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Introduction
Society and a Varying Climate System
As the glaciers of the last ice age receded and temperatures rose, humans moved into new territories and began to raise crops rather than seek them. The establishment of agriculture contributed to the gathering together of people in stable communities, and to the creation of early cities. In many regions, these agricultural communities were sensitive to the stability of the climate. While they might survive a singularly bad year or two, they were often vulnerable to prolonged or abrupt anomalies. Indeed, there is much circumstantial evidence to suggest that prolonged climatic variations contributed to the collapse of several well-established civilizations at certain times in the past (Weiss et al., 1993). For example, shifts in precipitation patterns in the early part of this millennium led to the demise of irrigation-based agriculture in Central America and the Peruvian highlands, causing starvation, population dispersal, and the end of once-prosperous civilizations.
Today we tend to think of society as well insulated from such catastrophes, yet with agriculture increasingly focused in certain regions of the globe, populations concentrated in large urban agglomerations, and world economic markets more responsive and competitive, our society's well-being and stability may be even more susceptible to global-scale climate change than were the societies of earlier civilizations. With the world's population nearing six billion, and continuing to increase at unprecedented rates, the security provided by a stable climate, and our potential vulnerability to its change, is becoming increasingly recognized.
With this increasing awareness comes greater evidence that climate has varied significantly in the past, and will continue to vary over time scales of decades to centuries. In 1992 and 1993, ice cores approximately 3 km long were extracted from the heart of the Greenland ice sheet, revealing changes in the Earth's climate system over the last 150,000 years or so (White et al., 1997a). One of the most remarkable revelations of these cores was the fact that the climate in the Holocene (the last 10,000 years)—a period that we might consider representative of our modem climate conditions—has undergone considerable natural variation. For instance, evidence from the tropics shows that large hydrologic changes have characterized much of the Holocene, with major impacts on biota and human societies. Prior to the Holocene, as the Earth warmed from the last glacial maximum (approximately 18,000 years ago), the climate system underwent large swings or cycles, and, even more surprisingly, abrupt temperature changes in decades or even shorter periods.
The long-held, implicit assumption that we live in a relatively stable climate system is thus no longer tenable. Furthermore, compounding the inevitable hazard of natural climate variations is the potential for long-term anthropogenic climatic alteration. The likelihood of changes arising from human influence adds another element of doubt to the possibility of predicting future climatic states and stability on these longer time scales; moreover, the uncertainty associated with the natural variability of the climate system precludes our ability to clearly assess human-induced climate change. Together, the evidence of natural variations and the potential for anthropogenic change have altered our way of viewing the climate system: Climate has changed and will continue to do so with or without anthropogenic influences, and a society that has been built around the perception of a stable climate system can only benefit by improving the understanding, assessment, prediction, and early detection of such change—both the natural variability and any possible anthropogenic changes.
Better understanding and prediction are particularly important for climate variability over long time scales, since such change has the potential for surpassing the significant social, economic, and political impacts of shorter-scale variations, which are often addressed through disaster relief. Over decades to centuries, the impacts of climate change can be considerable, and adaptation and mitigation (of both the forcing and the response) are likely to involve policy decisions
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and investments in infrastructure. Changes in frequency and intensity of extreme weather events may accompany such changes in climate (Karl et al., 1996), such as the devastating Midwestern floods that struck the United States in 1993 and again in 1997. The remarkable change in the flood frequency of the American River above Sacramento, California, is the subject of a current NRC study in the Water Science and Technology Board. The Folsom Dam was built in 1945 to provide flood protection for Sacramento. Eight floods greater than the largest flood in the 1905-1945 period have occurred since 1945. A similar situation exists for several of the other Sierra Nevada rivers in California. These high floods have led people to question the level of flood protection actually provided by the dam, and, more important, how flood risk should be analyzed.
Better information on likely climate change and the associated regional patterns—for example, the probability that such floods may occur in clusters, say six or seven times over a 20-year period—would not only permit the mitigation of negative impacts but afford the opportunity to exploit positive impacts. Governments and individuals alike would benefit from advance knowledge of any climate changes that would have a major impact on agriculture, energy production and utilization, water resources and quality, air quality, health, fisheries, forestry, insurance, recreation, and transportation—all fundamental to society's well-being, all vulnerable to any prolonged change or abrupt shift in our climate system. Not only would society benefit from increased climate-prediction skill by being better prepared to ward off adverse climatic consequences, but advance knowledge of climate variations would also enable society to capitalize on opportunities, such as increased geographical ranges for certain crops.
Unfortunately, the subtlety of slow changes over long time scales (relative to diurnal, seasonal, and interannual variations) tends to disguise their potential long-term severity, and thus limits society's willingness to address them in advance; this lack of urgency is exacerbated by the uncertainty in scientists' ability to forecast such change. Given the requisite understanding of climate variability, we hope to ultimately forecast and detect alterations in climate change (distinguishing natural variability from anthropogenic change), providing a rational basis for future policy and infrastructure-management decisions.
The limitations of the instrumental data on which our current state of understanding is based are readily exposed by evaluating their ability to help answer some of our most fundamental questions involving decadal or centennial change. For example, questions such as "Is the planet getting warmer? Is the hydrologic cycle changing? Are the atmospheric and oceanic circulations changing? Are the weather and climate becoming more extreme or variable? Is the radiative forcing of climate changing?" cannot yet be answered definitively. Each one of these apparently simple questions is actually quite complex, both because of its multivariate aspects and because global spatial and temporal sampling is required to address it adequately. The global observing systems needed to provide the answers are either inadequate or non-existent. For science to provide society with the information it needs, better data are essential. The models that will yield predictions require these data to improve our understanding of decade-to-century-scale climate change, its rate and range of variability, its likelihood and distribution of occurrence, and the sensitivity of the climate to changes in the forcing (natural and anthropogenic).
A U.S. Dec-Cen Science Strategy
The fundamental need to develop a good scientific understanding of climate variability and change over decade-to-century time scales, the inadequacy of our current understanding, and the limited resources available to increase this understanding all point to the need for a nationally recognized dec-cen science plan. The present report articulates the primary scientific issues that must be addressed in order to advance most efficiently toward the necessary understanding. In developing this plan, the members of the Dec-Cen panel have taken special care to recognize that research directed toward decade-to-century-scale change and variability will differ in two remarkable respects from research directed at shorter-time-scale variability.
First, research on these intermediate time scales is relatively new. As noted above, only recently have we obtained sufficiently long high-resolution paleoclimate records to allow the examination of past change on dec-cen time scales, and acquired faster computers and improved models that can perform the long simulations needed for studying such change. Consequently, we are on the steep slope of the learning curve, with new results and dramatic insights arising at an impressive rate. The fundamental scientific issues requiring our primary attention are evolving rapidly. Flexibility and adaptability in response to new opportunities and promising directions will be imperative if we are to optimally advance our understanding of medium- and long-range climate change and variability.
Second, the paradigm developed for the study of climate change on seasonal-to-interannual time scales cannot be applied to the study of climate problems on longer time scales. We have recently achieved considerable success in studying short-time-scale climate problems by generating hypotheses and models that are quickly evaluated and improved through analysis of the existing and rapidly expanding instrumental records. For longer-time-scale problems, the existing paleoclimate records are still too sparse and the historical records too short; as for future records, multiple decades will be required before even a nominal comparison with model predictions becomes possible. Furthermore, the change in atmospheric composition as a consequence of human actions represents a forcing whose future trends can be estimated
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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 scales—those of the human lifetime—are 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.
