Climate Change 2001:
Working Group II: Impacts, Adaptation and Vulnerability
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3.1. Definitions and Role of Scenarios

3.1.1. Introduction

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.

Box 3-1. Definitions

Projection. The term "projection" is used in two senses in this chapter. In general usage, a projection can be regarded as any description of the future and the pathway leading to it. However, a more specific interpretation was attached to the term "climate projection" throughout the Second Assessment Report (SAR) to refer to model-derived estimates of future climate.

Forecast/Prediction. When a projection is branded "most likely," it becomes a forecast or prediction. A forecast is often obtained by using deterministic models—possibly a set of such models—outputs of which can enable some level of confidence to be attached to projections.

Scenario. A scenario is a coherent, internally consistent, and plausible description of a possible future state of the world (IPCC, 1994). It is not a forecast; each scenario is one alternative image of how the future can unfold. A projection may serve as the raw material for a scenario, but scenarios often require additional information (e.g., about baseline conditions). A set of scenarios often is adopted to reflect, as well as possible, the range of uncertainty in projections. Indeed, it has been argued that if probabilities can be assigned to such a range (while acknowledging that significant unquantifiable uncertainties outside the range remain), a new descriptor is required that is intermediate between scenario and forecast (Jones, 2000). Other terms that have been used as synonyms for scenario are "characterization" (cf. Section 3.8), "storyline" (cf. Section 3.2), and

Baseline/Reference. The baseline (or reference) is any datum against which change is measured. It might be a "current baseline," in which case it represents observable, present-day conditions. It also might be a "future baseline," which is a projected

3.1.2. Function of Scenarios in Impact and Adaptation Assessment

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.

3.1.3. Approaches to Scenario Development and Application

The approaches employed to construct scenarios vary according to the purpose of an assessment. For instance, scenarios may be required for:

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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