3.1. Definitions and Role of Scenarios
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
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.
3.1.2. Function of Scenarios in Impact and Adaptation Assessment
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 modelspossibly a set of such modelsoutputs 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),
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
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 assessmentsespecially
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.
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:
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
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.