The SRES scenarios assume no climate policy intervention, but nations already are engaged in negotiations to reduce emissions of GHGs. Targets for stabilization of GHG concentrations in the atmosphere are being investigated by scientists and policymakers. TAR WGIII Chapter 2 reviews more than 120 mitigation scenarios, most of which aim to stabilize emissions of CO2 at some target level. Simple climate models, as well as some AOGCMs, have been used to estimate the climate and sea-level response to stabilization (see Harvey et al., 1997; TAR WGI Chapters 9 and 11). Relative to most reference emissions scenarios (e.g., the SRES scenarios), stabilization scenarios reduce global warming, especially beyond 2100. However, even for the lowest stabilization targets considered (450 ppm), based on long simulations by AOGCMs, the climate system and oceans may continue to respond for many centuries after stabilization of atmospheric concentrations of GHGs. Furthermore, because of regional variations in the time lag of response, regional patterns of climate change might be quite different from the unmitigated case (Whetton et al., 1998).
It is demonstrated throughout this report that changes in climatic variability and extremes often play a dominant role in climate change impacts. Moreover, the magnitude and frequency of extreme events can change rapidly with only relatively small changes in climatic averages (see Section 18.104.22.168). However, climate modelers have more confidence in estimates of changes in averages than in changes in variability and extremes (see TAR WGI Chapters 8-10 and 13). Thus, impact assessors need to look carefully at the extent to which changes in variability and extremes are covered implicitly by changes in averages; when this is not the case, they must incorporate possible changes in these phenomena into scenarios. Table 3-10 summarizes projected changes in several types of extreme climate events and their likelihood taken from TAR WGI Technical Summary (see Table 1-1 for a typology of extremes). Table 3-10 also provides representative examples, drawn from different sectors and regions, of impacts that would be expected with high confidence, conditional on the occurrence of a given change in climate extremes. All of this information is reported in other chapters in this report.
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