In assessing impacts, vulnerability, and adaptation to climate change, a large array of methods and tools pertain to specific sectors, scales of analysis, and environmental and socioeconomic contexts. In this chapter, the term methods refers to the overall process of assessment, including tool selection and application; the term tools refers to the formulated means of assessment. It is not the intent of this chapter to comprehensively canvas this full array of methods and tools; clearly, such appraisal falls more properly within the purview of the individual chapters in this volume. The purpose of this chapter is to address several overarching methodological questions that transcend individual sectoral and regional concerns. In so doing, this chapter focuses on five related questions:

• How can the current effects of climate change be detected? Is climate change already having a discernible effect? One of the key methodological problems is how to unequivocally identify a climate change signal in indicators of change in biotic and abiotic systems. This problem is exemplified in Section 2.2 by focusing on biological indicators and methodological advances that have been made since the Second Assessment Report (SAR).
  
• How can the future effects of climate change be anticipated, estimated, and integrated? Since the SAR, an explosion of climate change vulnerability and adaptation studies has occurred around the world, stimulated in large part by the United Nations Framework Convention on Climate Change (UNFCCC) and its national reporting requirements, as well as the availability of international donor support to non-Annex I countries. Section 2.3 reflects on methodological developments and needs for such vulnerability and adaptation studies, and Section 2.4 focuses on methods for regional and cross-sectoral integration.
  
• How can impacts and adaptations be valued and costed? Ultimately, decisions to avoid or reduce the adverse effects of climate change (or enhance the benefits) require some means of appraisal (monetary or otherwise) of projected impacts and alternative adaptation options. Section 2.5 reviews various methods for valuing and costing, including issues of nonmarket effects, equity, integration, and uncertainty.
  
• How can uncertainties be expressed and characterized? From the science of climate change to assessments of its impacts, uncertainties compound, resulting in a "cascade of uncertainty" that perplexes decisionmaking. Section 2.6 canvasses the problems of, and methods for, incorporating uncertainty into policy-relevant assessments.
  
• What frameworks are available for decisionmaking? Once adaptations have been valued, the choice of adaptation requires methods of weighing and balancing options. Section 2.7 summarizes the main decision analytic frameworks (DAFs) that can be used in this context.
  In addressing these questions, the following sections seek to furnish a brief description of the state of methods at hand, methodological developments that have occurred since the SAR, and needs and directions for applications and methods development for the future. Section 2.8 contains concluding remarks.

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