land-use census, and development of integrated economic and ecological data sets. These data will enhance understanding of abrupt climate change impacts and will aid development of adaptation strategies.

A major finding of this study is that the only thing we can be sure of is that there will be climatic surprises. Physical, ecological, and human systems are complex, nonlinear, dynamic and imperfectly understood. Climate changes are producing conditions outside the range of recent historical experience and observation, and it is unclear how the systems will interact with and react to the coming climatic changes.

Data Needed to Better Understand the Mechanisms and Triggers of Abrupt Climate Change

It is crucial to be able to recognize present or impending abrupt climate changes quickly. This capability will involve improved monitoring of parameters that describe climatic, ecological, and economic systems. Some of the desired data are not uniquely associated with abrupt climate change and, indeed, have broad applications. Other data take on particular importance because they concern properties or regions implicated in postulated mechanisms of abrupt climate change, such as the strength of the Atlantic thermohaline circulation. Research to increase our understanding of abrupt climate change should be designed specifically within the context of the various mechanisms thought to be involved. Focus is required to provide data for process studies from key regions where triggers of abrupt climate change are likely to occur, and to obtain reliable time series of climate indicators that play crucial roles in the postulated mechanisms. Observations could enable early warning of the onset of abrupt climate change. New observational techniques and data-model comparisons (data assimilation) will be required as appropriate.

Circulation of the ocean is inferred primarily by combining measurements of water properties with physical understanding of flow. Direct measurements of currents and tracer distributions confirm the inferences. Data limitations, such as the technical difficulty of measuring deep-ocean currents directly, have left some uncertainties, but recent technical advances now allow much improved observations. Collection of more direct data on ocean circulation, together with targeted tracer studies, would reveal characteristics of ocean circulation, including the strength of deep water sources. The new knowledge will help in understanding active processes, such as

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