cause. Chaotic processes in the climate system may allow the cause of such an abrupt climate change to be undetectably small.

Abrupt climate change generally refers to large-scale events (such as for an entire country or continent), of significant duration (for at least a few years), whose rate of change or variability is significantly greater than the recent variability of climate. From the point of view of societal and ecological impacts and adaptations, abrupt climate change can be viewed as a significant change in climate relative to the accustomed or background climate experienced by the economic or ecological system being subject to the change, having sufficient impacts to make adaptation difficult.

Another important consideration is that there is virtually no research on the economic or ecological impacts of abrupt climate change (Street and Glantz, 2000). Geoscientists are just beginning to accept and adapt to the new paradigm of highly variable climate systems, but this new paradigm has not yet penetrated the impacts community, particularly in economics and the other social sciences. The committee hopes that the diffusion of ideas among the different disciplines investigating climate change happens rapidly, so that research on the impacts of abrupt climate change can move ahead in a timely fashion.

Abrupt Climate Change and Abrupt Impacts

When investigating the impacts of climate change, it is natural to look first for the impacts of abrupt climate changes. An abrupt climate change—whether warming or cooling, wetting or drying—could have lasting and profound impacts on human societies and natural ecosystems. But it must be remembered that profound impacts are not limited to cases of abrupt climate change. Modest changes or increased variability of climate may be sufficient to produce severe impacts, giving the false appearance that these impacts were caused by an abrupt external forcing.

Abrupt impacts result from the fact that economic and ecological processes have adapted to specific climatic patterns and are therefore typically bounded by experience (in the case of society) or history (in the case of ecosystems). Abrupt impacts therefore have the potential to occur when gradual climatic changes push societies or ecosystems across thresholds and lead to profound and potentially irreversible impacts, just as slow geophysical forcing can cross a threshold and trigger an abrupt climate change. Consider that since the nineteenth century, Grand Forks, North Dakota, had successfully fought frequent floods up to a river stage of 49 feet. Then,

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