scales, data, validation, and integration. Procedures for assessing regional and local vulnerability and long-term adaptation strategies require high-resolution assessments, methodologies to link scales, and dynamic modeling that uses corresponding and new data sets. Validation at different scales often is lacking. Regional integration across sectors is required to place vulnerability in the context of local and regional development. Methods and tools to assess vulnerability to extreme events have improved but are constrained by low confidence in climate change scenarios and the sensitivity of impact models to major climatic anomalies. Understanding and integrating higher order economic effects and other human dimensions of global change are required. Adaptation models and vulnerability indices to prioritize adaptation options are at early stages of development in many fields. Methods to enable stakeholder participation in assessments need improvement.

Integrated Assessment
Integrated assessment is an interdisciplinary process that combines, interprets, and communicates knowledge from diverse scientific disciplines in an effort to investigate and understand causal relationships within and between complicated systems. Methodological approaches employed in such assessments include computer-aided modeling, scenario analyses, simulation gaming and participatory integrated assessment, and qualitative assessments that are based on existing experience and expertise.

Since the SAR, significant progress has been made in developing and applying these approaches to integrated assessment, globally and regionally. However, the emphasis in such integrated assessments, particularly in integrated modeling, has been on mitigation; few existing studies have focused on adaptation and/or determinants of adaptive capacity. Methods designed to include adaptation and adaptive capacity explicitly in specific applications need to be developed.

Costing and Valuation
Methods of economic costing and valuation rely on the notion of opportunity cost of resources used, degraded, or saved. Opportunity cost depends on whether the market is competitive or monopolistic and whether any externalities are present. It also depends on the rate at which future costs are discounted, which can vary across countries, over time, and over generations. The impact of uncertainty also can be valued if the probabilities of different possible outcomes are known. Public and nonmarket goods and services can be valued through willingness to pay for them or willingness to accept compensation for lack of them. Impacts on different groups, societies, nations, and species need to be assessed. Comparison of alternative distributions of welfare across individuals and groups within a country can be justified if they are made according to internally consistent norms. Comparisons across nations with different societal, ethical, and governmental structures cannot yet be made meaningfully.

No new fundamental developments in costing and valuation methodology have taken place since the SAR. Many new applications of existing methods to a widening range of climate change issues, however, have demonstrated the strengths and limitations of some of these methods. For example, many contingent valuation studies have raised questions about the reliability of such evaluations. Similarly, more attention is now paid to the limitations of methods that underlie efforts to reduce all impacts to one monetary value and/or to compare welfare across countries and cultures. Multi-objective assessments are preferred, but means by which their underlying metrics might more accurately reflect diverse social, political, economic, and cultural contexts need to be developed. In addition, methods for integrating across these multiple metrics are still missing from the methodological repertoire.

Treatment of Uncertainties
The Earth's linked climate and social-natural systems are very complex; thus, there are many unresolved uncertainties in nearly all aspects of the assessment of climatic impacts, vulnerabilities, and adaptation. Subjective judgments are inevitable in most estimates of such complex systems. Since the SAR, more consistent treatment of uncertainties and assessment of biases in judgments have been attempted. Progress also has been made in developing methods for expressing confidence levels for estimates, outcomes, and conclusions, based on more consistent quantitative scales or consistently defined sets of terms to describe the state of the science. Notable attempts to provide "traceable accounts" of how disaggregated information has been incorporated into aggregated estimates have been made, but more work is needed. Greater attention to eliminating inconsistent use of confidence terms or including a full range of uncertainty for key results is still needed in future assessments. Whereas significant progress on issues of uncertainty has been achieved in the context of impacts and vulnerability, a major challenge now lies in addressing uncertainties associated with adaptability.

Decision Analytic Frameworks
Policymakers who are responsible for devising and implementing adaptive policies should be able to rely on results from one or more of a diverse set of decision analytical frameworks. Commonly used methods include cost-benefit and -effectiveness analyses, various types of decision analysis (including multi-objective studies), and participatory techniques such as policy exercises, but there are many other possible approaches. Among the large number of assessments of climate change impacts reviewed in this volume, only a small fraction include comprehensive and quantitative estimates of adaptation options and their costs, benefits, and uncertainty characteristics. This information is necessary for meaningful applications of any decision analytical method. Very few cases in which decision analytic frameworks have been used in evaluating adaptation options have been reported. Greater use of methods in support of adaptation decisions is needed to establish their efficacy and identify directions for necessary research in the context of vulnerability and adaptation to climate change.

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