Climate Change 2001:
Working Group I: The Scientific Basis
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13.5 Representing Uncertainty in Climate Scenarios

13.5.1 Key Uncertainties in Climate Scenarios

Uncertainties about future climate arise from a number of different sources (see Figure 13.2) and are discussed extensively throughout this volume. Depending on the climate scenario construction method, some of these uncertainties will be explicitly represented in the resulting scenario(s), while others will be ignored (Jones, 2000a). For example, scenarios that rely on the results from GCM experiments alone may be able to represent some of the uncertainties that relate to the modelling of the climate response to a given radiative forcing, but might not embrace uncertainties caused by the modelling of atmospheric composition for a given emissions scenario, or those related to future land-use change. Section 13.5.2 therefore assesses different approaches for representing uncertainties in climate scenarios. First, however, five key sources of uncertainty, as they relate to climate scenario construction, are very briefly described. Readers are referred to the relevant IPCC chapters for a comprehensive discussion. Specifying alternative emissions futures

In previous IPCC Assessments, a small number of future greenhouse gas and aerosol precursor emissions scenarios have been presented (e.g., Leggett et al., 1992). In the current Assessment, a larger number of emissions scenarios have been constructed in the Special Report on Emissions Scenarios (SRES) (Nakic´enovic´ et al., 2000), and the uncertain nature of these emissions paths have been well documented (Morita and Robinson, 2001). Climate scenarios constructed from equilibrium GCM experiments alone (e.g., Howe and Henderson-Sellers, 1997; Smith and Pitts, 1997) do not consider this uncertainty, but some assumption about the driving emissions scenario is required if climate scenarios are to describe the climate at one or more specified times in the future. This source of uncertainty is quite often represented in climate scenarios (e.g., Section

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