search strategy, and this report seeks to identify the key supporting research needs.

DEFINITION OF ABRUPT CLIMATE CHANGE

What defines a climate change as abrupt? Technically, an abrupt climate change occurs when the climate system is forced to cross some threshold, triggering a transition to a new state at a rate determined by the climate system itself and faster than the cause. Chaotic processes in the climate system may allow the cause of such an abrupt climate change to be undetectably small.

To use this definition in a policy setting or public discussion requires some additional context, as is explored at length in Chapter 5, because while many scientists measure time on geological scales, most people are concerned with changes and their potential impacts on societal and ecological time scales. From this point of view, an abrupt change is one that takes place so rapidly and unexpectedly that human or natural systems have difficulty adapting to it. Abrupt changes in climate are most likely to be significant, from a human perspective, if they persist over years or longer, are larger than typical climate variability, and affect sub-continental or larger regions. Change in any measure of climate or its variability can be abrupt, including change in the intensity, duration, or frequency of extreme events. For example, single floods, hurricanes, or volcanic eruptions are important for humans and ecosystems, but their effects generally would not be considered abrupt climate changes unless the climate system is pushed over a threshold into a new state; however, a rapid, persistent change in the number or strength of floods or hurricanes might be an abrupt climate change.

The quintessential abrupt climate change was the end of the Younger Dryas interval about 11,500 years ago, when hemispheric to global climate shifted dramatically, in many regions by about one-third to one-half the difference between ice-age and modern conditions, with much of the change occurring over a few years (Alley, 2000). The changes affected many environmental parameters such as temperature and rainfall (Figure 1.2). Weaker, but still of hemispheric extent, was a short cooling spell 8,200 years ago that lasted for about 200 years (Alley et al., 1997). Although more regionally limited, the apparent change in El Niño behavior toward generally warmer and wetter conditions around 1976 (Nitta and Yamada, 1989; Trenberth, 1990; Graham, 1994) could also be considered an abrupt change. Thus, studies of abrupt climate change overlap with studies of ice



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