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Natural Climate Variability on Decade-to-Century Time Scales 1 INTRODUCTION
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Natural Climate Variability on Decade-to-Century Time Scales Variations in the earth's climate have had considerable impact on society—particularly agriculture, fisheries, water resources, and recreation—throughout recorded history. Such natural climate variability must be identified, quantified, and understood if ways are to be found to minimize its negative consequences and maximize its positive ones. In addition, human activities could significantly alter this natural variability, and indeed may already have done so. If we are to make informed decisions about our own future, it is essential that we assess the climate's sensitivity to a variety of factors, particularly on the decade-to-century time scales that are of most concern to human beings. Many information sources, including instrumental records, visual observations, and paleoclimate data, bear witness to substantial variability in the earth's climate on time scales from years to centuries. The notion of a stationary climate on these time scales has thus become untenable. While variability in the modern climate regime is small relative to the formidable changes that characterize transitions from glacial to interglacial periods, the rate of change is often similar or even greater. The relatively short instrumental record of climate clearly does not represent a steady background against which future variations can be gauged. Human-induced change will be difficult to assess unless the long-term natural variability of the climate system can be characterized. Natural variations with time scales of decades to centuries may well be masking anthropogenic climate changes that have already been effected, and will continue to do so. We must be able to recognize natural variability and its results if we are to make reasoned estimates as to whether a particular climate perturbation or trend is likely to have been induced by human activities, or simply represents a natural variation. These very basic issues give rise to four critical questions concerning climate change on time scales of decades to centuries: Can we characterize the climate system's variability on these scales, over both space and time? Can the causes of such climate variability be isolated? Can such climate changes be predicted? Can changes induced by human activities be distinguished from natural variability on these time scales? ASSESSING DECADE-TO-CENTURY-SCALE VARIABILITY To assess climate variability on intermediate scales, one must have some idea of what it, and the forcings, look like. Several possible forms of climate variation can be seen in Figure 1. For example, climate variability may involve periodic change (Figure 1a), which is similar in nature to a daily or annual cycle, but in this case has a cycle lasting tens to hundreds of years or longer. The climate may also undergo a sudden shift (Figure 1b) from its current state to a different state, possibly one characterized by significantly colder or warmer conditions. It may show a steady warming or cooling until a stable state is reached (Figure 1c). And last, the climate may maintain what appears to be a steady state, when characterized by a specific variable such as mean annual temperature, but variations in some other measure, such as seasonal temperature, diurnal range of temperature, snow or ice coverage, or storm frequency, may indicate FIGURE 1 Types of climate variations. (From Marcus and Brazel, 1984; reprinted with permission of the Office of the State Climatologist for Arizona.)
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Natural Climate Variability on Decade-to-Century Time Scales that significant change has taken place (Figure 1d). Many of the papers in this volume contribute insights and data on the form of such climate variations. The identification of the characteristics of climate variability involves several issues. Differentiating climate ''change" from climate "variability" is a matter of the time scale. What appears to be a trend, in a single decade's recording, may reveal itself as fluctuating variability over a period of a century. The Dust Bowl period in the central United States in the 1930s represents a short, decadal-scale natural variation in climate for a specific region. In contrast, the "little ice age", which lasted from the 1400s to the 1800s, represents a variation on a time scale of centuries. (During the little ice age the decadal variability was typical of that observed during modern conditions.) An additional complication arises because variation that occurs on one time scale may influence changes on other time scales. The well-documented El Niño / Southern Oscillation phenomenon, which is characterized by variation over 4- to 7-year periods, seems to be affected by longer-time-scale variations and, in turn, to modulate the variations themselves. Even after the time scale of interest has been defined, it may be difficult to recognize and classify natural variability as evidenced in modern instrumental observations, because anthropogenic change may already have contaminated them significantly. Limitations on data continuity (loss of records, changes in instrumentation, gaps, lack of data quality assurance) are also a serious handicap. Longer records, especially those based on proxy indicators of climate (i.e., indirect measures of climate such as the width of annual growth cycles in trees and corals), represent an excellent potential additional data set for estimating or extracting natural variability. An additional difficulty in assessing variability is that change and variation are often characterized by strong spatial dependency. That is, while some region or regions of the earth may be experiencing significant climate variation over a given period, other regions may show virtually no change. Similarly, a specific variable (e.g., global temperature) may show a marked change between decades, while another (e.g., global pressure fields) may not show any significant difference. Where and how climate is measured can influence the findings and conclusions. Our understanding of the causes of decade-to-century-scale variability is limited, in part because there are many possible causes, typically disguised by complex interactions. They include inherent variability (deterministic or random within individual components of the earth system), internal variability associated with the coupled ocean-atmosphere-cryosphere system, and forced variability such as solar variation and volcanic aerosol loading of the atmosphere. One key challenge is to isolate (if possible) the signatures of the various possible mechanisms so that we can discriminate between cause and effect as we examine the climate record. Also, a greater understanding of the magnitude of the forcing required to produce observed variations will allow us to focus on the mechanisms most likely to contribute to climate variability. It seems likely that several mechanisms, operating in concert, are responsible for the variations apparent in the observed climate record. Understanding the mechanisms that produce natural variability will require a hierarchy of climate models, including coupled models that are capable of addressing the interactions between the components of the earth system. The development of models that will yield simulations or predictions that can be verified and validated against the observational record, is another major challenge. Models that can describe the nature of decade-to-century-scale variations will serve not only as a measure of our understanding, but as a tool to increase this understanding. They provide our primary opportunity to predict climate variability, although ultimately prediction may be achieved through a variety of means. Given specified forcing scenarios, models may provide viable climate-response scenarios. But statistical characterization is another possible tool. For example, it would be useful to know that a particular extreme climate event (e.g., major regional flooding) tends to occur in clusters over a several-decade period rather than irregularly and infrequently. Making the distinction between natural variations and human-induced changes—and ultimately predicting future changes—will require more complete characterization of the climate system's variability in both space and time, a greater understanding of the causes of decade-to-century-scale climate variability and the mechanisms by which it is produced, and the development of more advanced predictive models. The papers included in this volume reveal significant progress in identifying the behavior of many key components of the climate system, their climate signatures, their internal modes of variability, and their interactions with different components of the earth system. The coupling of observations and models, together with the availability of both long-term, consistent, and high-quality observations and proxy data records, will be critical to this assessment of climate and its change. PURPOSE AND STRUCTURE OF THIS VOLUME The importance of climate variability to society motivated the organization of a workshop by the Climate Research Committee (CRC) of the National Research Council's Board on Atmospheric Sciences and Climate. Not only have climate variations on decade-to-century time scales received significantly less attention than seasonal and interannual climate variations, or even the glacial/interglacial periods, but for nearly two decades natural variations have been overshadowed by much-publicized concerns about the possibility of long-term changes caused by increases in
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Natural Climate Variability on Decade-to-Century Time Scales atmospheric concentrations of greenhouse gases. Both the workshop and the present volume cover a wide range of topics relevant to climate variability; they include the characteristics of the atmosphere and ocean environments as well as the methods used to describe and analyze them, such as proxy data and numerical models. The papers in this volume clearly demonstrate the range, persistence, and magnitude of natural variability as represented by many different climate indicators over the decade-to-century time scale. This book is not a simple "workshop proceedings". Not only have the 42 papers included been refereed and edited, but each paper is followed by a brief critique by the workshop discussion leader and by a condensed version of the lively discussion that followed each presentation. In addition, each of the major sections is introduced by an essay that provides a perspective for the reader, and completes the picture sketched by the individual papers. Chapter 2 of this volume presents the atmospheric side of natural climate variability. It begins with papers dealing with observational data, and then discusses current atmospheric modeling. Similarly, Chapter 3 presents ocean observations first, and then ocean models. Papers about coupled atmosphere and ocean models appear in Chapter 4, and Chapter 5 introduces a variety of sources of proxy data, from lake beds to ice cores. Chapter 6 contains the conclusions the CRC has drawn from the papers, commentaries, and discussions included. The introductory essays, which outline the significance of the papers included, in conjunction with Chapters 1 and 6 constitute a portrait of our current understanding of many aspects of climate variability on decade-to-century time scales. Chapter 7 presents the committee's recommendations for the direction of future research. Despite the volume's breadth, it is still not comprehensive. Most of the current research effort has involved the oceans and the atmosphere. Other, less studied components, such as the cryosphere, land-surface processes, and biogeochemical cycles, may also play substantial roles in natural variability. For example, the cryosphere, through snow cover, sea ice, and land ice, can influence climate over sub-seasonal to millennial time scales. It has been implicated in a number of important climate processes, including ice—albedo feedbacks, thermohaline circulation, and abrupt climate change. Land-surface processes influence the hydrological cycle and surface albedo. The biosphere affects climate on a variety of time scales through its influence on surface moisture fluxes, albedo, and the carbon cycle. The roles of these components require considerable attention still; they are under-represented in current studies. The CRC nonetheless feels that this book will serve as a foundation upon which research into climate variation on decade-to-century time scales can build. The ultimate goals—determining the characteristics of natural climate variability, predicting climate changes on decade-to-century scales, and assessing the effects of human activities—will be realized only through the cooperation of scientists and national and international agencies in all the fields represented. REFERENCE Marcus, M.G., and S.W. Brazel. 1984. Climate Changes in Arizona's Future. Arizona State Climate Publication No. 1, Office of the State Climatologist, Arizona State University, Tempe.