Executive Summary

Human activities increasingly influence all aspects—biological, chemical, and physical—of the planet on which we live. To better understand what is being affected and how, the scientific method requires us to partition the immense problem at hand into slightly more manageable pieces. One of those pieces is the natural climate variability on which any human-induced change is superimposed. Climate variability, with or without anthropogenic change, represents one of the most fundamental issues of scientific and social interest today. The purpose of the workshop held by the NRC's Climate Research Committee in September 1992 (at the National Academies' Beckman Center in Irvine, California) was to define natural climate variability on the time scale of a few human generations. This volume reflects not only the proceedings of that workshop but considerable intervening work, both by the invited authors and their colleagues, and by anonymous reviewers and the book's editorial committee.

THE CHALLENGE

Natural climate variability on decade-to-century time scales is best defined in terms of the bio-chemical-physical system that must be studied, the principal components of that system, the mechanisms active within each component, and the interactions between components. The main components of the earth system are the atmosphere, oceans, land surface, snow and ice at the surface of both oceans and land, and biota near the interfaces of atmosphere, ocean, and land. The natural mechanisms include radiative transfer, the planetary-scale circulation of the atmosphere and oceans, photochemical processes, and biogeochemical cycles of trace-gases and nutrients. The major interactions between the components of the climate system so defined are given by the exchanges of energy, momentum, water, and trace constituents, which take a large number of specific forms. For instance, ice-albedo feedback affects radiative transfer in the atmosphere and the heat exchange between it and the underlying high-latitude surfaces, and evaporation-wind stress affects the feedback between the tropical atmosphere and oceans.

Our current understanding of the climate system on these time scales is based on insufficient observations and imperfect models. Historically, both observations and models have addressed only one component of the system; the best (but still unsatisfactory) data sets and models are those available for the atmosphere, followed in order by those for the oceans and, more recently, the snow and ice, land surface, and biota. Fortunately, sophisticated global observation systems and model studies are now addressing all these components. Credible results have been obtained with coupled ocean-atmosphere models in the last decade for the interannual variability of the tropical Pacific Ocean and the overlying atmosphere. Similar results for the longer time scales and global components are only starting to become available at the time of this writing.

The community owes much to the remarkable foresight of the individuals and institutions whose persistence is responsible for the sets of long-term observations that are available to us today. Studies of climate variability on the interannual time scale have since been greatly stimulated



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Natural Climate Variability on Decade-to-Century Time Scales Executive Summary Human activities increasingly influence all aspects—biological, chemical, and physical—of the planet on which we live. To better understand what is being affected and how, the scientific method requires us to partition the immense problem at hand into slightly more manageable pieces. One of those pieces is the natural climate variability on which any human-induced change is superimposed. Climate variability, with or without anthropogenic change, represents one of the most fundamental issues of scientific and social interest today. The purpose of the workshop held by the NRC's Climate Research Committee in September 1992 (at the National Academies' Beckman Center in Irvine, California) was to define natural climate variability on the time scale of a few human generations. This volume reflects not only the proceedings of that workshop but considerable intervening work, both by the invited authors and their colleagues, and by anonymous reviewers and the book's editorial committee. THE CHALLENGE Natural climate variability on decade-to-century time scales is best defined in terms of the bio-chemical-physical system that must be studied, the principal components of that system, the mechanisms active within each component, and the interactions between components. The main components of the earth system are the atmosphere, oceans, land surface, snow and ice at the surface of both oceans and land, and biota near the interfaces of atmosphere, ocean, and land. The natural mechanisms include radiative transfer, the planetary-scale circulation of the atmosphere and oceans, photochemical processes, and biogeochemical cycles of trace-gases and nutrients. The major interactions between the components of the climate system so defined are given by the exchanges of energy, momentum, water, and trace constituents, which take a large number of specific forms. For instance, ice-albedo feedback affects radiative transfer in the atmosphere and the heat exchange between it and the underlying high-latitude surfaces, and evaporation-wind stress affects the feedback between the tropical atmosphere and oceans. Our current understanding of the climate system on these time scales is based on insufficient observations and imperfect models. Historically, both observations and models have addressed only one component of the system; the best (but still unsatisfactory) data sets and models are those available for the atmosphere, followed in order by those for the oceans and, more recently, the snow and ice, land surface, and biota. Fortunately, sophisticated global observation systems and model studies are now addressing all these components. Credible results have been obtained with coupled ocean-atmosphere models in the last decade for the interannual variability of the tropical Pacific Ocean and the overlying atmosphere. Similar results for the longer time scales and global components are only starting to become available at the time of this writing. The community owes much to the remarkable foresight of the individuals and institutions whose persistence is responsible for the sets of long-term observations that are available to us today. Studies of climate variability on the interannual time scale have since been greatly stimulated

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Natural Climate Variability on Decade-to-Century Time Scales by the cooperative efforts of dynamic meteorologists and physical oceanographers, sustained by long-term scientific coordination and by relatively healthy and stable governmental support, both national and international. The effort required to shed light on the global system's variability on decade-to-century time scales is proportionately greater, and atmospheric scientists, other oceanographers, glaciologists, biologists, and ecologists will be needed to join those meteorologists and oceanographers thus far engaged. The problem is further complicated by the fact that both natural and anthropogenic effects occur on the time scales of interest, and are hard to separate. It is thus important to establish an understanding of the natural variability, so that it can serve as a baseline against which possibly anthropogenic effects can be gauged. The NRC workshop papers were contributions toward this goal. FINDINGS FROM THE WORKSHOP PAPERS Essays at the beginning of each section of this volume discuss our progress in atmospheric observations, atmospheric modeling, ocean observations, ocean modeling, coupled systems, and climate proxy data. They set the stage for the 42 papers included. The preliminary results of these explorations of natural climate variability summarized in this volume are interesting and encouraging; the Climate Research Committee has extracted four main findings from them. First, the relatively short instrumental record of climate (the last 50 to 100 years) does not represent a stationary or steady record. The papers in this volume show that climate's natural propensity for change has manifested itself through periodic variations, sudden shifts, gradual changes, and changes in variability. Furthermore, such changes do not appear to be unique to this century, as proxy records such as ice cores and tree rings attest. Climate fluctuations over the past few millennia or so will need to be analyzed in greater detail to establish a baseline against which future variations can be gauged. Second, we are not yet certain why these changes in climate occur. Models must be used to test our hypotheses and to increase our understanding of the climate system. Models of the atmosphere, the ocean, and the coupled atmosphere-ocean system are beginning to yield insights into the causes of natural climate variations. For example, recent ocean modeling studies suggest that significant changes in the deep-water circulation may occur over time scales of decades to centuries, and that these changes might critically affect climate. A second ocean model finding is that the thermohaline circulation can oscillate between quasi-steady 'equilibrium' modes. Third, the Climate Research Committee feels that systematically combining observations and models, and ensuring the long-term continuity and sufficient quality of the data, will be critical to the assessment of climate variability and of the models that are used for climate simulation and prediction. The observations permit us to initialize, force, and diagnose models, providing reassurance that we are simulating the real world. Models not only serve as the measure of our understanding and the means of predicting, but are now good enough to help guide observation, monitoring, and data-management programs. Finally, additional data are needed to supplement and expand the currently sparse and sporadic record of past natural climate variability. Proxy data, historical records, current operational data, research data, and model simulations can all contribute significantly to our understanding. In some cases they are available but under-utilized; in others they must be obtained through special programs or refinement of existing collection programs. Consistent data quality and uniform data-management practices are essential, and all climate data should be standardized and made available to researchers worldwide. RECOMMENDATIONS The comfort and livelihood of future generations on this planet demand the vigorous pursuit of new insights into the characteristics of decade-to-century-scale climate variability. Significant benefits will be realized if this research yields reliable prediction capabilities. Such benefits have already been achieved for prediction on interannual time scales. In countries such as Peru, successful predictions of El Niño have enabled fisheries and agricultural communities to introduce adaptive measures in order to minimize its negative impacts. Evidence to date suggests that the magnitude of climate change is often proportional to the time scale over which it occurs, so that over the longer scales the potential benefits of accurate prediction could be even greater. The four recommendations below for making credible prediction methods a reality are discussed in detail in Chapter 7. Criteria must be established to ensure that key variables are identified and future observations are made in such a way that their results will yield the most useful data base for future studies of climate variability on decade-to-century time scales. Assurances of continuity and high data quality are essential. Modeling studies must be actively pursued, using a variety of models, in order to improve our skill in simulating and predicting the climate state, and to assess potential modes of variability. Closer links between models and observational studies will be necessary. Continuing efforts must be made to assimilate the data sets in dynamical models, so that ultimately the added value of dynamical consistency and dynamical interpolation may be realized.

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Natural Climate Variability on Decade-to-Century Time Scales Records of past climate change, particularly those reflecting the pre-industrial era, must be actively sought out and refined as a source of valuable new data on the natural component of climate variability. Climate data must be made freely available to researchers worldwide; data from many sources contribute to the solution of research problems.

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