By Jerry Melillo, The Ecosystems Center, Marine Biological Laboratory, Woods Hole, MA

The central goal of the National Assessment is to explore the potential consequences for the United States of climate changes, whether due to long-term trends in temperature and precipitation or short-term fluctuations and changes related to patterns of variability. We all recognize that impacts depend on the nature and extent of change in the biogeophysical system and the capacity of human societies to anticipate and respond to these changes. Consequently, when we assess impacts, we have to make assumptions about the future of the climate system, ecological systems, and socioeconomic systems.

Any thoughtful assessment of climate-change impacts must be done with the recognition that our prediction capabilities are limited for these three interrelated systems. Although our understanding of the climate system has progressed rapidly in recent years, we cannot yet accurately predict the rates and magnitudes of changes in local temperature and precipitation patterns. Despite substantial progress in short- and intermediate-term predictions of El Niño events, improvements are still needed to enable predictions of future changes in the variability of climate, particularly at small spatial scales. Extant ecological, social, and economic models can provide a general sense of the direction of changes, but are similarly limited and cannot adequately predict the fine-scale responses to climate changes in major "sectors" such as forestry, agriculture, and water resources. And finally, as a research community we are in the very early stages of interactively coupling the climate, ecological and socioeconomic systems to explore dependencies and feedbacks among them.

We have designed an approach to the National Assessment that allows consideration of these uncertainties. With respect to future climate, we are asking the regional and sectoral teams involved in the Assessment to consider a small number of "what-if futures." In addition, we have urged that the teams consider the alternative climate futures in the context of a short list of key socioeconomic dimensions. Some of this work will be done through new modeling studies. We are also encouraging each team to make use of the special insights of regional or sectoral experts who may be able to identify important feedbacks between systems not apparent from the current generation of modeling studies.

This issue of Acclimations focuses on climate science and predictions. Among the articles are overviews of the climate and socioeconomic scenarios that we are using in the National Assessment. In many ways, these scenarios are the foundation for subsequent analysis and synthesis. They provide a common framework from which comparisons of potential impacts on regions and sectors can be derived.