3.1.3.1. Exploratory Scenarios

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.

3.1.3.2. Normative Scenarios

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 similar premises.

The types of scenarios examined in this chapter are depicted schematically in Figure 3-1; they include scenarios of:

• Socioeconomic factors (Section 3.2), which are the major underlying anthropogenic cause of environmental change and have a direct role in conditioning the vulnerability of societies and ecosystems to climatic variations and their capacity to adapt to future changes.
• Land use and land cover (Section 3.3), which currently are undergoing rapid change as a result of human activities. Climate change itself may induce land-use and land-cover changes, with probable feedbacks to the climate system. Furthermore, future land cover may be influenced by efforts to sequester carbon and offset GHG emissions into the atmosphere.
• Other environmental factors (Section 3.4), which is a catch-all for a range of nonclimatic changes in the natural environment (e.g., CO₂ concentration, air pollution, stratospheric ozone depletion, and freshwater availability) that are projected to occur in the future and could substantially modify the vulnerability of a system or activity to impacts from climate change.
• Climate (Section 3.5), which is the focus of the IPCC and underpins most impact assessments reported in this volume.
• Sea level (Section 3.6), which generally is expected to rise relative to the land (with some regional exceptions) as a result of global warming-posing a threat to some low-lying coasts and islands.

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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