only with considerable uncertainty. Making progress in dec-cen climate prediction will require heavy reliance on improved and faster models, an expanded paleoclimate database, and assumed anthropogenic and natural forcing scenarios. The inherent slowness of obtaining new dec-cen time-scale climate observations necessitates the use of additional climate-data sources (e.g., paleoclimate proxy data) to most efficiently validate and improve the models used to assess dec-cen climate variability and change. Considerable effort is required to use such alternative means, because of the steps that must be taken to understand the limits and implications of the proxy indicators constituting the paleoclimate records. Considerable effort is also needed to monitor actual rates of anthropogenic emissions, as well as natural concentrations of radiatively active atmospheric constituents that force climate on dec-cen time scales. We can only begin collection of those data that will ultimately aid future generations of scientists in understanding decade-to-century-scale climate variability and change.
This Dec-Cen report identifies the fundamental science issues that must be addressed in order to realize the following ultimate goals:
• Characterize and assess natural climate variability. Achieving this objective will require a solid statistical grasp of natural variability that will serve as a baseline for gauging anthropogenic change. This will help to reduce a vast, complex system to manageable components that encapsulate its key aspects and allow us to evaluate its mechanisms and determine the likelihood of future changes. Meeting this goal will depend on the availability of greatly expanded paleoclimate and historical databases, and on believable simulations by comprehensive climate models.
• Design a comprehensive system to forecast change in the climatic mean and in climate variability. Developing such a predictive capability demands a good understanding of the climate system, tested through controlled hindcasting experiments. A forecasting system is required in order to assess the likely response to changes in the forcing, which will then permit us to address important questions regarding adaptation versus mitigation measures, especially for anthropogenic climate change. Some reliable indication of future change can be realized in the interim through existing models or statistical formulations.
• Develop a strategy for detecting climate change. This strategy will provide the basis for testing and refining our ultimate predictive capabilities, while the relevant observations will provide the ground truth for such predictions. Reaching this goal will require identification of the sensitive components of the climate system that must be monitored to evaluate both natural and anthropogenic climate change. Understanding and characterization of the natural variability of the climate system on dec-cen time scales are crucial if the anthropogenic "signal" is to be distinguished from the natural climatic "noise." All statements about detection of anthropogenic climate change imply knowledge of the background variability, so we must achieve greater certainty about the latter.
• Provide the physico-biogeochemical parameters or parameterizations required by social scientists for socioeconomic and environmental impact assessments and basic human-dimensions studies. The societal consequences of climate variability on dec-cen scalesthose of the human lifetimeare likely to be quite different from those of both shorter and longer time scales. Human-dimensions studies specific to the dec-cen time scale need to be performed, and scientists must be able to provide the necessary climate-related information.
Predicting and assessing the consequences of climate change and climate variability over dec-cen time scales will involve considerable scientific breadth: observing past, present, and future climate; understanding the processes of natural and anthropogenic change and variability; and modeling variability and change through a hierarchy of approaches. Potential consequences can be properly addressed only within the holistic perspective afforded by such breadth. This science strategy attempts to provide that perspective. Our strategy for achieving it is to include components that have already received considerable and widespread attention (e.g., those aspects of anthropogenic climate change highlighted in the recent Intergovernmental Panel on Climate Change document (IPCC, 1996a), while fleshing out the relevant issues of components that have received less institutional consideration (e.g., natural variability, and the interactions between natural and anthropogenic influences). Thus, the bulk of this report describes the latter, while including overviews of the former at the level needed to confer the necessary holistic dec-cen perspective.