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THE ATMOSPHERE



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Natural Climate Variability on Decade-to-Century Time Scales 2 THE ATMOSPHERE

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Natural Climate Variability on Decade-to-Century Time Scales Introduction Humankind lives at the bottom of the sea of air, and climate change is perceived by us mainly as a change in the overall conditions of this sea's lower layers. The atmosphere lies at the heart of decade-to-century climate change: It filters the sun's rays as they reach the surface of the earth and as they are reflected again into outer space, and it is the principal medium of exchange of heat, water, trace-gases, and momentum between the other components of the climate system—oceans, land surface, snow, ice masses, and the biosphere. The atmosphere and ocean are intimately coupled within the climate system, and are governed by similar physical laws. But the atmosphere has been explored in greater detail—in terms of available observations and of existing models—than any other component of the climate system. Thus it is natural to review the results of this exploration first, as we begin our examination of climate variability. Understanding of natural phenomena proceeds through a sequence of observations, experiments, and models. Given the complexity of the climate system, laboratory experiments can reproduce only very incompletely the system's major aspects, and have not been included in the present volume. Atmospheric observations have led, in past centuries, to very simple, purely descriptive models of atmospheric motions. In the second half of this century, advanced computer models of the atmosphere have simultaneously benefited from an increase in the number and quality of observations, and stimulated vast field programs designed to verify model results and yield the new details necessary for improving the models. The separate treatment of observations and models in this chapter and the next is, therefore, only a matter of expository convenience. The oldest instrumental records of atmospheric temperature—and, to some extent, precipitation—extend about 300 years into the past. The coverage and density of these measurements have grown more or less continuously, with a dramatic increase occurring in the 1940s and 1950s. This permits us to make a fairly informed assessment of past interannual variability, but we have considerably less confidence about interdecadal changes and only little or indirect information on the century-to-century time scale. The Atmospheric Observations section below starts with a paper by H.F. Diaz and R.S. Bradley that addresses head-on the question of how different the climate of this century has been from those of previous ones. Proceeding from temperatures (which tend to be more uniform in space and time) to precipitation (which is considerably less so), S.E. Nicholson looks at the socioeconomically critical issue of African rainfall variability on interannual and decadal time scales. J. Shukla provides complementary insight on the initiation and persistence of drought in the Sahel. The role of snow cover in the radiation balance at the surface makes it an important player in climate variability; this role is discussed by J.E. Walsh. Variability and trends of both liquid and solid precipitation over North America are reviewed by P.Ya. Groisman and D.R. Easterling. Returning to temperature, a careful study of the difference between trends in daily temperature maxima and minima is presented by T.R. Karl and his colleagues. C.D. Keeling and T.P. Whorf then analyze the decadal oscillations in global temperatures and in atmospheric carbon dioxide. These oscillations are at the heart of understanding natural variability on this time scale; they are also covered later in this section in the essay introducing atmospheric modeling.

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Natural Climate Variability on Decade-to-Century Time Scales The Southern Hemisphere has less instrumental coverage, in space and time, than the Northern Hemisphere. While many of the earlier papers in the section concentrate on the latter, D.J. Karoly describes the observed variability in the atmospheric circulation south of the equator. Atmosphere-ocean interaction is involved in the last two papers of this section. C. Deser and M.L. Blackmon review atmospheric climate variations at the surface of the North Atlantic, while D.R. Cayan and his associates study a general-circulation model simulation of the Pacific Ocean, driven by observed surface fluxes. Atmospheric models have a relatively long tradition in the climate community, going back to the 1950s. They span a spectrum, from simple radiative-convective models in one vertical dimension, through energy-balance models in one and two horizontal dimensions, to fully three-dimensional general-circulation models. These models are of interest in their own right as important tools for investigating climate change, either by themselves or coupled to models of other climate subsystems. They also provide an instructive example of the creation of a full suite of models for intercomparison and validation; their development is only now being followed, more or less closely, by the modeling enterprise in oceanography, hydrology, and other disciplines contributing to the climate-change enterprise. Modeling and observations are closely linked by the reciprocal problems of simulating the observed variability and validating the existing models. The Atmospheric Modeling section is thus appropriately begun by T.M.L. Wigley's and S.C.B. Raper's paper on modeling and interpreting paleoclimate data, with an emphasis on the greenhouse effect. G.R. North and K.-Y. Kim apply classic time-series analysis techniques to climate-signal detection. R.S. Lindzen then considers a few themes in these two papers from complementary viewpoints. A number of causes for climate variability on time scales of decades to millennia are reviewed by D. Rind and J.T. Overpeck. They emphasize the modeling approach to a study of these causes, while J.M. Wallace addresses similar issues by analyzing the climate record. The two sections of this chapter are introduced by essays, one by T.R. Karl and the other by M. Ghil. They provide an overview of the current state of the fields of atmospheric observations and atmospheric modeling, reviewing those topics not covered by the workshop and offering a perspective on the papers included. Following each paper, a commentary by the discussion leader and a condensation of the spirited discussion that took place at the workshop shed additional light on our current knowledge in both areas. The ocean is examined in Chapter 3, and atmosphere-ocean interaction is explored further in Chapter 4. Also of interest are the proxy records of climate variability, such as tree rings and coral reefs, which are covered in Chapter 5. The sponsoring committee's conclusions, which draw on the material in this chapter and in the other three, appear in Chapter 6.

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Natural Climate Variability on Decade-to-Century Time Scales Atmospheric Observations THOMAS R. KARL INTRODUCTION During the 1970s, the 1980s, and into the 1990s atmospheric scientists have accelerated research aimed toward identifying and explaining the presence of decade-to-century scale climate fluctuations. The motivation for special consideration of atmospheric climate fluctuations on these longer scales has its roots in the numerous major climate events whose occurrence is so well documented in the instrumental record. Probably the best-known of these fluctuations occurred about 1970, when rainfall suddenly decreased over the Sahel; the causes of this jump are discussed in this section by Nicholson (1995) and Shukla (1995). Other examples include the Dust Bowl years in North America during the 1930s, the diminished intensity of tropical storms affecting the East Coast of the United States during the 1960s, 1970s, and much of the 1980s, and the wet weather of the 1970s and 1980s over much of the United States. As access to climate records has broadened during the past few decades, researchers such as Hurst (1957), Mandelbrot and Wallis (1969), Mitchell (1976), Douglas (1982), and Lorenz (1986) have presented evidence suggesting that the notion of a static climate is no longer tenable, even on less-than-geological time scales. We have come to realize that 30 years of data, the length of time that has been used to compute temperature and precipitation "normals" (Court, 1968), is inadequate to define climate. It does not provide us with sufficient information either to minimize adverse climate impacts within such sectors as energy, water supply, transportation, environmental quality, construction, agriculture, etc., or to maximize the availability of the climate-governed resources, such as water and energy, on which both natural and man-made systems depend. Several climate fluctuations that have affected the United States serve to illustrate this point. Beginning about 1975, the interannual variability of mean winter temperatures and total precipitation averaged across the contiguous United States substantially increased (Figures 1a and 1b); this increase persisted at least through 1985. In contrast, the interannual variability had been very low during the previous 20 years. The mean temperature increased dramatically over the United States during the 1930s, coincident with a large decrease in summer precipitation (Figures 1c and 1d), before returning to more typical conditions. The wetness of the 1970s and the first half of the 1980s, which is clearly evident in Figure 1e, resulted in record-setting high lake levels and caused considerable economic damage and human suffering (Changnon, 1987; Kay and Diaz, 1985). Over the past decade another climate fluctuation has been evident over the United States; as Figure 1f shows, temperatures increased in a jump-like fashion during the early 1980s. Interestingly, the temperature discontinuity that is apparent between January and June is not reflected by the temperatures during the second half of the year. Due to the limited span of the instrumental climate record, evidence for climate fluctuations on decade-to-century time scales is biased toward higher frequencies. (The situation is much worse in the oceans; as Wunsch (1992) points out, the absence of comprehensive oceanographic data has led many oceanographers to focus on identi-

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Natural Climate Variability on Decade-to-Century Time Scales FIGURE 1 Time series of area-averaged seasonal and annual mean temperature and total precipitation for the contiguous United States. The smoothed curve is a nine-point binomial filter. The horizontal line reflects the mean over the period of record. fying a mean climate state.) Nonetheless, there is evidence that climate fluctuations on longer time scales occur throughout the world (Jones and Briffa, 1995; Diaz and Bradley, 1995). As Karoly (1995) shows, there is a dearth of the data that would permit the identification of decadal-scale circulation variability in the Southern Hemisphere, but analysis of the longer-term surface data available suggests that significant climate fluctuations persist at least through decadal time scales. The forcing agents of many climate fluctuations may have their origin in either anthropogenic or natural factors, or both. These fluctuations are important in terms both of their socioeconomic and biophysical effects and of our need to distinguish between natural climate variations and anthropogenic climate changes. In analyzing the climate record, the dangers of "data dredging" must be kept in mind. As access to climate data increases, the chance of finding trends and variations that appear to be significant also increases. For this reason decade-to-century-scale climate fluctuations have been referred to as the "gray area of climate change" (Karl, 1988). For instance, Keeling and Whorf (1995) find evidence for decadal fluctuations in the global temperature record that can be reproduced by assuming the existence of two oscillations with small differences in frequency that beat on time scales of about 100 years. The search for an explanation of this statistical result is a good example of the challenge presented by the existence of these decadal fluctuations. IDENTIFYING CLIMATE FLUCTUATIONS The instrumental record of atmospheric and related land and marine observations is fragmentary until at least the middle of the nineteenth century. Moreover, virtually all of

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Natural Climate Variability on Decade-to-Century Time Scales the observations used to study climate fluctuations are derived from observing programs that were developed not for this purpose, but rather to support day-to-day weather forecasting. Since the day-to-day variability of the atmospheric system in much of the world is far larger than the decadal climate variability, identification of multi-decadal climate fluctuations is often hampered by the poor quality and lack of homogeneity of the observations. For example, Groisman and Easterling (1995), in their study of the variability and trends of precipitation over North America, painstakingly document the many discontinuities and false jumps in the climate record that arise from changes in observing practices. These changes, if ignored, can and do overwhelm important long-term precipitation fluctuations. Other atmospheric quantities besides precipitation are also affected by problems in measurement. In this section, Robinson (1995) points out the large disparity in snow-cover extent between a NASA data set using microwave measurements and a NOAA data set using visible imagery. Karl et al. (1995) devote considerable discussion to jumps and trends introduced into the surface temperature record by such factors as urban heat islands, changes in irrigation practices, differences in instrumentation, and station relocations. Cayan et al. (1995) use surface marine observations to calculate the latent and sensible heat fluxes that are used as forcing agents of the sea surface temperature field in the North Pacific. Their analysis includes the basin-wide climate jump that began about 1976-1977. To account for known systematic biases related to trends in the wind field and the sea surface and marine air-temperature and moisture lapse rates (Ward, 1992; Cardone et al., 1990; Wright, 1988; Ramage, 1987), the global trend of each of these quantities is removed. As mentioned in Zebiak's commentary on Cayan's paper, it is unfortunate that such adjustments to the data are necessary. Often the corrections and adjustments that must be applied to a climate record are of such magnitude that it is difficult to be confident that the resulting time series adequately reflects the climate fluctuations. For example, Figure 2 illustrates the significant adjustments required to calculate global temperature fluctuations since the nineteenth century. Through the use of other data bases (e.g., changes in snow cover, alpine glaciers, or sea level) and the isolation of various components of the surface temperature record (e.g., marine air temperature, sea surface temperature, and land temperature), it is possible to gain more confidence in the adjustments applied. Each of the data sets has distinctly different problems related to long-term homogeneity and data quality that require independent adjustment procedures. When these independent data sets provide a physically consistent picture of decade-to-century climate fluctuations, their agreement can be very compelling. In fact, the analysis of quasi-periodic oscillations of surface winds, pressure, and ocean and marine air temperatures FIGURE 2 Smoothed global surface temperature variations, as derived from the original observations without adjustments for inhomogeneities in the climate record, compared to the same observations after adjustments for inhomogeneities as described by the Intergovernmental Panel on Climate Change (IPCC, 1990, 1992). by Deser and Blackmon (1995) relies entirely on physical consistency among independent variables. In order to improve our ability to discern climate fluctuations on decade-to-century time scales within the existing climate record, some federal agencies such as NSF, DOE, and NOAA are supporting data archeology efforts. Data archeology is the process of seeking out, restoring, correcting, and interpreting data sets. Such efforts are critical to identifying and understanding longer-scale climate fluctuations, since they often turn up information that reveals a pattern not otherwise obvious. The Comprehensive Ocean-Atmosphere Data Set (COADS), which is just one of several major data-archeology efforts, provides a good example of the type of benefits that can be expected from such efforts. Now, several years after the project's inception (Woodruff et al., 1987), scientists are identifying decadal-scale variations and changes in many climate elements previously thought to be too uncertain to document with any confidence (London et al., 1991; Flohn et al., 1990; Parungo et al., 1994). Interestingly, up to the present, satellite data have not figured prominently in the analysis of multi-decadal climate variability. Their short observing history and a lack of temporal homogeneity have hampered efforts to use them. Certainly, there are notable exceptions, such as NOAA's snow and sea-ice products and the Spencer and Christy (1990) microwave sounding-unit data. However, major temporal inhomogeneities in data sets, like those identified in the International Satellite Cloud Cover Project (ISCCP; see Klein and Hartmann, 1993), threaten the usefulness of much of the multi-decadal satellite data. The two major challenges over the next few decades for the United States will to be to ensure that global-change satellite-research projects such as EOS provide homogeneous data over their planned life-

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Natural Climate Variability on Decade-to-Century Time Scales times, and to lay the foundation for converting them into a long-term operational observing systems. Future observations will require more attention to data quality, homogeneity, and continuity if we are to understand the nature of decade-to-century-scale climate variability. Currently under consideration is a Global Climate Observing System (GCOS)1 that includes observations of terrestrial, atmospheric, and oceanic aspects of climate. Since GCOS would be built around existing observing networks and environmental problems, it is essential that scientists effectively convey long-term climate-monitoring needs to the agencies sponsoring space-based and conventional observing systems. Among the climate-monitoring issues that should be addressed in the near future are stability of network sites, intercomparability of instruments, and increased sampling in data-sparse regions. Two concerns for both present and future observations are the better collection and documentation of metadata on observing instruments and practices as well as processing algorithms, and improved data-archiving practices and data-management systems. All these will increase the value of climate monitoring for decade-to-century-scale research; most essential, of course, is assurance of a long-term commitment to observational objectives. THE FORCING AGENTS OF CLIMATE FLUCTUATIONS The climate record provides the key to developing, refining, and verifying our hypotheses regarding the forcing agents responsible for climate fluctuations, be they anthropogenic or natural, internal or external to the climate system, global or regional, or persisting one or many decades. Diaz and Bradley (1995) suggest that the many decade-to-century climate fluctuations evident in both observations and proxy records may indeed have natural origins, since similar fluctuations seem to occur in the climate record both before and after humans became capable of modifying climate. Often physically based models are the best means of testing our hypotheses about the cause of the fluctuations, as described in the modeling sections of this volume, but in addition they can be effectively used to help discern the physical consistency of apparent climate fluctuations and change. Cayan et al. (1995) demonstrate this approach; they use an ocean general-circulation model to help explain the climate jump over the North Pacific, as documented by Trenberth (1990; Trenberth and Hurrell, 1995, in this volume). Cayan et al. show how the atmosphere and ocean can act together to maintain decadal-scale climate fluctuations on a large spatial scale. Potentially important factors in explaining climate fluctuations on decade-to-century time scales are land-surface and atmospheric feedback effects. Walsh (1995) provides ample evidence that snow cover has important feedback effects on the climate system on short (daily and interannual) and long (thousands of years) time scales, but on the time scales of interest to us the impact of snow is still not well understood. However, Nicholson (1995) and Shukla (1995) present evidence that the feedback between land-surface characteristics and the atmosphere has led to the prolonged drought in the Sahel. Karl et al. (1995) document an asymmetric increase of the mean maximum and minimum temperatures over many portions of the global land mass. Although they cite a number of potential causes of this multi-decadal trend, such as increases of anthropogenic atmospheric sulfate aerosols (Charlson et al., 1992) and greenhouse gases, empirical evidence suggests that, at least in some regions, observed increases in cloud cover play an important role in modulating the surface temperature. The forcing responsible for the increase in cloud cover remains unknown. There are many important aspects of climate forcings and associated responses that cannot be covered here. Of particular relevance are the known changes in solar irradiance associated with the sunspot cycle. Recently Friis-Christensen and Lassen (1991) have used the length of the sunspot cycle to explain the decadal fluctuations and trends of land temperatures. Although a linear response of the surface temperature to the sunspot cycle length implies some unlikely responses of the temperature record in the early part of the time series (Kelly and Wigley, 1992), it is clear that changes in solar irradiance must continue to be monitored as a potential source of global climate fluctuations or change. Recently, Elliott et al. (1991) have found evidence for an increase of tropospheric water vapor since 1973, leading to an enhanced greenhouse effect. However, the many types of observing and data-processing inhomogeneities in the upper-air moisture record make it immensely difficult to separate spurious trends and discontinuities from the true climate signal, even when the signal may be as large as a 10 percent increase in specific humidity. A critical atmospheric quantity affecting surface temperature is the variability of cloud cover—its type, height, and spatial distribution. Using the ISCCP data base, Hartmann et al. (1992) showed the net forcing of various cloud types as a function of season and latitude. A recent study by Rossow (1995), however, reveals that this data set has serious biases that make it inappropriate for decadal-scale climate assessments. Meanwhile, conventional in situ data, analyzed on a national basis by a number of researchers, show a widespread general increase in total cloud cover 1   GCOS will develop a dedicated observation system designed specifically to meet the scientific requirements for monitoring the climate, detecting climate change, and predicting climate variations and change (ICSU/UNEP/UNESCO/WMO, 1993).

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Natural Climate Variability on Decade-to-Century Time Scales over the past several decades (Dutton et al., 1991; Henderson-Sellers, 1990; Karl and Steurer, 1990; London et al., 1991; McGuffie and Henderson-Sellers, 1989; Parungo et al., 1994). Clearly, a much more focused effort is required to better understand decadal and multi-decadal fluctuations of cloud cover, since they often have very important feedback effects on other climate quantities. For instance, cloud feedbacks work differently during the day from the way they do at night. They therefore differ at high and low latitudes, making generalizations speculative. Also, there is no discussion in this volume of the world's glaciers, in spite of their known tendency to fluctuate markedly on decade-to-century time scales. Long-term trends of climate change are integrated by mountain glaciers, and during the past century mountain glaciers all over the world have been declining (IPCC, 1990). The World Glacier Monitoring Service (1993) summarized these changes over the past several decades, and the USGS Satellite Image Atlas of the World discusses observed variations of hundreds of glaciers over the past several centuries. CONCLUSION AND RECOMMENDATIONS Identification and explanation of the forcings and feedbacks responsible for decade-to-century-scale climate fluctuations are essential to distinguishing between natural and anthropogenic impacts on the climate system, as well as to developing any predictive skill with respect to these phenomena. Further progress in understanding this gray area of climate change will depend upon our ability to address three topics. First, we must be able to document climate fluctuations without spurious discontinuities and trends. Second, we must ensure that we are adequately monitoring potential forcings and feedbacks internal and external to the climate system, so that we will have the data necessary for testing hypotheses about their operation. Last, data analysts must work closely with modelers (and vice versa) to test the hypotheses we formulate on a variety of climate models, ranging from simple one-dimensional climate models to complex coupled ocean-atmosphere general-circulation models. The re-analysis modeling projects of the United States (Kalnay and Jenne, 1991) and the European community are likely to enhance our confidence in both how and why climate has varied on decade-to-century time scales. (It must be emphasized, however, that the value of any re-analysis effort can be jeopardized by the use of data that are biased or, most troublesome, inhomogeneous in time.) Only by advancing in all three of these areas will we be able to overcome our ignorance about multi-decadal climate fluctuations and make predictions with any degree of assurance.

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Natural Climate Variability on Decade-to-Century Time Scales Documenting Natural Climatic Variations: How Different is the Climate of the Twentieth Century from That of Previous Centuries? HENRY F. DIAZ1 AND RAYMOND S. BRADLEY2 ABSTRACT Changes in decadal-mean surface temperature and its variance for different land areas of the Northern Hemisphere are examined. In the last 100 years, changes in surface air temperature have been greatest and most positive in the period since about 1970. Interannual variability, particularly at the largest spatial scales has also increased, although it differs according to regions. The most unusual decade of the last 100 years in the contiguous United States may have been the 1930s, although that of the 1980s is probably a close second, and in some regions perhaps the most anomalous. Both the 1930s and the 1980s experienced significant warming together with enhanced climatic variability. To incorporate a longer-term perspective than is obtainable from the modern instrumental record, we used summer temperature reconstructions based on tree-ring records, and on d18O ratios extracted from different ice-core records. Although none of these records is a simple or even direct temperature proxy, we present them as general indicators of prevailing environmental temperatures. The data were averaged by decades in order to focus on decadal-scale variability. With the exception of the data from tropical ice cores, the proxies indicate that the recent decades were not very unusual, either in regard to the mean or in terms of increased variability. While seasonal and annual temperature changes in the last two decades have been rather large in most areas of the Northern Hemisphere, the available paleoclimate evidence suggests that in many areas there have been decadal periods during the past several centuries in which reconstructed temperatures were comparable to those of the 1970s and 1980s, with climatic variability as large as any recorded in recent decades. Natural variability on decadal time scales is comparatively large—typically about half as large as the interannual variance. 1   NOAA Environmental Research Laboratories, Boulder, Colorado 2   Department of Geosciences, University of Massachusetts, Amherst, Massachusetts

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Natural Climate Variability on Decade-to-Century Time Scales INTRODUCTION Considerable effort has been expended over the last 25 years to improve our knowledge and understanding of climate processes and mechanisms associated with changes in climate (Saltzman, 1983; Trenberth, 1992). A major impetus for much of the recent climate research has been the global and regional climate projections made with physical climate models (the so-called general-circulation models, or GCMs) for a doubling of carbon dioxide concentration in the atmosphere. Most assessments of the climatic impact of increases in the ''greenhouse" gases (CO2, methane, nitrous oxide, chlorofluorocarbons) have concluded, more or less consistently, that global temperatures should rise (under a doubling of the most abundant of these greenhouse gases, CO2) from 1.5°C to 4.5°C (NRC, 1982; Bolin et al., 1986; IPCC, 1990; Schlesinger, 1984, 1991). Such changes, should they occur, will be superimposed on the natural variability of the climate, which is quite large at all time scales (Karl et al., 1989). Depending on a number of assumptions, the amount of global warming that should have been realized by now appears to be more consistent with the lower end of the above estimates (Bloomfield, 1992). Karl et al. (1991b) performed a variety of tests using output from three different GCMs to ascertain, given their different climate sensitivities, when a statistically significant greenhouse signal might be detected in the central United States. They concluded that it was likely that a greenhouse signal had been masked, to date, by natural climate variability, and that it would likely take another two to four decades before the greenhouse signal in temperature and precipitation might be unambiguously detected in this region. We should note the possibility that other human-induced factors may be acting to counteract the radiative effects of increased greenhouse-gas concentrations in the atmosphere. Recent work has indicated that sulfate aerosols, which act to increase the planetary albedo (Charlson et al., 1992; Hansen and Lacis, 1990; Wigley, 1989, 1991) may have counteracted to some degree the enhanced infrared warming (see Michaels and Stooksbury, 1992). These and other factors, whether anthropogenic in origin or not (e.g., changes in vulcanism), will undoubtedly "muddy" the climate picture and may make it more difficult to identify unequivocally a contemporary greenhouse signal. In this paper, we consider two aspects of climate-change detection that we feel bear strongly on the issue of natural versus anthropogenic climate variability. One aspect of the problem is, in effect, at what point one rejects the null hypothesis (no climate change) and accepts the premise that the climate of the last few decades belongs to a different sample population. The difficulty there arises because we are evaluating a relatively short observational record with the knowledge that the climate has fluctuated in the past few centuries by an amount that may be of the same magnitude as the fluctuations observed in the recent record (see Bradley and Jones, 1992). A second aspect of the problem of climate change versus natural variability, which we consider here, is temporal changes in that variability, i.e., in the variance. Changes in climatic variability are important, since they are likely to have a greater effect on a society's ability to mitigate and adapt to climatic changes than a slow alteration in the mean climate patterns. We have examined a variety of observational records of various lengths (typically 100 to 150 years), which are representative of different spatial scales (from hemispheric to regional basins). We will focus on decadal time scales, since one could argue that climatic changes will be better gauged at time scales that to some extent average out the high-frequency climatic "noise" associated with seasonal variability and other air-sea processes operating on annual time scales (e.g., the El Niño/ Southern Oscillation system). We compare these instrumental series with a suite of high-resolution climate proxy records—namely, tree rings and oxygen isotopes extracted from ice cores—spanning the last 300 to 800 years. Our aim is to give the reader some feeling for the range of this intermediate-frequency (decade-to-century-scale) climatic variability, as well as some appreciation of the uncertainties inherent in the existing records (instrumental, historical, and paleoenvironmental). We note that the "fingerprint" climate-change-detection technique (Barnett, 1986; Barnett and Schlesinger, 1987; Barnett et al., 1991) provides a very useful methodology for testing a hypothesis (the GCM climate-change projections) against observations. We will attempt here to highlight some aspects of climatic variability at decadal and longer time scales and will re-emphasize the conclusion of Barnett et al. (1991), who noted that the existence of a high degree of "unexplained" interdecadal climatic variability (see also Ghil and Vautard, 1991, and Karl et al., 1991b) will greatly complicate the detection of a greenhouse climate-change signal. THE MODERN (INSTRUMENTAL) RECORD Characteristics of Decadal-Scale Changes of Area-Mean Temperature Indices The available data suggest that during the past century decadal-scale changes in global-scale mean annual temperature are on the order of 0.1°C to 0.3°C (see Folland et al., 1990). Figure 1 illustrates the annual and seasonal temperature changes for the Northern Hemisphere land areas. The plotted data, from Jones et al. (1986a) with subsequent updates, are consecutive 10-year area-weighted averages from 1891-1900 through 1981-1990 of gridded temperature anomalies referenced to the 1951-1970 period. On this "dec-

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Natural Climate Variability on Decade-to-Century Time Scales FIGURE 1 Decadal-mean seasonal and annual temperature anomalies (in °C) for land areas of the Northern Hemisphere. Data is from Jones et al. (1986a), with subsequent updates. adal" time scale, the warming of the 1980s relative to the 1970s (0.32°C for the hemisphere) is comparable to the warming that occurred in the 1920s relative to temperatures in the 1910s (0.26°C). Some seasonal differences can be noted, with northern summer and fall showing the least warming trend and winter and spring showing the greatest warming trend over the last 100 years. The twentieth century contains many examples of decadal-scale climatic variations of sufficient magnitude to have significantly affected societies. The two-decade-long drought in the African Sahel is perhaps the best-known example. The words "climate change" have become overused, and climatic variations from seasonal to multi-year are often lumped together. Until the mid-1970s, "climate change" was used to refer to climatic changes on time scales of 104 to 106 years (essentially, Milankovitch time-scales). Variations of less than 10 years were generally considered to be part of the natural climate noise associated with interannual variations of the various components of the climate system and their nonlinear interactions, whereas variations on time scales of 10 to 103 years are now thought to be strongly driven by ocean-atmosphere heat exchanges resulting from changes in the ocean's thermohaline circulation. Because of the changes in the earth's radiative balance imposed by human activities during the past century, climatologists have been trying to detect climatic-change signals that hitherto have typically been associated with time scales of millennia. The climate record available for such studies, even with the aid of high-resolution paleoenvironmental records, is sufficient to define only continental to hemispheric changes on decadal time scales, and perhaps regional (less than about 105 km2) changes on century time scales. In the United States, perhaps the singular climatic event of this century was the severe, widespread, and persistent drought of the 1930s. This event has been widely chronicled in both scientific (e.g., Skaggs, 1975) and popular literature. Other climatic events that persisted for more than one year and affected relatively large areas of the United States were the drought of the 1950s in south central areas of the United States (Chang and Wallace, 1987), and the drought of the 1960s in the Northeast (Namias, 1966). Periodic drought of variable duration has affected all areas of the contiguous United States to varying degrees (Diaz, 1983). Indeed, multiyear climate anomalies are a characteristic feature of the climate of the United States and of other parts of the world (Karl, 1988; Folland et al., 1990). In Figure 2, decadal-mean seasonal (December-February for winter, March-May for spring, etc.) and annual temperature anomalies for various areas of the Northern Hemisphere are examined. Although Eurasia and North America (Figure 2a) have broadly similar seasonal trends over this period, differences can be noted in all seasons as well as in the annual averages. Figure 2b illustrates decadal temperature changes for the contiguous United States for the last century. The data used are from the adjusted divisional values (solid curve) described in Karl et al. (1984b). For comparison, the equivalent Jones et al. (1986a) gridded values are given as the dashed curve in Figure 2b. The decadal averages for the lower 48 states exhibit much smaller long-term trends than those noted for the continental-scale regions. Figures 2c and 2d illustrate the decadal temperature changes recorded in subregions of the United States, namely, the eastern and western halves of the United States (divided roughly along the 100th meridian), the state of Colorado, and a single state climatic division in Colorado where the city of Boulder is located. Again we note regional differences that can depart substantially from the temporal behavior of the larger spatial averages. We have tabulated the mean temperature change from one calendar decade to the next over the 100-year period 1891 to 1990 for each of the above regions. No particular pattern emerges, except that at the largest space scales, the greatest decade-to-decade warming occurs mostly from the 1970s to the 1980s, whereas the time of the largest such

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Natural Climate Variability on Decade-to-Century Time Scales PARKER ET AL., FIGURE 1a MOHSST5 anomalies (°C) (w.r.t. 1951-1980) without filled values, January 1878. PARKER ET AL., FIGURE 1b MOHSST5 anomalies (°C) with filled values, January 1878.

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Natural Climate Variability on Decade-to-Century Time Scales PARKER ET AL., FIGURE 1c Enhanced MOHSST5 anomalies (°C), January 1878. PARKER ET AL., FIGURE 1d GISST 1.0 anomalies (°C), January 1878. Sea ice is shaded black.

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Natural Climate Variability on Decade-to-Century Time Scales PARKER ET AL., FIGURE 3a MOHSST5 anomalies (°C) with filled values, January 1983. PARKER ET AL., FIGURE 3b GISST 1.0 anomalies (°C), January 1983. Sea ice is shaded black.

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Natural Climate Variability on Decade-to-Century Time Scales PARKER ET AL., FIGURE 4a MOHSST5 anomalies (°C) without filled values, January 1983, with coverage limited to that of January 1878. PARKER ET AL., FIGURE 4b MOHSST5 anomalies (°C) with filled values, January 1983, with coverage of input data limited to that of 1877-1878.

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Natural Climate Variability on Decade-to-Century Time Scales PARKER ET AL., FIGURE 4c Enhanced MOHSST5 anomalies (°C), January 1983, with coverage of input data limited to that of 1877-1878. PARKER ET AL., FIGURE 4d Globally complete SST anomalies (°C) and sea ice, January 1983, with coverage of input SST data limited to that of 1877-1878. Sea ice is shaded black.

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Natural Climate Variability on Decade-to-Century Time Scales PARKER ET AL., FIGURE 5a Bias (°C) of the reduced-sampling analysis of 1982-1983. PARKER ET AL., FIGURE 5b Root-mean-square differences (°C) between GISST 1.0 and the reduced-sampling analysis, 1982-1983.

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Natural Climate Variability on Decade-to-Century Time Scales PARKER ET AL., FIGURE 6 Correlations between GISST 1.0 and the reduced-sampling analysis, 1981-1990. PARKER ET AL., FIGURE 7 Bias (°C), GISST 1.1 minus Reynolds and Marsico (1993) analysis, 1982-1991.

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Natural Climate Variability on Decade-to-Century Time Scales GHIL, COMMENTARY ON DOUGLAS, FIGURE 1 Distribution of stations that show an oscillatory feature with periods from 18 to 30 months. (a) distribution of period; (b) distribution of the amplitudes of the quasi-biennial oscillation, as a percentage of the total variance at each station (courtesy of Y. Sezginer-Unal).

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Natural Climate Variability on Decade-to-Century Time Scales GHIL, COMMENTARY ON DOUGLAS, FIGURE 2 Distribution of the stations that show an oscillatory feature with periods from 36 months to 60 months. (a) distribution of the period; (b) distribution of the amplitudes of the low-frequency ENSO signal, as a percentage of the total variance at each station (courtesy of Y. Sezginer-Unal).

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Natural Climate Variability on Decade-to-Century Time Scales MUNK, DISCUSSION FOLLOWING LAZIER PAPER, FIGURE 1 Time series of range-averaged potential temperature profiles along four paths of the tomogra Total ice concentration in percent (from SSMM/I data) at the path endpoints are plotted below the images. Range-averaged profiles, computed from Seasonal Ice Zone E (SIZEX) data obtained on 15-16 March 1989 between moorings 1 and 6 and moorings 3 and 6 are given for comparison. (From SIZEX group, 1989; reprinted with permis Nauseu Remote Sensing Center.)

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Natural Climate Variability on Decade-to-Century Time Scales LEHMAN, FIGURE 8 Advanced VHR radiometer image of the eastern Norwegian Sea, showing the entry of warm Atlantic waters (warm colors) between the Faeroe and Shetland islands and the mixing of Atlantic waters into the North Sea. The site of Troll 3.1 is marked (). The dark, northeast-arching swath near the core site reflects cloud cover. (From Johannessen, 1986: reprinted with permission of Springer-Verlag.)