This chapter examines the development and application of scenarios required for assessment of climate change impacts, adaptation, and vulnerability. Scenarios are one of the main tools for assessment of future developments in complex systems that often are inherently unpredictable, are insufficiently understood, and have high scientific uncertainties. The central goals of the chapter are to set out the different approaches to scenario use, to evaluate the strengths and weaknesses of these approaches, and to highlight key issues relating to scenario application that should be considered in conducting future assessments.

Recognizing the central role of scenarios in impact and adaptation studies, scenarios are treated separately for the first time by Working Group II.¹ This chapter builds on Chapter 13 of the WGI contribution to the Third Assessment Report (TAR), which describes construction of climate scenarios, by embracing scenarios that portray future developments of any factor (climatic or otherwise) that might have a bearing on climate change vulnerability, impacts, and adaptive capacity. A distinction is drawn between climate scenarios, which describe the forcing factor of key interest in this report, and nonclimatic scenarios (e.g., of projected socioeconomic, technological, land-use, and other environmental changes), which provide the "context"—a description of a future world on which the climate operates. Many early impact assessments tended to focus on climate forcing without properly considering the context, even though this might have an important or even dominant role in determining future vulnerability to climate.

In addition to serving studies of impacts, scenarios are vital aids in evaluating options for mitigating future emissions of greenhouse gases (GHGs) and aerosols, which are known to affect global climate. For instance, projections of future socioeconomic and technological developments are as essential for obtaining scenarios of future emissions as they are for evaluating future vulnerability to climate (see TAR WGIII Chapter 2). Thus, although the focus of this chapter is on the development and use of scenarios in impact and adaptation assessment, reference to scenarios that have been developed for purposes of addressing mitigation is important and unavoidable.

There is a varied lexicon for describing future worlds under a changing climate; alternative terms often reflect differing disciplinary origins. Therefore, for the sake of consistency in this chapter, working definitions of several terms are presented in Box 3-1.

Selection and application of baseline and scenario data occupy central roles in most standard methodological frameworks for conducting climate change impact and adaptation assessment (e.g., WCC, 1993, 1994; IPCC, 1994; Smith et al., 1996; Feenstra et al., 1998; see Section 2.1). Many assessments treat scenarios exogenously, as an input, specifying key future socioeconomic and environmental baselines of importance for an exposure unit,² possibly with some aspects of adaptation potential also considered. Other assessments—especially those that use integrated assessment models (IAMs)—generate projections (e.g., of emissions, concentrations, climate, sea level) endogenously as outcomes, requiring only prior specification of the key driving variables (e.g., economic development, population). Outputs from such assessments might be applied themselves as scenarios for downstream analysis. Moreover, in IAMs, some of the original driving variables may be modified through modeled feedbacks.

Scenarios are widely used in climate change-related assessments. For some uses, scenarios are qualitative constructions that are intended to challenge people to think about a range of alternative futures that might go beyond conventional expectations or "business as usual" (BAU). Some of the socioeconomic and technological assumptions underlying GHG emissions scenarios are of this type (see TAR WGIII Chapter 2). For other uses, scenarios may be mainly quantitative, derived by running models on the basis of a range of different input assumptions. Most assessments of the impacts of future climate change are based on results from impact models that rely on quantitative climate and nonclimatic scenarios as inputs. Some scenario exercises blend the two approaches. However, not all impact assessments require a scenario component; in some cases, it may be sufficient that system sensitivities are explored without making any assumptions about the future.

• Illustrating climate change (e.g., by depicting the future climate expected in a given region in terms of the present-day climate currently experienced in a familiar neighboring region)
• Communicating potential consequences of climate change (e.g., by specifying a future changed climate to estimate potential future shifts in natural vegetation and identifying species at risk of local extinction)
• Strategic planning (e.g., by quantifying possible future sea-level and climate changes to design effective coastal or river flood defenses)
• Guiding emissions control policy (e.g., by specifying alternative socioeconomic and technological options for achieving some prespecified GHG concentrations)
• Methodological purposes (e.g., by describing altered conditions, using a new scenario development technique, or to evaluate the performance of impact models).

A broad distinction can be drawn between exploratory scenarios, which project anticipated futures, and normative scenarios, which project prescribed futures. In practice, however, many scenarios embrace aspects of both approaches.

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