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Understanding Climatic Change: A Program for Action (1975)

Chapter: SCOPE OF PRESENT RESEARCH ON CLIMATIC VARIATION

« Previous: PAST CLIMATIC VARIATIONS AND THE PROJECTION OF FUTURE CLIMATES
Suggested Citation:"SCOPE OF PRESENT RESEARCH ON CLIMATIC VARIATION." National Research Council. 1975. Understanding Climatic Change: A Program for Action. Washington, DC: The National Academies Press. doi: 10.17226/27501.
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Suggested Citation:"SCOPE OF PRESENT RESEARCH ON CLIMATIC VARIATION." National Research Council. 1975. Understanding Climatic Change: A Program for Action. Washington, DC: The National Academies Press. doi: 10.17226/27501.
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Suggested Citation:"SCOPE OF PRESENT RESEARCH ON CLIMATIC VARIATION." National Research Council. 1975. Understanding Climatic Change: A Program for Action. Washington, DC: The National Academies Press. doi: 10.17226/27501.
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Page 48
Suggested Citation:"SCOPE OF PRESENT RESEARCH ON CLIMATIC VARIATION." National Research Council. 1975. Understanding Climatic Change: A Program for Action. Washington, DC: The National Academies Press. doi: 10.17226/27501.
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Page 49
Suggested Citation:"SCOPE OF PRESENT RESEARCH ON CLIMATIC VARIATION." National Research Council. 1975. Understanding Climatic Change: A Program for Action. Washington, DC: The National Academies Press. doi: 10.17226/27501.
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Page 50
Suggested Citation:"SCOPE OF PRESENT RESEARCH ON CLIMATIC VARIATION." National Research Council. 1975. Understanding Climatic Change: A Program for Action. Washington, DC: The National Academies Press. doi: 10.17226/27501.
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Page 51
Suggested Citation:"SCOPE OF PRESENT RESEARCH ON CLIMATIC VARIATION." National Research Council. 1975. Understanding Climatic Change: A Program for Action. Washington, DC: The National Academies Press. doi: 10.17226/27501.
×
Page 52
Suggested Citation:"SCOPE OF PRESENT RESEARCH ON CLIMATIC VARIATION." National Research Council. 1975. Understanding Climatic Change: A Program for Action. Washington, DC: The National Academies Press. doi: 10.17226/27501.
×
Page 53
Suggested Citation:"SCOPE OF PRESENT RESEARCH ON CLIMATIC VARIATION." National Research Council. 1975. Understanding Climatic Change: A Program for Action. Washington, DC: The National Academies Press. doi: 10.17226/27501.
×
Page 54
Suggested Citation:"SCOPE OF PRESENT RESEARCH ON CLIMATIC VARIATION." National Research Council. 1975. Understanding Climatic Change: A Program for Action. Washington, DC: The National Academies Press. doi: 10.17226/27501.
×
Page 55
Suggested Citation:"SCOPE OF PRESENT RESEARCH ON CLIMATIC VARIATION." National Research Council. 1975. Understanding Climatic Change: A Program for Action. Washington, DC: The National Academies Press. doi: 10.17226/27501.
×
Page 56
Suggested Citation:"SCOPE OF PRESENT RESEARCH ON CLIMATIC VARIATION." National Research Council. 1975. Understanding Climatic Change: A Program for Action. Washington, DC: The National Academies Press. doi: 10.17226/27501.
×
Page 57
Suggested Citation:"SCOPE OF PRESENT RESEARCH ON CLIMATIC VARIATION." National Research Council. 1975. Understanding Climatic Change: A Program for Action. Washington, DC: The National Academies Press. doi: 10.17226/27501.
×
Page 58
Suggested Citation:"SCOPE OF PRESENT RESEARCH ON CLIMATIC VARIATION." National Research Council. 1975. Understanding Climatic Change: A Program for Action. Washington, DC: The National Academies Press. doi: 10.17226/27501.
×
Page 59
Suggested Citation:"SCOPE OF PRESENT RESEARCH ON CLIMATIC VARIATION." National Research Council. 1975. Understanding Climatic Change: A Program for Action. Washington, DC: The National Academies Press. doi: 10.17226/27501.
×
Page 60
Suggested Citation:"SCOPE OF PRESENT RESEARCH ON CLIMATIC VARIATION." National Research Council. 1975. Understanding Climatic Change: A Program for Action. Washington, DC: The National Academies Press. doi: 10.17226/27501.
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Page 61

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2S SCOPE OF PRESENT RESEARCH ON CLIMATIC VARIATION The overview of the problem of climatic variation presented in the preceding chapters and in the technical appendixes contains only those references to the literature that were helpful in the illustration of a particular viewpoint or necessary to document a specific source of in- formation. In the course of its deliberations, however, the Panel found it necessary to survey present research on climatic variation, as rep- resented by the more recently published literature and by selected on- going activities. Inasmuch as this information may serve as a useful background to the Panel’s recommendations for a national and inter- national program of climatic research, it is summarized here. Even this survey, in which emphasis is given to material published since 1970, must be considered incomplete and necessarily gives precedence to sources of information most readily available to the Panel. Further use- ful references on various aspects of the problem of climatic variation are to be found in other recent publications (Committee on Polar Research, 1970; National Science Board, 1972; Wilson, 1970, 1971). CLIMATIC DATA COLLECTION AND ANALYSIS Here the current status of efforts to assemble climatic data for both the atmosphere and ocean is summarized, and the various observational field programs directed to the collection of specific data of climatic interest are described. 46

SCOPE OF PRESENT RESEARCH ON CLIMATIC VARIATION 47 Atmospheric Observations Climatological data banks are maintained by NOAA’s National Climatic Center (NCC) and National Meteorological Center (NMC) and by the military operational weather services, particularly the Air Force’s En- vironmental Technical Applications Center (ETAC) and the Navy’s Fleet Numerical Weather Central (FNWC). Using data from these sources, atmospheric data sets specifically for climatic studies have been assembled by the National Center for Atmospheric Research, the Geo- physical Fluid Dynamics Laboratory, MIT, and other institutions. Efforts to assemble the rapidly accumulating data from meteorological satellites have also been made by NOAA’s National Environmental Satel- lite Service (NESS) and by the University of Wisconsin. Sustained efforts to assemble and systematically analyze such data for the use of the climatic research community are important tasks for the future. In addition to the standard compilations of climatological statistics prepared on a routine basis by governmental agencies, new summaries of upper-air data have been prepared (Crutcher and Meserve, 1970; Taljaard et al., 1969); these have permitted the initial construction of the average monthly global distributions of the basic meteorological variables of pressure, temperature, and dew points at selected levels. The analysis of such data in terms of the various statistics of the global circulation is less advanced, although intensive studies of a five-year period in the northern hemisphere have recently been completed (Oort, 1972; Oort and Rasmusson, 1971; Starr and Oort, 1973). These studies provide the most quantitative analyses of the annual climatic variations of the atmosphere yet made, and plans are under way for their extension to additional five-year periods. Studies of the spatial patterns of observed variability over longer time periods are almost entirely confined to surface variables in the northern hemisphere (Hellerman, 1967; Kutzbach, 1970; Wagner, 1971). Such studies should be extended to other portions of the atmosphere and broadened to include other, less comprehensively observed climatic elements. An observational analysis of the tropical and equatorial circulation has been completed (Newell et al., 1972), and statistics for the strato- spheric climate are becoming increasingly available (Newell, 1972). Comprehensive data on the components of the global atmospheric energy balance are only beginning to be available (Newell et al., 1969), although many rely on older and indirect data for the unobserved ele- ments of the heat balance at the earth’s surface (Budyko, 1956, 1963; Lvovitch and Ovtchinnikov, 1964; Moller, 1951; Posey and Clapp,

48 UNDERSTANDING CLIMATIC CHANGE 1964). More recent direct observations from satellites, however, are providing valuable new insight into both the time and space variations of the overall radiation budget of the earth (Vonder Haar and Suomi, 1971) and promise to provide further data of climatic importance as newer and more versatile satellite observational capabilities develop (Chahine, 1974; COSPAR Working Group 6, 1972; Raschke et al., 1973; Smith et al., 1973). Oceanic and Other Observations The observational data base for the oceans is much less developed than that for the atmosphere, and oceanic climatic summaries are based largely on observations that are more widely scattered in both space and time. Even for the more traveled parts of the oceans, these data are sufficient only to indicate the average large-scale features of the ocean’s structure and circulation (Fuglister, 1960; Hellerman, 1967; Sverdrup et al., 1942; U.S. Navy Hydrographic Office, 1944). Updated com- pilations of surface stress (Hellerman, 1967)) and sea-surface tem- peratures (Alexander and Mobley, 1973; Washington and Thiel, 1970) have been made, and summaries of the observed subsurface temperature structure have recently become available for selected oceans (Born et al., 1973; Robinson and Bauer, 1971). Significant repositories of oceanic data useful for climatic purposes exist at a number of institutions, although a comprehensive oceanic data inventory has not yet been prepared. The Navy’s Fleet Numerical Weather Central, the Scripps Institution of Oceanography, the Woods Hole Oceanographic Institution, and the National Marine Fisheries Service, for example, all have specialized oceanographic data banks, as well as data from individual cruises and expeditions. Guides to the oceanic data services of the Environmental Data Service (1973) of NOAA are also available. An increasing amount of data on oceanic surface conditions is becom- ing available from satellite observations and other remote-sensing techniques (Munk and Woods, 1973; Shenk and Salomonson, 1972) and offer the promise of routine global monitoring of sea-surface tem- perature and sea-ice distribution. Satellite data collected by NEss also permit the determination of the snow and ice extent over land; this and other glaciological data are being accumulated by the U.S. Geological Survey. The further extension of oceanographic, sea-ice, and glaci- ological observations by satellites, buoys, and ships is under active consideration in connection with the FGGE (GARP, 1972; Stommel, 1973) and is part of other large-scale programs as well (International

SCOPE OF PRESENT RESEARCH ON CLIMATIC VARIATION 49 Decade of Ocean Exploration, 1973; International Glaciological Pro- gramme for the Antarctic Peninsula, 1973; Kasser, 1973; Mid-ocean Dynamics Experiment-one, Scientific Council, 1973; Joint U.S. POLEX Panel, 1974). Observational Field Programs Many observational data of importance to climatic research have been acquired in special field programs. Some of these are directly related to GARP itself (AMTEX Study Group, 1973; GARP Joint Organizing Com- mittee, 1972, 1973; Houghton, 1974; Kondratyev, 1973), such as the Complete Atmospheric Energetics Experiment (CAENEX), the Air-Mass Transformation Experiment (AMTEX), the GARP Atlantic Tropical Ex- periment (GATE), and the Arctic Ice Dynamics Joint Experiment (AIDJEX). Others are part of the NSF’s International Decade of Ocean Exploration (IDOE) (1973) program (Mid-ocean Dynamics Experi- ment-one, Scientific Council, 1973), such as the Geochemical Ocean Sections Study (GEOSECS), the Mid-ocean Dynamics Experiment (MODE), the North Pacific Experiment (NORPAX), and the Climate, Long-range Investigation, Mapping, and Prediction (CLIMAP) project. Other field programs are aimed at the monitoring of atmospheric com- position and aerosols, such as those of NCAR, the Environmental Pro- tection Agency, and NoAa’s Environmental Research Laboratories. Each of these programs is focused on physical processes of im- portance in particular geographical regions and is a valuable source of experience and information. There are also international programs of this sort in various stages of planning, such as the Polar Experiment (POLEX) (Joint U.S. PoLEXx Panel, 1974), the International Glaci- ological Program for the Antarctic Peninsula (IGPAP) (1973), and the International Southern Ocean Studies (isos) programs (isos Planning Committee, 1973). Cooperative programs such as these will be neces- sary for the comprehensive future monitoring, analysis, and modeling of climate and climatic variation. STUDIES OF CLIMATE FROM HISTORICAL SOURCES The record of past climates as contained in various historical documents, writings, and archeological material has been increasingly recognized as an important source of information (Bryson and Julian, 1963; Butzer, 1971; Carpenter, 1965; LeRoy Ladurie, 1971; Lamb, 1968, 1972; Ludlam, 1966, 1968). These sources permit the study of historical climates over the past several thousand years. A systematic compilation

50 UNDERSTANDING CLIMATIC CHANGE of material of this sort is being undertaken by the Climatic Research Unit of the University of East Anglia (Lamb, 1973b). STUDIES OF CLIMATE FROM PROXY SOURCES The assembly of paleoclimatic information from proxy data sources has attained new importance in recent years with the development of new methods of dating and of new techniques of quantitative climatic in- ference. In the following, the various efforts in this aspect of climatic research are briefly summarized. General Syntheses Two broad surveys of paleoclimatology have appeared in recent years (Funnel! and Riedel, 1971; Schwartzbach, 1961), along with textbooks (Flint, 1971; Washburn, 1973) and symposia (Black et al., 1973; Turekian, 1971), which emphasize the glacial processes during the late Cenozoic period. Other recent paleoclimatological syntheses have been concerned with the broad range of Quaternary studies (Wright and Frey, 1965), with the relationships between Pleistocene geology and biology (Butzer, 1971; West, 1968), and with more recent paleoclimatic fluctuations from a meteorological viewpoint (Lamb, 1969). The review of the full range of paleoclimatic events on all time scales given in Appendix A of this report has been made possible by the recent application of improved dating methods to the stratigraphic record of ocean sediments and uplifted reefs. This synthesis illustrates the essential need for an accurate time scale in the interpretation of proxy climatic data. Chronology The methods of dendrochronology (Ferguson, 1970; LaMarche and Harlan, 1973), the radiocarbon method (Olsson, 1970; Wendland and Bryson, 1974), and other isotopic dating methods have recently been used to infer the chronology of climate over the past several hundred thousand years (Broecker and van Donk, 1970; Matthews, 1973; Mesolella et al., 1969). Biostratigraphic and paleomagnetic correlations between the marine and continental records have provided a reasonably accurate chronology of the past 60 million years by the use of potassium— argon and other isotopes (Berggren, 1971, 1972; Hays et al., 1969; Kukla, 1970; Ruddiman, 1971; Sancetta et al., 1973; Shackleton and Kennett, 1974a, 1974b; Shackleton and Opdyke, 1973).

SCOPE OF PRESENT RESEARCH ON CLIMATIC VARIATION 51 Monitoring Techniques Following the initial efforts to estimate paleotemperatures from isotopic time series (Emiliani, 1955, 1968), recent work has made it possible to separate the effects of temperature from those of ice-volume change (Shackleton and Opdyke, 1973). Multivariate statistical techniques have recently been developed that permit the quantitative estimation of climatic parameters from the concentration of fossil plankton in deep- sea sediments (Imbrie and Kipp, 1971; Imbrie et al., 1973; Kipp, 1974), the growth record of tree rings (Fritts et al., 1971), and the continental distribution of fossil pollen (Webb and Bryson, 1972). These methods have since been applied to the reconstruction of paleo- ocean temperatures (Luz, 1973; McIntyre et al., 1972a; Pisias et al., 1973; Sachs, 1973), as well as to pressure and precipitation anomalies (Fritts, 1972). Isotopic studies of cores taken in the polar ice caps provide measures of the air temperature at the time of ice formation (Dansgaard et al., 1971). Further refinements of such monitoring tech- niques will help to fill in the paleoclimatic record, especially when several independent methods are available for the same period. Proxy Data Records and Their Climatic Inferences Proxy data come from a wide variety of sources; potentially, any bio- logical, chemical, or physical characteristic that responds to climate may provide proxy data useful in the reconstruction of past climates. One of the more prolific sources of long-term climatic information has been the extensive collection of deep-sea cores, obtained routinely over the years on various oceanographic expeditions and more recently from the Deep-Sea Drilling Project (Douglas and Savin, 1973; Shackle- ton and Kennett: 1974a, 1974b). Analysis of the fossil flora and fauna in such cores, with chronology provided from their isotopic content and paleomagnetic stratigraphy, has been performed for all the princi- pal oceans of the world (Emiliani, 1968; Gardner and Hays, 1974; Hunkins et al., 1971; Imbrie, et al., 1973; Kellogg, 1974; Kennett and Huddlestun, 1972; Moore, 1973) and provides a preliminary docu- mentation of the temperature and large-scale displacements of the surface waters during the last few hundred thousand years (McIntyre et al., 1972b; Shackleton and Opdyke, 1973). Other characteristics of the sediment cores, such as the presence of volcanic ash (Ruddiman and Glover, 1972), also indicate climatically important events, as well as providing valuable core dating horizons. For periods of particular interest, such as the glacial maximum of about 18,000 years ago, de-

52 UNDERSTANDING CLIMATIC CHANGE tailed reconstructions of seasonal sea-surface temperature and salinity have been made for the North Atlantic (McIntyre et al., 1974) and more recently have been extended to the world ocean under the CLIMAP program. The concentration of fossil pollen and the record of soil types in relatively undisturbed continental sites is another source of proxy data on terrestrial paleoclimates. In recent years, pollen data have been analyzed from a number of continental areas (Bernabo et al., 1974; Davis, 1969; Heusser, 1966; Heusser and Florer, 1974; Livingstone, 1971; Swain, 1973; Tsukada, 1968; van der Hammen et al., 1971) and provide a preliminary documentation of the surface vegetational changes during the late Cenozoic and Quaternary periods (Leopold, 1969; Wright, 1971). Soil records have been studied less extensively but provide corroborative evidence of surface climatic conditions (Frye and Willman, 1973; Kukla, 1970; Sorenson and Knox, 1973). In many ways analogous to the records from deep-sea cores, proxy climatic data from ice cores have recently been obtained from sites in both Antarctica and Greenland (Dansgaard et al., 1969, 1971). Such ice-core records provide a detailed history of atmospheric conditions over the ice during the last hundred thousand years (Dansgaard et al., 1973; Johnsen et al., 1972; Langway, 1970). The drilling of deeper cores are planned, and their analysis and correlation with other proxy data will contribute significantly to the reconstruction of global climatic history. Further climatic inferences are obtained from proxy data on marine shorelines. By assembling data on dated terraces at selected continental and island sites, and with the necessary adjustments for eustatic changes in the earth’s crust, the record of sea-level variations over the last 150,000 years is becoming established (Bloom, 1971; Currey, 1965; Matthews, 1973; Mesolella et al., 1969; Milliman and Emery, 1968; Steinen et al., 1973; Walcott, 1972), particularly as regards the timing of high stands. Closely related to the questions of ice, soil, and sea-level changes are the proxy data from glacial fluctuations themselves. Considerable attention has been given in recent years to the reconstruction of the glacial history of the most recent major ice age in North America (Andrews et al., 1972; Black et al., 1973; Dreimanis and Karrow, 1972; Frye and Willman, 1973; Paterson, 1972; Porter, 1971; Richmond, 1972), as well as the relatively small but significant fluctuations in mountain glaciers over the past 10,000 years (Denton and Karlén, 1973). Although local glacial margins fluctuate primarily in response to the glacier’s net mass accumulation, their overall pattern provides

SCOPE OF PRESENT RESEARCH ON CLIMATIC VARIATION 53 evidence of larger-scale and longer-period climatic responses. When these changes are combined with the more limited data on the glacial history of the Antarctic ice sheet, a number of worldwide relationships in the major fluctuations of glacial extent begin to emerge (Denton et al., 1971; Hughes, 1973). In the postglacial period, important proxy data on climatic variations over the continents also come from the records of tree rings and closed- basin lakes. Both of these features respond directly to the hydrologic and thermal balances at the surface and when properly calibrated for local effects can provide a record of climate over thousands of years. With the introduction of new dating and analysis methods, the records of tree-ring width variations from both living and fossil trees provide an annually integrated record of climatic changes, especially in arid regions (Ferguson, 1970; Fritts, 1971, 1972; LaMarche, 1974; La- Marche and Harlan, 1973). The radiocarbon dating of debris in selected arid lakes provides further evidence of climatic variations, particularly as they affect the local water balance (Broecker and Kaufman, 1965; Butzer et al., 1972; Farrand, 1971). Institutional Programs Much of the present research on paleoclimates is performed in con- junction with other glaciological and geological programs, such as those of the U.S. Geological Survey, the Illinois Geological Survey, the Lamont-Doherty Geological Observatory of Columbia University, and the Army’s Cold Regions Research and Engineering Laboratory. Other efforts are conducted within the larger oceanographic research labora- tories, such as the Scripps Institution of Oceanography of the University of California, the Woods Hole Oceanographic Institution, the U.S. Naval Oceanographic Laboratory, and the marine research laboratories of the University of Miami, the University of Rhode Island, and Oregon State University. In recent years, more specialized paleoclimatic re- search efforts have been developed at a number of other universities, joining the long-established Laboratory of Tree-ring Research of the University of Arizona and the Institute for Polar Research at The Ohio State University. These include the Quaternary Research Centers at the University of Washington and the University of Maine, the Center for Climatic Research at the University of Wisconsin, the Institute of Arctic and Alpine Research at the University of Colorado, and the paleoclimatic research programs in the geology and geophysics depart- ments of Brown University and Yale University. Notable among the many cooperative activities of these and other

54 UNDERSTANDING CLIMATIC CHANGE institutions are the NSF’s IDOE programs, including the CLIMAP and NORPAX projects. Such cooperative programs have been instrumental in developing an effective collaboration among the paleoclimatic, oceanographic, and meteorological research communities and should be broadened in the future. PHYSICAL MECHANISMS OF CLIMATIC CHANGE Although the problem of climatic change has been the subject of speculation for over a century, recent research has concentrated on the study of specific physical processes and on the interactions among various components of the climatic physical system. Here the more recent of such efforts are briefly surveyed, together with a review of associated empirical, diagnostic, and theoretical studies. Physical Theories and Feedback Mechanisms Of particular interest in the problem of climatic change is the question of the cause of the ice ages. Among the recent attempts to answer this question are hypotheses that focus upon the roles of sea ice (Donn and Ewing, 1968) and ice shelves (Wilson, 1964), the carbon dioxide balance (Plass, 1956), and the ocean’s salinity (Weye, 1968). Other hypotheses emphasize the roles of variations of external boundary con- ditions, particularly the incoming solar radiation (Alexander, 1974; Budyko, 1969; Clapp, 1970; Manabe and Wetherald, 1967) and the volcanic dust loading of the atmosphere (Lamb, 1970). It is generally believed that the astronomical variations of seasonal solar radiation play a role in longer-period climatic changes (Milanko- vitch, 1930; Mitchell, 1971b; Vernekar, 1972), although there is no agreement on the physical mechanisms involved. Recent studies have also been made of the long-standing question of possible short-term relationships between the climate and solar activity itself (Roberts, 1973; Roberts and Olson, 1973). Other hypotheses of climatic change reckon with the possibility that much of the observed variations of climate are essentially the result of the natural, self-excited variability of the internal climatic system (Bryson, 1974; Mitchell, 1966, 1971b; Sawyer, 1966). Of the many feedback processes involved in climate (Schneider and Dickinson, 1974) the role of aerosols has recently received particular attention (Chylék and Coakley, 1974; Joseph et al., 1973; Mitchell, 1971a, 1974; Rasool and Schneider, 1971; Schneider, 1971). Although our knowledge of the physical properties and global distribution of

SCOPE OF PRESENT RESEARCH ON CLIMATIC VARIATION 55 aerosols is limited, these studies indicate that the climatic effects may be substantial (Rasool and Schneider, 1971; Yamamoto and Tanaka, 1972). Several research programs on aerosols are under way, including the Global Atmospheric Aerosol Research Study (GAARS) of NCAR and the Soviet CAENEX program (Kondratyev, 1973) previously noted. Attention has also been focused on the regulatory roles of cloudiness (Cox, 1971; Mitchell, 1974; Schneider, 1972) and air—sea interaction (Namias, 1973; White and Barnett, 1972) in the global climatic sys- tem. In both cases, however, an adequate quantitative understanding has not yet been achieved. Diagnostic and Empirical Studies Related to the search for physical climatic theories and mechanisms are many empirical and diagnostic studies of various aspects of climatic change. Particular attention has been given to the analysis of the large- scale variations of the atmospheric circulation that have been observed during the past few decades (Angell et al., 1969; Bjerknes, 1969; Davis, 1972; Namias, 1970; Wahl, 1972; Wahl and Lawson, 1970; White and Walker, 1973) and to their relationship to regional anomalies of temperature and rainfall (Landsberg, 1973; Namias, 1972b; Winstanley, 1973a, 1973b). Satellite observations of the large-scale variations of surface albedo and seasonal snow cover have brought new attention to these features of the climatic system (Kukla and Kukla, 1974; Wagner, 1973), as well as necessitating a significant revision of the atmospheric radiative energy budget (London and Sasamori, 1971) and the estimated oceanic energy transport (Vonder Haar and Oort, 1973). Several recent diagnostic and empirical studies have also focused on aspects of the atmosphere—ocean interaction on seasonal, annual, and decadal time scales (Lamb and Ratcliffe, 1972; Namias, 1969, 1971b, 1972a) and have prompted new attention to their relevance to long- range forecasting (Ratcliffe, 1973; Ratcliffe and Murray, 1970). The larger-scale variations of ocean-surface temperature and sea level have also been studied and have led to the identification of apparent tele- connections with the atmospheric circulation (Namias, 1971a; Wyrtki, 1973, 1974). New studies of mesoscale oceanic features have been made (Bern- stein, 1974) and provide further evidence of the dominance of this scale in the oceanic energy spectrum (in agreement with the preliminary results of the MODE program). Other oceanic studies have concentrated on the empirical evaluation of the turbulent fluxes of momentum, heat,

56 UNDERSTANDING CLIMATIC CHANGE and water vapor across the air—sea interface (Holland, 1972; Paulson et al., 1971, 1972). The difficulties of estimating the transport of even the strongest ocean currents or the heat balance over ice-covered seas with the present data base have also received renewed emphasis (Fletcher, 1972; Niiler and Richardson, 1973; Reid and Nowlin, 1971). Predictability and Related Theoretical Studies An important problem in climatic variation is the determination of the degree of predictability that is inherent in the natural system, as well as that which is achievable by simulation. A number of recent studies of simplified models have shown that multiple climatic solutions may exist under the same external conditions (Budyko, 1972b; Faegre, 1972; Lorenz, 1968, 1970) in a manner suggestive of certain features of the observed climatic record. There is also evidence from simplified models that the completely accurate specification of a climatic state is not achievable in any case, because of the same kind of nonlinear error growth that limits the accuracy of weather prediction (Fleming, 1972; Houghton, 1972; Leith, 1971; Lorenz, 1969; Robinson, 1971a). Analyses of selected climatic time series indicate only limited pre- dictability on yearly and perhaps decadal time scales (Kutzbach and Bryson, 1974; Leith, 1973; Lorenz, 1973; Vulis and Monin, 1971), while the general white-noise character of higher-frequency fluctuation has been confirmed in model simulations (Chervin et al., 1974). Further studies of climatic predictability are needed in order to identify both the intrinsic and practical limits of climatic prediction. NUMERICAL MODELING OF CLIMATE AND CLIMATIC VARIATION The accurate portrayal of global climate is the scientific goal of much of the atmospheric and oceanic numerical modeling effort now under way (Smagorinsky, 1974). When such models are coupled, the direct numerical simulation of at least the shorter-period climatic variations becomes a realistic possibility. The study of longer-period climatic variations, however, may require the construction of increasingly parameterized models. Here the more recent modeling research in both of these approaches is briefly reviewed. Atmospheric General Circulation Models and Related Studies Studies with global atmospheric general circulation models (GCM’s) have focused on the simulation of seasonal climate, with emphasis on

SCOPE OF PRESENT RESEARCH ON CLIMATIC VARIATION 57 the analysis of the surface heat and hydrologic balances (Gates, 1972; Holloway and Manabe, 1971; Kasahara and Washington, 1971; Manabe, 1969a, 1969b; Manabe et al., 1972; Somerville et al., 1974). As de- scribed more fully in Appendix B, simulations of average January climate have now been achieved by several Gcm’s. Although additional global Gcm’s are under development (Corby et al., 1972), only two at this writing have been integrated over time longer than one year (Manabe et al., 1972, 1974b; Mintz et al., 1972). Global atmospheric models have also recently been applied to the simulation of specific regional circulations, such as those in the tropics (Manabe et al., 1974). In such applications the model’s parameteriza- tion of processes in the surface boundary layer is of particular importance (Deardorff, 1972; Delsol et al., 1971; Sasamori, 1970). Con- siderable recent interest has also been shown in the simulation of strato- spheric climate with global Gcm’s (Kasahara and Sasamori, 1974; Kasahara et al., 1973; Mahlman and Manabe, 1972). An overview of global atmospheric (and oceanic) GCM’s is given in Appendix B; more detailed reviews of these and other models have recently been prepared (Robinson, 1971b; Schneider and Kellogg, 1973), while others are in preparation (GARP Joint Organizing Committee, 1974; Schneider and Dickinson, 1974). Statistical—Dynamical Models and Parameterization Studies Research on the development of dynamical climate models (in which the transfer properties of the large-scale eddies are statistically parameterized rather than resolved as in the GcmM’s) has accelerated in recent years (Willson, 1973). These models include those that address only the surface heat balance (Budyko, 1969; Faegre, 1972; Sellers, 1969, 1973), those that consider the time-dependent zonally averaged motion (MacCracken, 1972; MacCracken and Luther, 1973; Saltzman and Vernekar, 1971, 1972; Wiin-Nielsen, 1972; Williams and Davies, 1965), and those in which the statistical eddy fluxes are represented in terms of the large-scale motions themselves (Dwyer and Petersen, 1973; Kurihara, 1970, 1973). A key problem in such models is the correct parameterization of the heat and momentum transports by the large-scale eddies. While a completely adequate formulation has not yet been achieved, research is continuing by a variety of methods (Clapp, 1970; Gavrilin and Monin, 1970; Green, 1970; Saltzman, 1973; Smith, 1973; Stone, 1973). Because of the generally longer time scales involved in the oceanic general circulation, relatively less attention has been given to the corresponding formulation of statistical-dynamical

58 UNDERSTANDING CLIMATIC CHANGE ocean models (Adem, 1970; Petukhov and Feygel’son, 1973; Pike, 1972). This problem, however, will assume greater importance with the increased development of coupled ocean—atmosphere systems re- viewed below. Oceanic General Circulation Models Although generally less advanced than their atmospheric counterparts Oceanic GCM’s have recently been developed to the point where suc- cessful simulations of the seasonal variations of ocean temperature and currents have been achieved in both idealized basins (Bryan, 1969; Bryan and Cox, 1968; Haney, 1974) and in selected ocean basins with realistic lateral geometry (Cox, 1970; Galt, 1973; Holland and Hirsch- man, 1972; Huang, 1973). The numerical simulation of the complete world ocean circulation has only recently been achieved with baroclinic models (Alexander, 1974; Cox, 1974; Takano et al., 1973); this shows significant improvement over earlier global simulations with homoge- neous wind-driven models (Bryan and Cox, 1972; Crowley, 1968). As noted earlier, such models have not yet been able to resolve the energetic oceanic mesoscale eddies, although a number of experimental calcula- tions to that end are under way. Recent studies have also shown the importance of improving the models’ treatment of the oceanic surface mixed layer (Bathen, 1972; Denman, 1973; Denman and Miyake, 1973) and sea ice (Maykut and Untersteiner, 1971) and of incorporating bottom topography (Holland, 1973; Rooth, 1972) and the abyssal water circulation (Kuo and Veronis, 1973). Coupled General Circulation Models Although preliminary numerical calculations with a model of the coupled atmosphere—ocean circulation were performed several years ago (Manabe and Bryan, 1969; Wetherald and Manabe, 1972), it is only recently that a truly globally coupled model has been achieved (Bryan et al., 1974; Manabe et al., 1974a). These calculations underscore the importance of the ocean’s participation in the processes of air—sea interaction and in the maintenance of large-scale climate. These and other such coupled models now under construction will lay the basis for the systematic exploration of the dynamics of the atmosphere—ocean system and its role in climatic variation. The necessary calibration and testing of coupled GcMm’s will require a broad data base and access to the fastest computers available.

SCOPE OF PRESENT RESEARCH ON CLIMATIC VARIATION 59 APPLICATIONS OF CLIMATE MODELS The uses of climate models extend across a wide range of applications, including the reconstruction of past climates and the projection of future climates. Here the more recent use of models for such studies is briefly reviewed, as distinguished from the research on model design and calibration reviewed above. Simulation of Past Climates By assembling the boundary conditions appropriate to selected periods in the past, numerical models may be applied to the simulation of paleoclimates. The climate of the last ice age has recently received increased attention, both through the application of parameterized and empirical models (Alyea, 1972; Lamb and Woodroffe, 1970; Mac- Cracken, 1968) and through the use of atmospheric Gcm’s (Kraus, 1973; Williams et al., 1973). In the latter case, the specification of the distribution of glacial ice and sea-surface temperature represents a strong thermal control over the simulated climate. In order to provide realistic information on the near-equilibrium ice-age climatic state, these conditions should be constructed on the basis of the appropriate proxy climatic records, while other portions of this same paleoclimatic data base serve as verification. An initial effort of this sort is now under way as part of the CLIMAP program. At the present time, the simulation of the time-dependent evolution of past climates over thousands of years can only be achieved with the more highly parameterized models; the design and calibration of such models of the air—sea—ice system are largely tasks for the future. Climate Change Experiments and Sensitivity Studies Numerical climate models also permit the examination of the climatic consequences of a wide variety of possible changes in the physical sys- tem and its boundary conditions; such studies, in fact, are a primary motivation for the development of the climatic models themselves. As previously noted, a number of experiments on the effect of solar radia- tion changes have been performed with simplified models (Budyko, 1969; Manabe and Wetherald, 1967; Schneider and Gal-Chen, 1973; Sellers, 1969, 1973), and further studies of this kind with global models are under way. A number of recent experiments have been made with atmospheric GCM’s on the effects of prescribed sea-surface temperature anomalies on the large-scale atmospheric circulation (Houghton et al.,

60 UNDERSTANDING CLIMATIC CHANGE 1973; Rowntree, 1972; Spar, 1973a, 1973b), while others have been concerned with the climatic effects of thermal pollution (Washington, 1972) and of sea ice (Fletcher 1972). Although these experiments indicate that the models display a re- sponse over several months’ time to small changes in the components of the surface heat balance, their longer-term climatic response is not known. Such experiments serve to emphasize the need for extended model integrations, preferably with coupled models, and underscore the importance of determining the models’ sensitivity and the conse- quent noise levels in model-generated climatic statistics. The reduction of this climatic noise has an important bearing on the determination of the significance of climatic variations (Chervin eft al., 1974; Gates, 1974; Gilman et al., 1963; Leith, 1973). This question is also closely related to the problem of long-range or climatic prediction (Brier, 1968; Kukla et al., 1972; Lamb, 1973a). Studies of the Mutual Impacts of Climate and Man Although the influence of man’s activities on the local climate has long been recognized, renewed attention has been given in recent years to the possibility that man’s increasingly extensive alteration of the environment may have an impact on the large-scale climate as well (Sawyer, 1971). Here the more recent of such studies are briefly re- viewed, along with studies of the parallel problem of climate’s impact on man’s activities themselves. Aside from the numerical simulations of anthropogenic climatic effects noted earlier, there have been a number of recent studies of the climatic consequences of atmospheric pollution (Bryson and Wend- land, 1970; Mitchell, 1970, 1973a, 1973b; Newell, 1971; Yamamoto and Tanaka, 1972) and of the possible effects of man’s interference with the surface heat balance, primarily through changes of the surface land character (Atwater, 1972; Budyko, 1972a; Landsberg, 1970; Sawyer, 1971). Aside from local climatic effects, such as those due to urbanization, these studies have not yet established the existence of a large-scale anthropogenic climatic impact (Machata, 1973). Like their numerical simulation counterparts, such studies are made more diffi- cult by the high levels of natural climatic variability and by the lack of adequate observational data. A longer-range question receiving increased attention is the problem of disposing of the waste heat that accompanies man’s production and consumption of energy. When projected into the next century, this effect poses potentially serious climatic consequences and may prove

SCOPE OF PRESENT RESEARCH ON CLIMATIC VARIATION 61 to be a limiting factor in the determination of acceptable levels of energy use (Haefele, 1973; Lovins, 1974). These and other aspects of man’s impact on the climate have been considered extensively in the SCEP and SMIC reports (Wilson, 1970, 1971). Recent attention has also focused on the effects of climatic varia- tions on man’s economic and social welfare. From a general awareness of these effects (Budyko, 1971; Johnson and Smith, 1965; Maunder, 1970) research has turned to the representation of climatic anomalies in terms of the associated agricultural and commercial impacts (Pittock, 1972) and to the development of climatic impact indices (Baier, 1973). Further studies are necessary in order to develop comprehensive climatic impact simulation models, with both diagnostic and predictive capability.

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he increasing realization that man’s activities may be changing the climate, and mounting evidence that the earth’s climates have undergone a long series of complex natural changes in the past, have brought new interest and concern to the problem of climatic variation. The importance of the problem has also been underscored by new recognition of the continuing vulnerability of man’s economic and social structure to climatic variations. Our response to these concerns is the proposal of a major new program of research designed to increase our understanding of climatic change and to lay the foundation for its prediction.

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