There is now a substantial literature on the potential impacts of gradual climate change. This was recently reviewed by interdisciplinary teams for the Intergovernmental Panel on Climate Change in its Third Assessment Report (Intergovernmental Panel on Climate Change, 2001a; Ch. 18 is especially relevant). There are also numerous studies of the impacts of climate variability on environment and society at the regional scale, such as caused by El Niño, and some of the more extreme variability considered in these studies may be analogous to abrupt change. There have, however, been few studies that directly analyze the impact of global abrupt climate change. Therefore, the question arises as to whether studies of gradual climate change are useful to assess the impacts of abrupt climate change. In certain areas, existing studies are likely to be instructive. For example, it is likely that those sectors of economies and ecosystems most vulnerable to gradual change are also likely to be vulnerable to abrupt change and, to some extent, the converse should also be true. Agriculture and forests have been identified as systems that are vulnerable to gradual changes, and they are likely to be even more vulnerable to abrupt changes. On the other hand, many manufacturing processes are thought to be relatively robust to climate change at any temporal scale.

For natural systems, it is well documented that gradual climate change can affect species distribution, population abundance, morphology, and behavior, ultimately affecting community structure (Easterling et al., 2000). Although less is known about the links between small- and large-scale climate processes and ecosystems during extremes in climate variability, it is clear that ecosystem structure and function are both affected by disturbances associated with weather extremes such as floods, tornadoes, and tropical storms (Pickett and White, 1985; Walker and Willig, 1999). For example, in the 1950s, widespread drought in New Mexico was responsible for a significant shift of the boundary between ponderosa pine forest and pinyon-juniper woodland (Allen and Breshears, 1998).

In some economic and ecologic sectors, where climatic impacts are of a smooth and linear nature, it is likely that the influence of abrupt climate change will be to accelerate the effects of climate change rather than to qualitatively change the impact. The case of slow sea-level rise discussed earlier provides an example where rapid and unanticipated changes appear to cause only a modest increase in damages. However, under some circumstances, abrupt climate change may not only exacerbate the impacts of gradual climate change but may lead to qualitatively different and more severe impacts.

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