Working Group I: The Scientific Basis

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13.5.2 Approaches for Representing Uncertainties

There are different approaches for representing each of the above five generic sources of uncertainty when constructing climate scenarios. The cascade of uncertainties, and the options for representing them at each of the five stages, can result in a wide range of climate outcomes in the finally constructed scenarios (Henderson-Sellers, 1996; Wigley, 1999; Visser et al., 2000). Choices are most commonly made at the stage of modelling the climate response to a given forcing, where it is common for a set of climate scenarios to include results from different GCMs. In practice, this sequential and conditional approach to representing uncertainty in climate scenarios has at least one severe limitation: at each stage of the cascade, only a limited number of the conditional outcomes have been explicitly modelled. For example, GCM experiments have used one, or only a small number, of the concentration scenarios that are plausible (for example, most transient AOGCM experiments that have been used for climate scenarios adopted by impacts assessments have been forced with a scenario of a 1% per annum growth in greenhouse gas concentration). Similarly, regionalisation techniques have been used with only a small number of the GCM experiments that have been conducted. These limitations restrict the choices that can be made in climate scenario construction and mean that climate scenarios do not fully represent the uncertainties inherent in climate prediction.

In order to overcome some of these limitations, a range of techniques has been developed to allow more flexible treatment of the entire cascade of uncertainty. These techniques manipulate or combine different modelling results in a variety of ways. If we are truly to assess the risk of climate change being dangerous, then impact and adaptation studies need scenarios that span a very substantial part of the possible range of future climates (Pittock, 1993; Parry et al., 1996; Risbey, 1998; Jones, 1999; Hulme and Carter, 2000). The remainder of this section assesses four aspects of climate scenario development that originate from this concern about adequately representing uncertainty:

  1. scaling climate response patterns across a range of forcing scenarios;
  2. defining appropriate climate change signals;
  3. risk assessment approaches;
  4. annotation of climate scenarios to reflect more qualitative aspects of uncertainty.

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