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or other mechanisms responsible for driving dec-cen climate variability and its spatial patterns? A related set of questions is: How do processes and changes within one component influence other aspects of the climate system, how are changes in one region transmitted to other regions, and what components and time scales are involved in such telecommunication? For example, how do large-scale dec-cen changes in the atmospheric circulation influence the seasonal-to-interannual variability of severe storms? How do the details of the planetary boundary-layer physics and bio-geochemistry, and the Earth's surface characteristics, influence the propagation of climatic variability or the transfer of greenhouse gases between various components of the climate system?

A U.S. Dec-Cen Program

In view of dec-cen climate variability's intrinsic scientific interest, its direct importance to society, and its involvement with variability on other time scales, the NRC Dec-Cen panel recommends the initiation of a national program designed to increase understanding of this topic. The initial design of this program would address the issues that are outlined above, while maintaining flexibility and adaptability so that new directions and opportunities can be pursued as our understanding is improved and research directions are refined.

It is essential that all of these elements be present, because the paradigm developed for the study of climate variability on seasonal-to-interannual time scales cannot be applied to the study of dec-cen climate problems. Studies of short-time-scale climate problems have generally used a process of generating hypotheses and models that can be quickly evaluated and improved through analysis of existing historical records or observations of near-term climate variability. For dec-cen problems, the paleoclimate records are still too sparse and the historical records too short for this process to be applied; as for future records, multiple decades of observations are required before even a nominal comparison to model predictions can be made. Furthermore, the change in atmospheric composition as a consequence of anthropogenic emissions represents a forcing whose future trends can be estimated only with considerable uncertainty. As a result, making progress in dec-cen-scale prediction will require heavy reliance on improved and faster models, an expanded paleoclimate data base, and assumed scenarios for anthropogenic emission. Without the benefit of real-time observations for constant model validation and improvement, a substantial effort will be needed to validate models through alternate means, to improve understanding of the limits and implications of the proxy indicators constituting the paleoclimate records, and to monitor actual rates of emissions and atmospheric concentrations of radiatively active atmospheric constituents. As for future observations, we can only now begin collection of the data that will ultimately aid future generations of scientists in further understanding dec-cen climate variability and change.

The ultimate research objective of a U.S. Dec-Cen Program would be to define, understand, and model dec-cen climate variability and change (natural and anthropogenic), so that the extent to which they are predictable can be determined. If it can be shown that they are indeed predictable, the ultimate practical aims of a Dec-Cen Program would be to design and implement a complete prediction system, building on the emerging seasonal-to-interannual prediction systems now being constructed; to predict future decadal-to-centennial variations to the extent possible; and to learn to use these predictions for the benefit of all. The program also would provide a means for detecting climate change, which will be necessary for national and international policy decisions.

Climate variability and change on dec-cen time scales involve all of the elements of the U.S. Global Change Research Program: natural and anthropogenic variability and change; past, present, and future observational networks and data bases; modeling; and the physical, chemical, and biological sciences, with their implications for society. A U.S. Dec-Cen Program should encompass the dimensions of these elements pertaining to dec-cen climate variability and change, with particular attention to important aspects that have not received coordinated programmatic support, such as the effects of aerosols on climate, and coupled ocean-atmosphere modeling of long-term climate change, among others. The U.S. Dec-Cen Program would serve as the primary contribution of the United States to the DecCen and Anthropogenic Climate Change (ACC) components of the international Climate Variability and Predictability (CLIVAR) Programme of the World Climate Research Programme (WCRP) and would provide invaluable input to the Intergovernmental Panel on Climate Change (IPCC). It must also be well coordinated with CLIVAR's seasonal-to-interannual climate-variability component (GOALS) and other WCRP activities such as GEWEX, ACSYS, and WOCE, as well as with the components of the International Geosphere-Biosphere Programme, such as the PAGES (Past Global Changes) program. A successful Dec-Cen Program