Working Group II: Impacts, Adaptation and Vulnerability

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2.6.4. Aggregation and the Cascade of Uncertainty

Figure 2-2:
Range of major uncertainties that are typical in impact assessments, showing the "uncertainty explosion" as these ranges are multiplied to encompass a comprehensive range of future consequences, including physical, economic, social, and political impacts and policy responses (modified after Jones, 2000, and "cascading pyramid of uncertainties" in Schneider, 1983)
A single aggregated damage function or a "best guess" climate sensitivity estimate is a very restricted representation of the wide range of beliefs available in the literature or among lead authors about climate sensitivity or climate damages. If a causal chain includes several different processes, the aggregate distribution might have very different characteristics than the various distributions that constitute the links of the chain of causality (see Jones, 2000). Thus, poorly managed projected ranges in impact assessment may inadvertently propagate uncertainty. The process whereby uncertainty accumulates throughout the process of climate change prediction and impact assessment has been described as a "cascade of uncertainty" (Schneider, 1983) or the "uncertainty explosion" (Henderson-Sellers, 1993). The cascade of uncertainty implied by coupling the separate probability distributions for emissions and biogeochemical cycle calculations to arrive at concentrations needed to calculate radiative forcing, climate sensitivity, climate impacts, and valuation of such impacts into climate damage functions has yet to be produced in the literature (see Schneider, 1997, Table 2). When the upper and lower limits of projected ranges of uncertainty are applied to impact models, the range of possible impacts commonly becomes too large for practical application of adaptation options (Pittock and Jones, 1999). This technique is less explicitly applied in assessments where two or more scenarios (e.g., M1 to M4 in Figure 2-1) are used and the results expressed as a range of outcomes. If an assessment is continued through to economic and social outcomes, even larger ranges of uncertainty can be accumulated (see Figure 2-2).

Because of the lack of consistent guidance on the treatment of uncertainties, diversity of subject areas, methods, and stage of development of the many fields of research to be assessed in the SAR, it was not possible to agree on a single set of terms to describe the confidence that should be associated with the many outcomes and/or processes that had been assessed. Thus, the uncertainties guidance paper (see also Box 1-1) suggests that the TAR authors agree on two alternative sets of terms from which writing teams can select (see Figures 2 and 3 in Moss and Schneider, 2000). As noted in the decision analysis literature (e.g., Morgan and Henrion, 1990), it is important to attach a quantitative range to each verbal characterization to assure that different users of the same language mean the same degree of confidence.

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