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to play a role in climate change. On short time scales, while the properties of the upper layer of the ocean play an important role in climate variations, as demonstrated by El Niño predictions, the deep ocean currents have little direct influence, given their slow rate of movement and change. However, on longer and longer time scales, these slow, vast deep-ocean currents may play a more important role, perhaps even a dominant one.

Similarly, the anthropogenic increase of greenhouse gases is not likely to produce a perceptible difference in climate from one year to the next, so it should have little impact on seasonal or interannual predictions for the next few years. Over longer periods, however, the effects can accumulate to produce a significant change. Likewise, climate processes that introduce very small alterations on a year-to-year basis, such as subtle yearly changes in the thickness of sea ice in polar regions (affecting the surface temperature of the ice and its albedo), might show long-term accumulative influences extending well beyond their immediate region. Therefore, although such processes might be ignored for climate prediction on seasonal-to-interannual time scales, they must be considered for longer-term climate prediction, so that the resulting "drift" of climate away from its previous state is taken into account.

In addition to emphasizing different physical components, short- and long-term climate predictions have different implications for society. Predictions involving short-time-scale events such as El Niño can be used to formulate short-term mitigating actions for any negative impacts, such as reinforcing sea walls, preparing for excessive rain, or increasing local disaster-relief funds. Anticipated variations in conditions from one season or year to the next also could be taken advantage of through short-term adaptive measures, such as altering the crops planted, adjusting water-resource management strategies, or reallocating energy resources.

On longer time scales, climate variations can lead to prolonged droughts, or can alter the frequency and distribution of severe weather events for many years. They also can influence the nature of short-term events, such as the frequency with which El Niño occurs, or their duration or severity. Such long-term changes have the potential for greatly surpassing shorter-scale variations in their societal, economic, and political impacts. Areas of the economy that could be significantly affected include agriculture, energy production and utilization, fisheries, forestry, insurance, recreation, and transportation. Water resources and quality, air quality, human health, and natural ecosystems also could suffer or improve. Consequently, responses to climate changes on decade-to-century time scales may involve investments in infrastructure and changes in policy.

For example, if it becomes clear that midwestern floods like those of 1993 and 1997 will occur more often in the next few decades, or that El Niño events will continue to be as frequent as in the 1990s, or that sea level will keep rising, then efforts to mitigate such effects will certainly involve both policy (e.g., modifications of building codes) and infrastructure changes (e.g., construction of protective and adaptive structures such as dikes and irrigation systems). Such predictions also might enable society to capitalize on opportunities, like improved planning for water-resource management, expansion or relocation of agricultural regions, prediction of prime fishing sites and times, or adjustment of insurance rates to better reflect the true risks over the lifetime of a policy.

The aforementioned differences, along with the practical need to subdivide the climate problem into more manageable units, at least for organizational purposes, have led to this initial division of climate-prediction efforts on the basis of time scale. This report represents the first stage in developing a coherent national effort for addressing decadal to centennial climate variability and change. It does this in the form of a science strategy that articulates the fundamental scientific issues that must be addressed, outlines the concepts underlying an observational and modeling program, and recommends the development of a formal, national dec-cen program that would ensure a balanced, effective approach to the scientific issues and the observational and modeling needs.

A Dec-Cen Science Strategy

The science strategy proposed in this volume focuses on six of the attributes of the Earth' s environmental system that are considered to be most directly relevant to society and that have displayed variations on dec-cen time scales in the past, and on six components of the climate system that control these attributes. The attributes are precipitation and water availability, temperature, solar radiation, storms, sea level, and ecosystems. The controling climate components are the atmospheric composition and radiative forcing, atmospheric circulation, hydrologic cycle, ocean circulation, land and vegetation, and cryosphere.

This report describes each attribute and component, illustrates how they have varied in the past over decade-to-century time scales, explains the mechanisms involved and the interactions among them, describes their predictabilities, and presents the remaining issues and questions that surround them. The report also discusses the current state of knowledge of the climate patterns identified so far, and the outstanding issues related to such patterns. This approach maintains a scientifically sound yet socially relevant focus while naturally leading to an overall science strategy for future research, since the issues and questions serve to define and justify the science requirements presented in the plan.

Dec-Cen Issues

The outstanding scientific questions related to dec-cen