census were undertaken periodically, trends in fragmentation could be tracked, and potential steps to reduce the vulnerability to fragmentation could be better evaluated. The construction of databases to integrate economic, ecological, archeological, and climatic data would help to improve the understanding of the effects of past abrupt climate changes.
Some data collection required for better monitoring and understanding of the causes, magnitudes, and effects of abrupt climate change could be done in the private sector under a granting mechanism; other datasets might require federal data-collection efforts.
Recommendation 2. New modeling efforts that integrate geophysical, ecological, and social-science analyses should be developed to focus on investigating abrupt climate changes. In addition, new mechanisms that can cause abrupt climate change should be investigated, especially those operating during warm climatic intervals. Understanding of such mechanisms should be improved by developing and applying a hierarchy of models, from theory and conceptual models through models of intermediate complexity, to high-resolution models of components of the climate system, to fully coupled earth-system models. Model-data comparisons should be enhanced by improving the ability of models to simulate changes in quantities such as isotopic ratios that record past climatic conditions. Modeling should be used to generate scenarios of abrupt climate change with high spatial and temporal resolution for assessing impacts and testing possible adaptations. Enhanced, dedicated computational resources will be required for such modeling.
Developing theoretical and empirical models to understand abrupt climate changes and the interaction of such changes with ecological and economic systems has a high priority. Modeling is essential for collaborative research between physical, ecological, and social scientists, and much more effort in the development of accurate models will be necessary to produce a useful understanding of abrupt climate processes. Model analyses help to focus research on possible causes of abrupt climate change, such as human activities; on key areas where climatic thresholds might be crossed; and on fundamental uncertainties in climate-system dynamics. Most analyses have considered only gradual climate change; given the accumulating evidence of