Exploratory (or descriptive) scenarios describe how the future might unfold, according to known processes of change or as extrapolations of past trends. They are sometimes described as BAU scenarios; often they involve no major interventions or paridigm shifts in the organization or functioning of a system but merely respect established constraints on future development (e.g., finite resources, limits on consumption). However, the term "business-as-usual" may be misleading because exploratory scenarios also can describe futures that bifurcate at some point (an example might be uptake or rejection of a new technology) or that make some assumptions about regulation and/or adaptation of a system. The simplest model is a direct extrapolation of past trends (e.g., projection of future agricultural crop productivity often is based on extrapolation of recorded increases in productivity; Mela and Suvanto, 1987; Alexandratos, 1995). Most climate scenarios considered in this report can be regarded as exploratory: They are future climates that might occur in the absence of explicit policies of GHG reduction.
Normative (or prescriptive) scenarios describe a prespecified future, presenting "a picture of the world achievable (or avoidable) only through certain actions. The scenario itself becomes an argument for taking those actions" (Ogilvy, 1992). Normative scenarios span a wide spectrum, according to their degree of prescriptiveness. At one end of the spectrum are scenarios that are constrained in only one or a few dimensions. For example, scenarios that lead to a substantial degree of climate change sometimes are used as a reference for assessing the "worst case" as far as impacts are concerned (e.g., scenarios that explore extreme events and tails of frequency distributions).
At the other extreme of the spectrum are comprehensive, multidimensional normative
scenarios that are constructed to meet the constraints of a prescribed target
world. Examples are scenarios that constrain emissions within bounds ("safe
emissions corridors") that avoid inducing a critical climate change, defined
according to a subjectively selected impact criterion (Alcamo and Kreileman,
1996). Most of the emissions stabilization scenarios explored by the IPCC in
recent assessments (IPCC, 1996a; Schimel et al., 1997a) are founded on
The types of scenarios examined in this chapter are depicted schematically in Figure 3-1; they include scenarios of:
Issues that are common to all scenarios concerning scenario consistency and the interactions and feedbacks between scenarios are treated in Section 3.7. Characterizations of future climate and related conditions during the 21st century, based on the new IPCC emissions scenarios, are introduced in Section 3.8, and the chapter closes with a brief examination of key gaps in knowledge and emerging new methods of scenario development.
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