cial disruption for Mayan culture (Hodell et al., 1995; Gill 2000) and that abrupt climate shifts played a role in the collapse of Mesopotamian civilization (Weiss et al., 1993). Recognizing the potential for abrupt changes in climate has constituted a paradigm shift for the research community, but many questions concerning the processes that cause and mediate abrupt climate change remain, including the following.
What are the patterns of environmental variability associated with abrupt climate change in the tropics and high latitudes?
What is the role of freshwater cycling in abrupt climate change?
Will warmer climates influence the occurrence of abrupt climate change?
Might climate changes occur that are unassociated with a change in external forcing?
What feedback processes are dominant, and what is their role in causing the persistence of abrupt changes including droughts?
Recent research has shown that human activities are affecting climate, but it is often difficult to separate human-induced changes from those occurring naturally (Intergovernmental Panel on Climate Change, 2001b). The question arises whether anthropogenic influences will trigger abrupt climate change. It is not now possible to answer that question, because the processes that cause abrupt climate change are not sufficiently understood.
There is little doubt that the rate, magnitude, and regional extent of abrupt transitions to different climate states could have far-reaching implications for society and ecosystems. Research has shown that, in response to gradual changes in climate, much of the economic capital stock1 and some plants and animals may adjust without major disruption. But rapid changes in climate could have major effects, disrupting ecological or economic systems in a manner that prevents their timely replacement, repair, or adaptation.
Ecological systems are particularly vulnerable to abrupt climate change because they are long-lived and relatively immobile. In addition, these systems often have low adaptive capability. Their vulnerability is increased by human activities that alter ecosystems, reducing species abundance and composition and blocking migration. One reason for the vulnerability of eco-