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Executive Summary

The evidence of natural variations in the climate system—which was once assumed to be relatively stable—clearly reveals that climate has changed, is changing, and will continue to do so with or without anthropogenic influences. Such variability influences society through a multitude of impacts while operating over a continuum of time scales, from seasonal through centennial and longer. Variability in climate (the time-averaged weather) manifests itself in a variety of forms, such as trends, cycles, and complex regional interactions.

Recent progress in the documentation of climate variability, and in our ability to predict certain climatic events and their regional impacts, has had a significant beneficial effect on society. Climate prediction, typically a season to a year or so in advance, has been successful to some extent for certain regions and climatic phenomena—for example, El Niño and its remote effects, tropical rainfall in Africa and Brazil, and precipitation in northwestern Europe and western North America. Some climatic properties, such as temperature or pressure, tend to preserve their spatial structure through time, or assume a limited number of related shapes, while their amplitude, phase, and sometimes geographic position change. Some of these patterns have also begun to be recognized and documented. The apparent persistence of such patterns, even allowing for their slow evolution, indicates that it may be possible to exploit these ''signals'' to help us understand and predict future climate variability and change.

The success of the short-term climate predictions mentioned above, together with the growing documentation of coherent climate patterns, provides some confidence that improved understanding might lead to broader and longer-range forecasting skills, with commensurate benefits to society. The extension of forecasts beyond seasonal-to-interannual time scales is not, however, a simple undertaking. This difficulty reflects the complexity of the Earth's climate system, which involves the atmosphere, ocean, land, biomass, and cryosphere, together with their various interactions. The complexity of this system is further enhanced by anthropogenic and natural changes in radiative forcing, as well as the anthropogenic changes that result from increased production of greenhouse gases. Furthermore, the slow evolution of climate associated with variability on decade-to-century scales may lead to gradual, but compounded changes in short-time-scale phenomena. Therefore, simply maintaining our present prediction capabilities may require continued investment in developing our understanding of the interaction of climate phenomena operating on different time and space scales.

The international scientific community and policymakers recognize the importance of the current successes and the potential for building on them, as well as the complexity of the problem. They have undertaken to organize global efforts to identify the most important problems yet to be resolved, and to target key research areas and regions. The present report is motivated by this international effort, as well as by our national interest in most efficiently using limited research dollars to improve the understanding of climate change, establish the means for detecting climate change, and ultimately realize broader and longer climate predictions.

Decade-to-Century-Scale Climate Variability

This report focuses on decade-to-century-scale (dec-cen) climate variability and change. A separate report by the NRC's Global Ocean-Atmosphere-Land System Panel (GOALS) deals with seasonal-to-interannual prediction (NRC, 1998a). Certainly the physics of climate change extend across all these time scales. There are, however, fundamental differences in the nature of dec-cen and seasonal-to-interannual change: the dominant processes driving the change, the manner in which they affect society, and the manner in which society might use the predictive information. For example, on longer time scales, more of the slower components of the climate system can typically be expected



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Page 1 Executive Summary The evidence of natural variations in the climate system—which was once assumed to be relatively stable—clearly reveals that climate has changed, is changing, and will continue to do so with or without anthropogenic influences. Such variability influences society through a multitude of impacts while operating over a continuum of time scales, from seasonal through centennial and longer. Variability in climate (the time-averaged weather) manifests itself in a variety of forms, such as trends, cycles, and complex regional interactions. Recent progress in the documentation of climate variability, and in our ability to predict certain climatic events and their regional impacts, has had a significant beneficial effect on society. Climate prediction, typically a season to a year or so in advance, has been successful to some extent for certain regions and climatic phenomena—for example, El Niño and its remote effects, tropical rainfall in Africa and Brazil, and precipitation in northwestern Europe and western North America. Some climatic properties, such as temperature or pressure, tend to preserve their spatial structure through time, or assume a limited number of related shapes, while their amplitude, phase, and sometimes geographic position change. Some of these patterns have also begun to be recognized and documented. The apparent persistence of such patterns, even allowing for their slow evolution, indicates that it may be possible to exploit these ''signals'' to help us understand and predict future climate variability and change. The success of the short-term climate predictions mentioned above, together with the growing documentation of coherent climate patterns, provides some confidence that improved understanding might lead to broader and longer-range forecasting skills, with commensurate benefits to society. The extension of forecasts beyond seasonal-to-interannual time scales is not, however, a simple undertaking. This difficulty reflects the complexity of the Earth's climate system, which involves the atmosphere, ocean, land, biomass, and cryosphere, together with their various interactions. The complexity of this system is further enhanced by anthropogenic and natural changes in radiative forcing, as well as the anthropogenic changes that result from increased production of greenhouse gases. Furthermore, the slow evolution of climate associated with variability on decade-to-century scales may lead to gradual, but compounded changes in short-time-scale phenomena. Therefore, simply maintaining our present prediction capabilities may require continued investment in developing our understanding of the interaction of climate phenomena operating on different time and space scales. The international scientific community and policymakers recognize the importance of the current successes and the potential for building on them, as well as the complexity of the problem. They have undertaken to organize global efforts to identify the most important problems yet to be resolved, and to target key research areas and regions. The present report is motivated by this international effort, as well as by our national interest in most efficiently using limited research dollars to improve the understanding of climate change, establish the means for detecting climate change, and ultimately realize broader and longer climate predictions. Decade-to-Century-Scale Climate Variability This report focuses on decade-to-century-scale (dec-cen) climate variability and change. A separate report by the NRC's Global Ocean-Atmosphere-Land System Panel (GOALS) deals with seasonal-to-interannual prediction (NRC, 1998a). Certainly the physics of climate change extend across all these time scales. There are, however, fundamental differences in the nature of dec-cen and seasonal-to-interannual change: the dominant processes driving the change, the manner in which they affect society, and the manner in which society might use the predictive information. For example, on longer time scales, more of the slower components of the climate system can typically be expected

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Page 2 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

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Page 3 climate variability are fairly extensive and disparate. The panel's selection of principal issues was both guided and constrained by a few overarching questions: • What are the patterns in both space and time of dec-cen variability, and what mechanisms give rise to them? • What is the relationship between natural dec-cen variability and observed global warming? For example, what do we have to know about natural variability in order to detect anthropogenic change? • How does variability of forcing (natural and anthropogenic) affect dec-cen variability? • What is the role of the interactions among the climate components in generating and sustaining dec-cen variability? • To what extent is dec-cen variability predictable, and what unresolved issues must be addressed to realize that predictability? A few of the issues raised in the report are briefly described below. Climate Patterns and Long-Term Climate Change It has become clear that much of the accelerated global warming that has occurred since the mid-1970s is associated with a low-frequency relative phasing of two of the predominant patterns of the Northern Hemisphere: the Pacific-North American (PNA) teleconnection and the North Atlantic Oscillation (NAO). This phasing produces a spatial warming pattern that is similar to one of the "greenhouse fingerprints" predicted by models. This similarity raises several questions regarding the interaction between anthropogenic change and natural variability. For example, is the accelerated warming the result of natural variability caused by an unusually persistent coincidence of the NAO and PNA, or the result of the modification of natural modes (patterns) by anthropogenic changes in radiative forcing that alter the phasing, or some combination of both of these? Likewise, there appears to have been a distinct change in the character (frequency and severity) of El Niño and La Niña events during this period of accelerated warming. Is this a consequence of the influence of anthropogenic change on the dominant natural modes of climate variability, or is it a natural, low-frequency (dec-cen) modulation of a high-frequency (interannual) mode? In addition to answering these specific questions, it is clear that we must determine the longevity of the current patterns, as well as their spatio-temporal variability; identify the best ways of characterizing the patterns; identify new patterns, especially in data-poor regions such as the Southern Hemisphere; determine which ones are statistical products of our analysis tools, rather than true dynamic modes; and determine the mechanisms and underlying physics that control the patterns, their sensitivities, and their evolution in space and time, including their interaction with anthropogenic and natural changes in radiative forcing, and their interaction amongst themselves. Radiative Forcing The redistribution of greenhouse gases between the ocean, atmosphere, and biosphere, and the manner in which these fluxes influence tropospheric greenhouse-gas concentrations on dec-cen time scales, are of central importance to dec-cen climate change. Accurate predictions of changes in green-house-gas concentrations and their effects on radiative forcing will require an improved understanding of the complex interactions between the physics (including radiation), biology, and chemistry (including photochemistry) of the climate system that are driving the fluxes, redistributions, and changes. For some of these gases, such as methane, N2O, and extratropical ozone, it is not even clear what has caused the recent changes. Better understanding of the cloud/water-vapor/radiation processes and feedbacks is needed as well, because they will strongly influence climatic response to the increased radiative forcing associated with higher green-house-gas concentrations. Understanding of the radiation, chemistry, and dynamic coupling in the upper troposphere and lower stratosphere must be improved in order to more accurately predict how stratospheric ozone will recover from depletion by anthropogenic halocarbons, and to determine how aerosols influence dec-cen climate variability. A number of issues must be addressed regarding the potential role of changes in solar radiation on climate. For instance, how representative are proxies for solar activity (e.g., sunspots) of actual total solar irradiance on dec-cen time scales? To what extent are dec-cen climate changes related to changes in the sun's output? Other Issues A variety of discipline-specific issues require attention in order to better understand dec-cen climate variability. Many of these issues require inter- and multi-disciplinary perspectives and efforts. A better understanding needs to be obtained of the nature of dec-cen changes and variations that have occurred in the portions of the climate system that are especially sensitive to climate changes. These sensitive components include cloud cover, sea-ice fields, terrestrial water, and the subsurface ocean. Improved understanding is also needed of how the various components of the climate system interact to produce dec-cen climate variability, as distinguished from the variability that is a product of variations internal to a single component. The atmosphere works on very fast time scales, so coherent decadal-to-centennial variability within the atmosphere is expected to be attributable, at least in part, to interactions with the other, slower components of the climate system, or else to be a response to changes in the radiative forcing. (Nonlinear internal feedbacks are a distinct possibility, though). On the other hand, modeling studies suggest that the ocean is susceptible to cycles of variability on dec-cen and longer scales that are caused by mechanisms internal to the ocean alone. Are these

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Page 4 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.

In order to address the key issues, the U.S. Dec-Cen Program must include: • a long-term, stable observing system; • a hierarchical modeling program; • appropriate process studies; and • a means for producing and disseminating long-term proxy and instrumental data sets. 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

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Page 5 should lead to a deeper knowledge of the interconnections between seasonal-to-interannual variability and dec-cen variability, an improved ability to detect anthropogenic climate change, and an improved understanding of how natural dec-cen variability is anthropogenically modified. Decadal-to-centennial climate variability is a subtle phenomenon. It proceeds too slowly to be perceptible to our senses, but its cumulative effects ultimately define the life prospects of future generations. Informed stewardship of the Earth's resources for the generations to come must draw on the insights afforded by model predictions and extrapolation of observations that can give us a glimpse of the future. A U.S. Dec-Cen Program will be the first step toward assuming this responsibility.