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APPENDIX A SURVEY OF PAST CLIMATES INTRODUCTION The earthâs climates have always been changing, and the magnitude of these changes has varied from place to place and from time to time. In some places the yearly changes are so small as to be of minor inter- est, while in others the changes can be catastrophic, as when the monsoon fails or unseasonable rain delays the planting and harvesting of basic crops. On a longer time scale, certain decades have striking and anomalous characteristics, such as the severe droughts that affected the American Midwest during the 1870âs, 1890âs, and 1930âs and the high temperatures recorded globally during the 1940âs. And on still longer time scales, the climatic regimes that dominated certain centuries brought significant changes in the global patterns of temperature, rain- fall, and snow accumulation. For example, northern hemisphere winter temperatures from the midfifteenth to the midnineteenth centuries were significantly lower than they are today. The late nineteenth century represented a period of transition between this cold intervalâsometimes known as the Little Ice Ageâand the thermal maximum of the 1940âs. Some idea of the magnitude of the climatic changes that characterized the Little Ice Age can be gained from a study of proxy or natural records of climate, such as those of alpine glaciers. As shown in Figure A.1, as late as the midnineteenth century the termini of these glaciers were still advanced well beyond their present limits. The practical as well as the purely scientific value of understanding 127
128 UNDERSTANDING CLIMATIC CHANGE fe me ae he FIGURE A.1 The Argentiére glacier in the French Alps. (a) An etching made about 1850, showing the extent of the glacier during the waning phase of the Little Ice Age. (b) Photo- graph of the same view taken in 1966. [From LeRoy Ladurie (1971).]
APPENDIX A 129 the processes that bring about climatic change is self-evident. Only by understanding the system can we hope to comprehend its past and to predict its future course. This objective can be achieved only by study- ing the workings of the global climate machine over a time span ade- quate to record a representative range of conditions in natureâs own laboratory, and for this the record of past climates is indispensable. From the evidence discussed below and summarized in Figure A.2 we conclude that a satisfactory perspective of the history of climate can be achieved only by the analysis of observations spanning the entire time range of climatic variation, say, from 10-' to 10° years. Near the short end of this range there is a rich instrumental record to collate and analyze, although as discussed elsewhere in this report, awkward gaps exist in our knowledge of many parts of the airâ-seaâice system during even the past hundred years. As the time scale of observations is lengthened to include earlier centuries, the direct instrumental record becomes less and less adequate. A continuous time series of observa- tions as far back as the seventeenth century is available for only one area. For earlier times the instrumental record is blank, and indirect means must be found to reconstruct the history of climate. The science of paleoclimatology is concerned with the earthâs past climates, and that branch which seeks to map the reconstructed climates may be referred to as paleoclimatography. So defined, the science of paleoclimatology does far more than satisfy manâs natural curiosity about the past; it provides the only source of direct evidence on pro- cesses that change global climate on time scales longer than a century. When calibrated and assembled into global arrays, these data will be essential in the reconstruction of paleoclimates with numerical models. Nature of Paleoclimatic Evidence The subject of ancient climates may conveniently be approached in terms of the nature of the climatic record, whether from human (historical) recordings or from proxy or natural climatic indicators. It is therefore convenient to identify historical climatic data and proxy climatic data as sources of paleoclimatic evidence. Prior to the period of instrumental record, historical climatic data are found in books, manuscripts, logs, and other documentary sources and provide valuable (although fragmentary) climatic evidence before the advent of routine meteorological observations. Lamb (1969) has pioneered the collection of such data and has charted the main course of climate over Western Europe during the past 1000 years [Figure A.2(b)]. Where the historical or manuscript record overlaps the instru-
130 UNDERSTANDING CLIMATIC CHANGE AIR TEMPERATURE coro WARM coLo WwaRM 1960 woof OS - LEGEND a -1700F J 1. THERMAL MAXIMUM OF 1940s ° 5 1 9 19207 < 1500+ - 44 2. LITTLE ICE AGE q â Y B80 q '300F 1 3. YOUNGER DRYAS COLD INTERVAL us r 4 Op ag 7 OPT EASTERN 4. PRESENT INTERGLACIAL (HOLOCENE) are 900 5. LAST PREVIOUS INTERGLACIAL (E EMIAN) (a) THE LAST 10* YEARS se (b) THE- LAST 103 years ©. EARL TER F LEISTOCENE INTERGLACIALS MID-LATITUDE AIR TEMPERATURE GLOBAL ICE VOLUME COLO WARM COLO WARM MAX MIN. if oF 4 <q â AL wâ 0 0.2F & lo âo £50 2 PY w L ~N wT u 75 a aâ vn {co o W 0.4 + 220+ â o a nN z 100 Sos 5 o o 9 257 © 125 50.6} - ~- 3 307 150! = 0.7+ }ââââ________ ~10°C -ââ____â+ ~10 °C o.et (c) THE LAST 104 YEARS (d) THE LAST 10° YEARS 0.9 ~52x10'* m?® (e) THE LAST 10° YEARS FIGURE A.2 Generalized trends in global climate: the past million years. (a) Changes in the five-year average surface temperatures over the region 0-80 °N during the last 100 years (Mitchell, 1963). (b) Winter severity Index for eastern Europe during the last 1000 years (Lamb, 1969). (c) Generalized midiatitude northern hemisphere air-temperature trends during the last 15,000 years, based on changes in tree lines (LaMarche, 1974), marginal fluctuations in alpine and continental giaciers (Denton and Karién, 1973), and shifts in vegetation patterns recorded in pollen spectra (van der Hammen et al., 1971). (d) Gen- eralized northern hemisphere air-temperature trends during the last 100,000 years, based on midlatitude sea-surface temperature and pollen records and on worldwide sea-level records (see Figure A.13). (e) Fluctuations in global ice-volume during the last 1,000,000 years as recorded by changes in isotopic composition of fossil plankton In deep-sea core V28â-238 (Shackleton and Opdyke, 1973). See legend for identification of symbols (1) through (6).
APPENDIX A 131 mental record, the climatic reconstructions may be confirmed and calibrated by the latter. In contrast, the proxy record of climate makes use of various natural recording systems to carry the record of climate back into the past. Records from well-dated tree rings, annually layered (or varved) lake sediments, and ice cores resemble the historical data in that values can be associated with individual years and may be calibrated with modern data to extend the climatic record for many centuries, and in certain favored sites for as long as 8000 to 10,000 years. Other record- _ ing systems, such as the pollen concentration in lake sediments and fossil organisms and oxygen isotopes in ocean sediments, have less resolution but may provide continuous records extending over many tens of thousands of years. These and other characteristics of proxy climatic data sources are summarized in Table A.1. In general, the older geological records provide only fragmentary and generally qualitative information but constitute our only records extending back many millions of years. For the past one million years, however, and especially for the past 100,000 years, the record is rela- tively continuous and can be made to yield quantitative estimates of the values of a number of significant climatic parameters. These in- clude the total volume of glacial ice (and its inverse, the sea-level), the air temperature and precipitation over land, the sea-surface temperature and salinity for much of the world ocean, and the general trend of air temperature over the polar ice caps. Like sensing systems made by man, each natural paleoclimatic indicator must be calibrated, and each has distinctive performance characteristics that must be understood if the data are to be interpreted correctly. In discussing these sources it is useful to distinguish between those paleoclimatic indicators that are more or less continuous re- corders of climate, such as tree rings and varves, and those whose records are episodic, such as mountain glaciers. We should also con- sider the minimum attainable sampling interval that is characteristic of a particular paleoclimatic indicator (see Table A.1). Thus, tree rings, varves, and some ice cores can be sampled at intervals of one year, pollen or other sedimentary fossil samples only rarely represent less than about 100 years, and many geological series are sampled over intervals representing a thousand years or more. These figures reflect differences in the resolving power of each proxy indicator. Climate- induced changes in a plant community as reflected in pollen concentra- tions, for example, are relatively slow; the high-frequency information is lost, but low-frequency changes are preserved. In contrast, tree-ring
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134 UNDERSTANDING CLIMATIC CHANGE records and isotopic records in ice cores respond yearly and even seasonally in favored sites. Each proxy record also has a characteristic chronologic and geo- graphic range over which it can be used effectively. Tree-ring records go back several thousand years at a number of widely distributed con- tinental sites, pollen records have the potential of providing synoptic coverage over the continents for the past 12,000 years or so, and a nearly complete record of the fluctuating margins of the continental ice sheets is available for about the last 40,000 years. Planktonic and benthic fossils from deep-sea cores can in principle provide nearly global coverage of the ocean going back tens of millions of years, al- though sampling difficulties have thus far limited our access to sediments deposited during the last several hundred thousand years. Although instrumental records best provide the framework neces- sary for the quantitative understanding of the physical mechanisms of climate and climatic variation with the aid of dynamical models, the increasingly quantitative and synoptic nature of paleoclimatic data will add a much-needed perspective. As discussed elsewhere in this report, it is therefore important that the historical and proxy records of past climate be systematically assembled and analyzed, in order to provide the data necessary for a satisfactory description of the earthâs climates. Instrumental and Historical Methods of Climate Reconstruction Over the past three centuries, the development of meteorological instruments and appropriate âplatformsâ for sensing the state of the atmosphereâhydrosphereâcryosphere system has produced an important storehouse of quantitative information pertaining to the earthâs climates. Time series of these records show that climate undergoes considerable variation from year to year, decade to decade, and century to century. From a practical viewpoint, much of this information mirrors the economically more important climatic variations, as found, for example, in the changes of crop and animal production, the patterns of natural flora and fauna, and the variations of the levels of lakes and streams and the extent of ice. Generally the term âclimateâ is understood to describe in some fashion the ââaverageâ of such variations. As discussed in Chapter 3, a complete description of a climatic state would also include the variance and extremes of atmospheric behavior, as well as the values of all parameters and boundary conditions regarded as ex- ternal to the climatic system. In discussing the reconstruction of climates from instrumental data, several characteristics of past and present observational systems should
APPENDIX A 135 be considered. First, instrumental observations have been obtained for the most part for the purpose of describing and forecasting the weather. Hence, although extensive records of such weather elements as tempera- ture, precipitation, cloudiness, wind, and observations are available, they are inadequate for many climatic purposes. There exist few direct measurements related to the thermal forcing functions of the at- mosphereâhydrosphereâcryosphere system, such as the solar constant; the radiation, heat, and moisture budgets over land and ocean surfaces; the vegetative cover; the distribution of snow and ice; the thermal structure of the oceanic surface layer; and atmospheric composition and turbidity. Second, observational records may be expected to contain errors due to changes in instrument design and calibration and to changes in instrument exposure and location. There is therefore a need to establish and maintain conventional observations at reference climatological stations and a need to identify, insofar as possible, âbenchmarkâ records of past climate. Such observations are needed to supplement the climatic monitoring program described elsewhere (see Chapter 6). Third, the time interval over which portions of the climatic system need to be described are very different. If, for example, the fluctuations in the volume and extent of the polar ice caps are to be studied, a time interval of order 100,000 years (the maximum residence time of water in the ice caps) is required. Or if atmospheric interaction with the deep oceans is to be considered, then a time interval of the order 500 years (the residence time of bottom water) is required. It is there- fore apparent that the period of instrumental records covering the past century or two is long enough only to have sampled a portion of such climatic responses, and that our information on older climates must come from historical sources and from the various natural (proxy) indicators of climate described earlier. Although such records will al- ways be fragmentary, we should recognize their unique value in describ- ing the past behavior of the earthâs climatic system. For practical reasons, it has been convenient to compute climatic statistics over relatively short intervals of time, such as 10, 20, or 30 years, and to designate the 30-year statistics as climatic ânormals.â It is important to note, however, that the most widely accepted climatic ânormalsâ (for the period 1931-1960) represent one of the most ab- normal 30-year periods in the last thousand years (Bryson and Hare, 1973). As noted elsewhere in this report, the entire last 10,000 years are themselves also abnormal in the sense that such (interglacial) climates are typical of only about one tenth of the climatic record of the last million years. While continuous observations of atmospheric pressure, temperature,
136 UNDERSTANDING CLIMATIC CHANGE and precipitation are available at a few locations from the late seven- teenth century, such as the record of temperature in Central England assembled by Manley (1959), it is only since the early part of the eighteenth century that the spatial coverage of observing stations has permitted the mapping of climatic variables on even a limited regional scale. These and other scattered early observations of rainfall, wind direction, and sea-surface temperature have been summarized by Lamb (1969). Only since about 1850 are reliable decadal averages of surface pressure available for most of Europe, and only since about 1900 are there reliable analyses for the midlatitudes of the northern hemisphere, as shown in Figure A.3. And only since about 1950 does the surface observational network begin to approach adequate coverage over the continents; large portions of the oceans, particularly in the southern hemisphere, remain inadequately observed. For the climate of the free atmosphere, the international radiosonde network permits reliable analyses for the midlatitudes of the northern hemisphere only since the 1950âs, and less than adequate coverage exists over the rest of the globe. Beginning in the 1960âs, routine ob- servations from satellite platforms have begun to make possible global observations of a number of climatic variables, such as cloudiness, the planetary albedo, and the planetary heat budget. Yet many important quantities, such as the heat and moisture budgets at the earthâs surface and the thermal structure and motions of the oceanic surface layer, remain largely unobserved on even a local scale. Biological and Geological Methods of Climate Reconstruction During the first three decades of the nineteenth century, Venetz in Switzerland and Esmark in Norway inferred the existence of a pre- historic ice age from the study of vegetation-covered moraines and other glacial features in the lower reaches of mountain valleys. After a century of effort, the literature of paleoclimatology has become so diverse, and so burdened by stratigraphic terminology, that it is useful to provide a summary of paleoclimatic techniques. The quantitative description of past climates as determined by bio- logical and geological records requires the development of paleoclimatic monitoring techniques and the construction of time scales by suitable chronometric or dating methods. In general, the second of these prob- lems is the more difficult. Beyond the range of "*C dating (the past 40,000 years), it is only since about 1970 that the main chronology of the climate of the past 100,000 years has become clear; and only since 1973 that the main features of the chronology of the past million years have been estab-
APPENDIX A 137 FIGURE A.3 Growth of the network of surface pressure observa- tions and of the area that can be covered by reliable 10-year average isobars (Lamb, 1969). (a) 1750-1759, (b) 1850-1859, (c) 1950-1959.
138 UNDERSTANDING CLIMATIC CHANGE lished. Key discoveries in these time ranges have been in the sea-level records of oceanic islands and in the sedimentary records of deep-sea cores. In preparing this survey, the chronology of these records has been used as a framework into which the data from more fragmentary or poorly dated records have been fitted. Monitoring Techniques The problem of developing a paleoclimatic monitoring techniqueâ or finding something meaningful to plotâmay be broken down into three subproblems. A natural climatic record must be (a) identified, (b) calibrated, and (c) obtained from a stable recording medium. Identification of Natural Climatic Records A number of different monitoring techniques that can provide data for paleoclimatic inference are summarized in Table A.1 and are based on observations of fossil pollen, ancient soil types, lake deposits, marine shore lines, deep-sea sediments, tree rings, and ice sheets and mountain glaciers. The tech- niques that are emphasized here are those that in general yield more or less continuous time series. Other types of proxy data are also useful in the reconstruction of climatic history (see, for example, Flint, 1971, or Washburn, 1973). Calibration of Paleoclimatic Records Many proxy records must be calibrated to provide an estimate of the climatic parameter of interest. The elevation of an ancient coral reef, for example, is a record of a previous sea level; but before it can be used for paleoclimatic purposes the effect of local crustal uplift or subsidence must be removed (Bloom, 1971; Matthews, 1973; Walcott, 1972). Another example may be cited from paleontology, where the taxo- nomic composition of fossil assemblages and the width of tree rings are known to reflect the joint influence of several ecological and en- vironmental factors of climatic interest. Here appropriate statistical techniques are used to define indices that give estimates of the individual paleoclimatic parameters, such as air temperature, rainfall, or sea- surface temperature and salinity. In the case of tree rings, although each tree responds only to the local temperature, moisture, and sun- light, for example, by averaging over many sites, the treesâ response may be related to the large-scale distribution of rainfall and surface tempera- ture. In this way a statistical relationship may be established with a variety of parameters, even though they may not be direct causes of tree growth. When such tree-ring data are carefully dated they can
APPENDIX A 139 thus provide estimates of the past regional variations of climatic elements such as precipitation, temperature, pressure, drought, and stream flow (Fritts et al., 1971). These methods yield what are called transfer func- tions, which serve to transform one set of time-varying signals to another set that represents the desired paleoclimatic estimates. In addition to their application to tree-ring data, multivariate statistical-analysis techniques have been successfully applied to marine fossil data (Imbrie and Kipp, 1971; Imbrie, 1972; Imbrie et al., 1973) and to fossil pollen data (Bryson et al., 1970; Webb and Bryson, 1972). Typical results indicate, for example, that average winter sea-surface temperatures 18,000 years ago in the Caribbean were about 3°C lower than today, while those in midlatitudes of the North Atlantic were about 10°C below present levels. The oxygen isotope ratio '*O/1%O as it is preserved in different ma- terials is used in three separate paleoclimatic monitoring techniques. Although the results are interpreted differently, in each technique the ratio is measured as the departure 5'*O from a standard, with positive values indicating an excess of the heavy isotope. One technique ex- amines the ratio in polar ice caps, where the values of 8'8O are generally on the order of 30 parts per thousand lower than in the oceanic reservoir, because of the precipitation and isotopic enrichment that accompanies the transport of water vapor into high latitudes. As shown by Dans- gaard (1954) and by Dansgaard et al. (1971) the value of 5?®O in each accumulating layer of ice is closely related to the temperature at which precipitation occurs over the ice. Although complicating effects make it impossible to convert the 88O curve into an absolute measure of air temperature, the isotopic time series are extraordinarily detailed. Another isotopic technique records 5!*°O in the carbonate skeletons of planktonic marine fossils (Emiliani, 1955, 1968). Here the ratio is determined by the isotopic ratio and temperature in the near-surface water in which the organisms live. Work by Shackleton and Opdyke (1973) demonstrates that the observed ratio is predominantly influ- enced by the isotopic ratio in the seawater. Hence the isotopic curve reflects primarily the changing volume of polar ice, which, upon melting, releases isotopically light water into the ocean. A third technique measures the isotopic ratio in benthic fossils whose skeletons reflect conditions prevailing in bottom waters. By making the assumption that the temperature of bottom water underwent little change over the past million years, the difference between the isotopic ratio observed in benthic and planktonic fossils can be used to estimate changes in surface-water temperatures. Initial application of this tech- nique (Shackleton and Opdyke, 1973) provides an independent con-
140 UNDERSTANDING CLIMATIC CHANGE firmation of the previously cited estimate of glacial-age Caribbean temperatures obtained by paleontological techniques. Over time spans on the order of tens of millions of years, measurements of 5'*O in benthic fossils offer a means of tracing changes in bottom water in which the effects of changing polar temperatures and ice volumes are combined (Douglas and Savin, 1973). Evaluation of the Recording Medium All paleoclimatic techniques require that ambient values of a climatic parameter be preserved within individual layers of a slowly accumulating natural deposit. Such de- posits include sediments left by melting glaciers on land; sediments accumulating in peat bogs, lakes, and on the ocean bottom; soil layers; layers accumulating in polar ice caps; and the annual layers of wood formed in growing trees. Ideally, a recording site selected for paleo- climatic work should yield long, continuous, and evenly spaced time series. The degree to which these qualities are realized varies from site to site, so that distortions and nonuniformities in each record must be identified and removed. The stratigraphic techniques by which this screening is accomplished will not be discussed here, although the reader should be aware that (with the exception of tree rings) some degree of chronological distortion will occur in all paleoclimatic curves where chronometric control is lacking. To enable the reader to form his own judgments as to the chronology of past climatic changes, most of the paleoclimatic curves given in this report show explicitly the time control points between which the data are spaced in proportion to their relative position in the original sedi- mentary record. This procedure assumes that accumulation was con- stant between the time controls, which is a reasonable assumption in favorable environments. In other cases this assumption introduces a distortion in the signal and a consequent uncertainty in the timing of the inferred climatic variations. Each of the recording media used in paleoclimatography has char- acteristic limitations and advantages. As summarized in Table A.1, the reconstruction of past climates requires evidence from a variety of techniques, each yielding time series of different lengths and sampling intervals and reflecting variations in different regions. The tree-ring record, for example, provides evenly spaced and continuous annual records, but only for the past few thousand years. The ice-margin record of both valley and continental glaciers is discontinuous, especially prior to about 20,000 years ago, because each major glacial advance tends to obliterate (or at least to conceal) the earlier evidence. Records of lake levels and sea levels are also discontinuous. The former rarely
APPENDIX A 141 extend back more than 50,000 years, although the latter extend back several hundred thousand years. Soil sequences display great variability in sedimentation rate but provide continuous climatic information for sites on the continents where other records are not available (or are discontinuous); in favored sites, the soil record extends back about a million years. Pollen records are usually continuous but are rarely longer than 12,000 years. Deep-sea cores provide material for the study of fossils, oxygen isotopes, and sedimentary chemistry. These records are relatively continuous over the past several hundred thousand years and are distributed over large parts of the world ocean. Their relatively uniform but low deposition rates, however, generally limit the chronological detail obtainable. Cores taken in the continental ice sheets provide a detailed and generally continuous record for many thousands of years, although their interpretation is handicapped by the lack of fully adequate models of the ice flow with its characteristic velocityâtemperature feedback. Chronometric Techniques The problem of constructing a paleoclimatic chronology has been ap- proached by four direct methods and one indirect method. Dendrochronology The most accurate direct dating is achieved in tree-ring analysis, in which many records with overlapping sets of rings are matched. With sufficient samples, virtual certainty in the dates of each annual layer may be obtained, and a year-by-year chronology can be established for periods covered by the growth records of both living and fossil trees. Such records are especially valuable for studying variations of climate during the last few hundred years and can be extended to many of the land regions of the world. Analysis of Annually Layered Sediments In favored locations, lakes with annually layered bottom sediments provide nearly the same time control as do tree rings. Some ice cores and certain marine sediment cores from regions of high deposition rates also contain distinct annual layers. These data, along with tree rings and historical records, are the only source of information on the high-frequency portion of the spectrum of climatic variation. Radiocarbon Dating The advent of the ?*C method in the early 1950's was a major breakthrough in paleoclimatography, for it made possible the development of a reasonably accurate absolute chronology of the
142 UNDERSTANDING CLIMATIC CHANGE past 40,000 years in widely distributed regions. Prior knowledge was essentially limited to dated tree-ring sequences (for the past several thousand years) and to varve-counted sequences in Scandinavia (ex- tending back to about 12,000 years). The 1*C method has an accuracy of about +5 percent of the age being determined; that is, material 10,000 years old could be dated within the range 9500-10,500 years. The calibration of âC ages against those determined from dendro- chronology gives insight into the variations of atmospheric *C produc- tion rates over the past 7000 years (Suess, 1970). Decay of Long-Lived Radioactivities | These methods employ daughter products of uranium decay or the production of *°Ar through potassium decay. Used under favorable circumstances, one of the uranium methods (the decay of **°Th) can provide approximate average sedimentation rates in deep-sea cores. The other method (the growth of ?°°Th) can be used successfully on fossil corals to provide discrete dates for shore- line features recording ancient sea levels. Together, these techniques have provided a reasonably satisfactory chronology of the past 200,000 years with a dating accuracy of about +10 percent. Our chronology for older climatic records is based on the well-known K/Ar technique, applied to terrestrial lava flows and ash beds. This technique has pro- vided, for example, the important dates for paleomagnetic reversal boundaries. Stratigraphic Correlation with Dated Sequences Much of the absolute chronology of climatic sequences is supplied by an indirect method, namely, the stratigraphic correlation of specific levels in an undated sequence with dated sequences from another location. For example, a particular glacial moraine that lacks material for âC dating may be identified with another formed at the same time that has datable ma- terial. Such correlation by direct physical means is limited to relatively small regions, however, and stratigraphic correlation techniques must be used. Three such methods form the backbone of the chronology of paleoclimate: biostratigraphy, isotope stratigraphy, and paleomag- netic stratigraphy. The techniques of biostratigraphy use the levels of extinction or origin of selected species as the basis for correlation. This method has enabled Berggren (1972), for example, to devise a time scale of the past 65,000,000 years that is widely used as a basis for historical inter- pretation. Isotope stratigraphy, applicable only to the marine realm, makes use of the fact that the record of oxygen isotope variationsâ
APPENDIX A 143 which reflects chiefly the global ice volumeâhas distinctive char- acteristics that permit the correlation of previously undated sequences. The application of paleomagnetic correlation techniques has revolu- tionized our approach to the climatic history of the past several million years. Their importance stems from the fact that the principal magnetic reversal boundaries, which have occurred irregularly about every 400,000 years, are recorded in both marine and continental sedimentary sequences. Regularities in Climatic Series On the assumption that climatic changes are more than just random fluctuations, paleoclimatologists have long sought evidence of regu- larities in proxy records of the earthâs climatic history. Many have found what they believe to be firm evidence of order and refer to the chronological patterns as âcycles.â Although the number of records is limited, and hard statistical evidence is sometimes lacking, it is never- theless convenient to describe some of the larger climatic changes in terms of quasi-periodic fluctuations or cycles with specified mean wave- lengths or periods, in the sense that they describe the apparent repetitive tendency of certain sequences of climatic events. For example, many aspects of the global ice fluctuations during the last 700,000 years may be summarized in terms of a 100,000-year cycle [see Figure A.2(e)]. Each such period is marked by a gradual transition from a relatively ice-free climate (or interglacial) to a short, intense glacial maxima and followed by an abrupt return to ice-free climate. No two such cycles are the same in detail, however, and should not be construed as indicating strict periodicities in climate. Some paleoclimatic cycles may be periodic, or at least quasi-periodic, and rest on evidence that is exclusively or mainly chronological. The best example is the approximate 100,000-year cycle found from the spectral analysis of time series, such as that shown in Figure A.4. For the 100,000-year cycle, as well as some of the higher-frequency fluctua- tions that modify it, there is circumstantial evidence to suggest that these have in some way been induced by secular variations of the earthâs orbital parameters, which are known to alter the latitudinal pat- tern of the seasonal and annual solar radiation received at the top of the atmosphere. For the 2500-year (and shorter) fluctuations suggested by some proxy data series, the causal mechanism is unknown. With the possible exception of the approximately 100,000-year quasi- periodic fluctuation referred to above, the quasi-biennial oscillation (of
144 UNDERSTANDING CLIMATIC CHANGE PERIOD IN THOUSANDS OF YEARS co §6100 50 33 25 20 17 #14 #12 «11 + «10 18 âT T.6OCOT T T T T T T 1.6 â 1.4- 7 1.2 7 oc > - 5 1.0 a. uJ 2_ < 08- â uJ oc 0.6r â 0.4 â 0.2 - = Ot i j { i 1 4 0 1 2 3 4 5 6 7 8 9 10 CYCLES PER 100,000 YEARS FIGURE A.4 Power spectrum of climatic fluctuations during the last 600,000 years according to Imbrie and Shackleton (1974). The data analyzed are time-series observations of 5°O in fossil plankton in the upper portion of a deep-sea core in the equatorial! Pacific, inter- polated at intervals of 2500 years (Shackleton and Opdyke, 1973). This ratio reflects fluctuations in global ice-volume.
APPENDIX A 145 2â3 year period) is the only quasi-periodic oscillation whose statistical significance has been clearly demonstrated. This is not to say that other such fluctuations in climate are absent but rather that much further analysis of proxy records is required. A question of equal importance is the shape of the continuum variance spectrum of climatic fluctua- tions. A uniform distribution of variance as a function of frequency (or âwhite noiseâ) would imply a lack of predictability in the statistical sense or a lack of âmemoryâ of prior climatic states. A âred-noiseâ spectrum, on the other hand, in which the variance decreases with in- creasing frequency, implies some predictability in the sense that suc- cessive climatic states are correlated. The existence of nonzero auto- correlations in such a spectrum implies that some portion of the climatic system retains a âmemoryâ of prior states. In view of the relatively short memory of the atmosphere, it seems likely that this is provided by the oceans on time scales of years to centuries and by the worldâs major ice sheets on longer times scales. An initial estimate of the variance spectrum of temperature has been made from the fluctuations on time scales from 1 to 10,000 years by Kutzbach and Bryson (1974) and is shown in Figure A.5. This spectrum has been constructed from a combination of calibrated botanical, chemi- cal, and historical records, along with instrumental records in the North Atlantic sector. As may be seen in Figure A.5(a), the variance spectral density increases with decreasing frequency (increasing period) over the entire frequency domain but is most pronounced for periods longer than about 30 years. In Figure A.5(b), the spectrum of the same time series is shown with frequency on a logarithmic scale and the ordinate as spectral density (V) times frequency (f), so that equal areas repre- sent equal variance. Again, for periods longer than about 30-50 years, the observed temperature spectrum is seen to depart significantly from the white-noise continuum associated with the high-frequency portion of the spectrum. The determination of the character of the variance spec- trum of the various climatic elements remains largely a task for the future. We will use the term âcycleâ in the following paragraphs to designate such quasi-periodic sequences of climatic events, since there appears to be no other word or phrase that conveys the concept of a series of generally similar events spaced at reasonably regular intervals in time. Although our knowledge of the record of past climates has improved greatly during the last decade, a much broader paleoclimatic data base is clearly required. Only then can adequate spectral analyses be per- formed and the spatial and temporal structure of paleoclimatic variations firmly established.
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148 UNDERSTANDING CLIMATIC CHANGE CHRONOLOGY OF GLOBAL CLIMATE Period of Instrumental Observations A variety of meteorological indices have been used to characterize the climate and its temporal variations during the past century or more of extensive observations. Global- or hemisphere-averaged indices such as the surface temperature index shown in Figure A.6 are often used for this purpose. This index clearly suggests a worldwide warming begin- ning in the 1880âs, followed by a cooling since the 1940âs. The warming may be recognized as the last part of a complex but recognizable trend that has persisted since the end of the seventeenth century [see Figure A.2(b)]. âThe geographic patterns of temperature change during these overall warming and cooling epochs show considerable variability, with the largest changes concentrated in the polar regions of the northern hemisphere. Mitchell (1963) has shown that the pattern of temperature change during recent decades is consistent with concomitant changes in the large-scale atmospheric circulation as reflected in sea-level pressure. Less attention has been given to the more complex relationships be- tween circulation variation and changing precipitation patterns, although Kraus (1955a, 1955b) and Lamb (1969) have considered this aspect of the problem. Lamb and Johnson (1959, 1961, 1966) and Lamb (1969) have made an extensive analysis of certain features of atmospheric circulation eS â QO 2 â= 0.4 4 © <= 0.2 4 QO uJ a 0 Eex -0.2 â\ â ud a =s -0.47- 7 - -0.6 i { i I l | i L A | rT | AL I A of i 1880 1900 1920 1940 1960 YEAR FIGURE A.6 Recorded changes of annual mean temperature of the northern hemisphere as given by Budyko (1969) and as updated after 1959 by H. Asakura of the Japan Meteorological Agency (unpublished results).
APPENDIX A 149 based on the observed and historically reconstructed surface pressure maps for individual months since about 1750. They have extracted such indices as the strength of the zonal and meridional flow, the position and wavelength of troughâridge patterns, and the position and strength of subtropical pressure systems. The year-to-year and decade-to-decade changes in these indices reflect changing large-scale circulation pat- terns, which in turn are associated with changing patterns of tempera- ture and precipitation. From the instrumental era for the North Atlantic sector, the typical variability over 20- to 30-year intervals of the low- level westerlies is +1-2 m sec~', and that for the planetary-scale circulation features (such as large-scale troughs and ridges) is +1-â2 deg latitude and + 10-20 deg longitude. Although changes in the position, pattern, and intensity of the gen- eral circulation are interrelated, such empirical studies suggest that longitudinal shifts have the most significant effects on the climatological temperature and precipitation patterns, at least for middle and higher latitudes. Examples of such shifts are shown in Figure A.7. In tropical and subtropical latitudes, on the other hand, latitudinal shifts appear to be more closely related to regional climatic variations, as indicated by the data of Figure A.8. The Last 1000 Years To obtain an indication of the climate in the northern hemisphere for the last 1000 years, Lamb (1969) has compiled manuscript references on the character of European weather and has developed an index of winter severity, as shown in Figure A.2(b) and A.9. Although different longitudes show somewhat different results, the trends shown by this index (the excess number of unusually mild or unusually cold winter months over months of opposite character) for the period since about 1700 have been validated by comparison with thermometer records. Other portions of the record have been cross-checked with data on glacial fluctuations, oxygen isotope variations, and tree growth, so that the main characteristics of European climate during this period are reasonably well known. LaMarche (1974) has constructed temperature and moisture records from the ring-width variations in trees at high- altitude arid sites in California [see Figure A.9(c)]. Comparisons of his data with those from Europe shown in Figure A.9(d) indicate a degree of synchrony in the major fluctuations of temperature between the west coast of North America and Western Europe during the last 1000 years. The early part of the last millenium (about a.p. 1100 to 1400) is sometimes called the Middle Ages warm epoch but was evidently not as
150 UNDERSTANDING CLIMATIC CHANGE 1790-1829 ;â 1800-1839 |â 1810-1849 râ Trough Ridge Trough 1820-1859 |â 1830-1869 |[â 1840-1879 |â 1850-1889 /â 1860-1899 [â- 1870-1909 |â 1860-1919 }-â 1890-1929 |â 1900-1939 |â 1910-1949 -â 1920-1959 1 | 1 l 60°w 50 40 30 20 10 0 10 20°E a 1780-1819 [ 1800-1839 |â 1820-1859 |-â Trough Ridge 1840-1879 |â 1860-1899 [â 1880-1919 -â 1900-1939 [â 1920-1959 [- | | | L | | | | 70°wW 60 50 40 30 20 10 0 10°E FIGURE A.7_ Forty-year running means of the longitudes of the semipermanent surface pressure troughs and ridges In the North Atlantic (Lamb, 1969). (a) at 45 °N in January; (b) at 55 °N in July.
APPENDIX A 151 110. ; So OCiâiâ S a (a) ae sg 9- L Tt T T v ev T T T T T T 204 : °. wey e eee ° e :, a (b) 100- °. . fi ° ° ee 2 â e, = §0 T âT , YT oe | T Y 7 7 T T v 120- ee o *e, a 2â " (c) =f eo % ° *o 8%, ot Feces, ecoe ; a = 100- . 100 =SiCid1}Otiâ(â COCK (atié«s (ts«SO YEAR FIGURE A.8 Twenty-year running means of selected climatic indices (Winstanley, 1973). (a) Frequency (days per year) of westerly weather type over the British Isles (from Lamb, 1969); (b) winter-spring rainfall (mm) at 14 stations in North Africa and the Middle East; (c) summer monsoon rainfall (mm) at eight stations in the Sahel of North Africa and northwest India. warm as the first half of the twentieth century. The period from about 1430 to 1850 is commonly known as the Little Ice Age, and some records indicate that this period had cold maxima in the fifteenth and seventeenth centuries. From such evidence we infer that the atmospheric circulation may have been more meridional than at present and char- acterized in western Europe and western North America by short, wet summers and long, severe winters. During the Little Ice Age many glaciers in Alaska, Scandinavia, and the Alps advanced close to their maximum positions since the last major ice age thousands of years ago. A visual impression of these events in the French Alps was shown in Figure A.1. The expansion of the Arctic pack ice into North Atlantic waters caused the Norse colony in southwest Greenland to become isolated and perish; and in Iceland, grain that had grown for centuries could no longer survive.
152 UNDERSTANDING CLIMATIC CHANGE PARIS-LONDON = GREENLAND ICE CORE HISTORIC 80â (av00) WINTER SEVERITY PRESENT 8 8 8 g BEFORE YEARS TREE CHANGE IN RING WIDTH MEAN ANNUAL MM" TEMPERATURE 2 3 4 5 6 7 00 Q lO 15 oO FIGURE A.9 Climatic records of the 1900 past 1000 years. (a) The 50-year moving BEFORE PRESENT average of a relative index of winter severity compiled for each decade from 1700 documentary records in the region of Paris and London (Lamb, 1969). (b) A 8 1500 record of 5'8O values preserved in the ice core taken from Camp Century, Greenland (Dansgaard et al., 1971). (c) i YEAR Records of 20-year mean tree growth © 8 at the upper treeline of bristlecone YEARS pines, White Mountains, California (La- 3 ooâ ooâ ca 1100 Marche, 1974). At these sites tree growth is limited by temperature with low growth reflecting low temperature. = l 900 (d) The 50-year means of observed and estimated annual temperatures over central England (Lamb, 1966). The Last 5000 Years As indicated in Figure A.2(c), the period from 7000 to 5000 years ago was marked by temperatures warmer than those that prevail today [and is thus sometimes known as the hypsithermal interval (Flint, 1971 )]. The last 5000 years is characterized by generally declining temperatures and a trend toward more extensive mountain glaciation (but not ice sheets) in all parts of the world (Porter and Denton, 1967). Close
APPENDIX A 153 fo) © J TREE GROWTH WIOTH(mm) FLUCTUATIONS AT on Oo a Li UPPER TREELINE > = z m Oo 4 RING L 40.3 HOLOCENE EXPANSION ¢ GLACIER CONTRACTION | FLUCTUATIONS eee e terete cheat teat beesett cates ates (Denton& Karlén,1973) 3000 2000 1000 pc AD. 1000 1971 FIGURE A.10 Climatic records of the past 5000 years. (a) Average (100-year mean) ring widths of bristiecone pine at the upper treeline in the White Mountains of California (LaMarche, 1974). Positive growth departures indicate warm-season (AprilâOctober) temperatures above the long-term mean, with a total temperature range of about 4°F. (b) Records of the advance and retreat of Holocene Alaskan glaciers (Denton and Karlén, 1973). examination of the records of mountain glaciers, treelines, and tree rings suggests that this general cooling trend was itself punctuated in many parts of the world by cold intervals centered at about 5300, 2800, and 350 years ago, as shown in Figure A.10. Much further analysis of proxy climatic records during this period is needed, including the evidence available from historical sources. The Last 25,000 Years The climatic record of the last 25,000 years is largely concerned with the present interglacial interval (or Holocene) and the terminal phases of the last major glaciation [see Figure A.2(d)]. Although the maxi- mum ice extent occurred between about 22,000 and 14,000 years ago (see Figure A.11) the curves of ice accumulation and decline are not identical for the various ice sheets. The Laurentide ice sheet (which covered parts of eastern North America) and the Scandinavian ice sheet (which covered parts of northern Europe) reached their maximum extent between 22,000 and 18,000 years ago, while the Cordilleran ice sheet achieved its maximum only 14,000 years ago. The maximum areas of the northern hemisphere ice sheets during the past 25,000 years were about 90 percent of the maxima during the last million years of the Pleistocene (see Table A.2). Widespread deglaciation began rather abruptly about 14,000 years
154 UNDERSTANDING CLIMATIC CHANGE VARIATIONS IN FLUCTUATION IN THE MARGINS OF THREE CARIBBEAN NORTHERN HEMISPHERE ICE SHEETS PLANKTON (Core VI2-I22) (Erie Lobe of Lourentide Ice sheet) (Cordilleran ice Sheet in Froser - Puget Faunal Index Tw 2000 â 1690 _â*t090 590 9 o A I ad (Eastern Sector of Scandinavian ice Sheet $ 3 __% a naa 70 : x ) | a: +t 10 a 2 | 20 WwW 430 +4 a. a 2 LL wl. i +05 -05 -15 -25 1s00 1000 300 0 \d 4 1 i al a 2 A a A fA lS 80" (%0) 5050180000600 600° 400 = 200 0 Central km Hudson km Ohio Bay DISTANCE (km) FROM CENTER OF OUTFLOW @= c'* dates in Vi2-I22 (ice margin fluctuation chronology controlled by numerous C' data) FIGURE A.11 Climatic records of the past 40,000 years. (a) Fluctuations in Caribbean plankton (core V12â122) interpreted as a record of sea-surface temperature in °C. (b) Fluctuations in the Isotopic composition of Caribbean plankton interpreted as a record of changing giobal ice-volume. Both records are from Imbrie et al. (1973). Curves (c), (d), and (e) are time-distance plots of changes in the margins of three northern hemisphere ice sheets. Curve (c) is from Dreimanis and Karrow (1972), curves (d) and (e) are due to G. H. Denton, University of Maine (unpublished). The chronology of curves (a) and (b) is controlled by â¢âC dates shown by solid circles; the ice-margin curves are controlled by numerous 1âC dates. ago, and the waning phases of the continental ice sheets were char- acterized by substantial marginal fluctuations (Dreimanis and Karrow, 1972), as shown in Figure A.11. The Cordilleran ice sheet, which had just attained its maximum extent, melted rapidly and was gone by 10,000 years ago. The Scandinavian ice sheet lasted only slightly longer and retreated at the rate of about 1 km per year between about 10,000 and 9000 years ago. The climatic instability suggested by these fluctuations in the margins of the northern hemisphereâs major ice sheets is corroborated by the records from fossil pollen, deep-sea cores, ice cores, and sea-level variations, as shown in Figure A.12, and by lacustrine records in western North America and Africa. By 8500 years ago the ice conditions in Europe had reached essentially their present
APPENDIX A 155 TABLE A.2 Characteristics of Existing Ice Sheets and of the Maximum Quaternary Ice Cover * Area (10'? m?) Existing Glaciers Greenland 1.80 Spitsbergen + Iceland 0.07 Canadian Archipelago 0.15 North America 0.08 Europe + Asia 0.17 South America 0.03 Antarctica 12.59 TOTAL AREA 14.99 (3% of earthâs surface) TOTAL ICE VOLUMEâ 2.5 10° km? EQUIVALENT SEA-LEVEL CHANGE 70m Maximum Quaternary Glaciation Greenland 2.30 Spitsbergen + Iceland 0.44 Alaska 1.03 Cordillera 1.58 Laurentide 13.39 Scandinavia 6.67 Europe 0.09 Asia 3.95 South America 0.87 Antarctica 13.81 Other 0.04 TOTAL AREA 44.17 (9% of earthâs surface) TOTAL ICE VOLUMEâ 7.5 10° km* EQUIVALENT SEA-LEVEL CHANGE 210m @ From Flint (1971). > Based on the present ice thickness of 1700 m in Greenland and Antarctica. state, and in North America the ice sheets had shrunk to about their present extent by about 7000 years ago. How widespread and synchronous these fluctuations were is not yet known, but evidence is growing that there were several periods of widespread cooling and glacial expansion in the regions bordering the Atlantic Ocean [see Figure A.2(c)], spaced about 2500 years apart. One of these glacial advances (the Younger Dryas event, about 10,800 to 10,100 years ago) was a climatic event of unparalleled abruptness in Europe, establishing itself within a century or less and lasting for some 700 years. Northern forests that had advanced during the pre-
156 UNDERSTANDING CLIMATIC CHANGE MINNESOTA NORTH ATLANTIC GREENLAND DATED SHORE- POLLEN CORE CORE V-23-8I ICE CORE LINE FEATURES wy= I2 15-43 o -38 -33 -26 25 50-â+ââ_+ 75 100 O Wi a a. @ e TS 2aj @ T â u oO + g 15] 1S % | oO = 20} 4 20 = © <q 25 t â;â____4â___+ ââ_â}â__-~ - + + 4 â + + + 3 63 66 69 72 6 9 (2 IS -43-38 -33 28 25 50 75 100 Floral Index Faunal index 8 0'8 (%) (% rise since 16,000 YBP ) Tj Ts SEA LEVEL FIGURE A.12 Climatic records of the last 25,000 years. (a) A fioral index reflecting changes in vegetation in Minnesota, as documented by pollen counts in a bog core (Webb and Bryson, 1972). The index is an estimate of July air temperature in °F. (b) A faunal Index reflecting changes In foraminiferal plankton in a core west of Ireland, from C. Sancetta, Brown University (unpublished). The index is an estimate of August sea- surface temperature in °C. (c) Values of 5'°O in the ice-core of Camp Century, Greenland (Dansgaard et al., 1971). The isotope ratio is judged to reflect air-temperature variations over the ice cap, with the more negative values assoclated with colder temperatures. (d) Generalized curve of numerous sea-level records (Bloom, 1971). Chronology of curves (a) and (b) is established by 1'C dates (solid circles) and stratigraphic correlation with 4C dates (open circles). Chronology for curve (c) above arrow (12,700 years ago) taken from Dansgaard et al., (1971); below arrow, the chronology of Dansgaard et al. has been modified by stratigraphic correlation with dated records in North Atlantic deep-sea cores (Sancetta et al., 1973). Curve (d) is controlled by numerous ''C dates. ceding warm interval were destroyed in many places. Such vegetation records suggest that by the end of the Younger Dryas event, European climate had returned to about its present state. The rise in sea level during the last 18,000 years indicated in Figure A.12(d) is generally ascribed to the melting of northern hemi- sphere continental ice sheets. Details of the sea-level curve, however, do not correspond to the chronology of deglaciation just described: while the continental ice sheets had essentially disappeared by about 7000 years ago, the worldwide stand of sea level has reached its maxi- mum only during the last few thousand years or is still slowly rising
APPENDIX A 157 (Bloom, 1971). One possibility is that the West Antarctic ice sheet is unstable and has been disintegrating during the entire interval in ques- tion. Further research is clearly needed to settle this question, although it serves to illustrate the global interrelationships among the elements of the climatic system. The Last 150,000 Years In order to find an ancient counterpart to the warm, ice-free condi- tions of the past 10,000 years (the Holocene or present interglacial), it is necessary to go back some 125,000 years to an interval known as the Eemian interglacial (see Figure A.2). As shown by the proxy data of Figure A.13, the warmest part of this period lasted about 10,000 NORTH ATLANTIC TIRANEAN GREENLAND SHORELINE FEATURES PLANKTON (Core V23-82) iuicedonla Lake) (Comp Century Core) (Core SG TOPE (Bermuda, Borbadece, New Guinea) ow 8> |!+ 4.9 6 60 90 °o-44 oO O05Ff -O2+ -09-16 + ~ 0 0F -70 + -40 40+ oO 5 e w IST e ° +15 = e ws 30+ e > + 30 : 457 + 40 % 60 + » + 60 <mM 75 4 ° ° ° 1 78 4S 90+ ° > 90 iâ n Qo > o ° > > +105 S 120 Tt ° - ° â> > t120 135 + 368 â_+-â_+â_+ b t +âââ4 r ¢ +ââ+ 5 $ is4 =6 O 3% 60 90 -44 -38 -32 -28 O5 -02 -09 -6 -l00-70 -40 -10 FAUNAL INDEX % ARBORE AL ICE CORE %e SO® in SEA LEVEL T, POLLEN 80 (%ee) PLANKTON SHELL METERS FIGURE A.13 Climatic records of the last 135,000 years. (a) A faunal index refiecting changes in foraminiferal plankton in a core west of Ireland. The Index is an estimate of August sea-surface temperature in °C (Sancetta et al., 1973). (b) The percentage of tree pollen accumulated in a Macedonian lake. High values indicate warmer and some- what dryer conditions (van der Hammen et al., 1971). (c) Oxygen isotope ratio expressed as 5150 In an ice core at Camp Century, Greeniand. This is interpreted as indicating changing air temperatures over the Ice cap (Dansgaard et al., 1971). (d) Oxygen isotope ratio in skeletons of pianktonic foraminifera in a Caribbean core, interpreted as changes in global ice volume. High negative values reflect the melting of ice containing isotopicaily light oxygen (Emiliani, 1968). (e) Generalized sea-level curve. Portion younger than 20,000 years is representative of a large number of sites (Bloom, 1971); see also Figure A.12. Older segments are from elevated coral reef tracts on Barbados, Bermuda, and New Guinea (Mesolelia et al., 1969; Veeh and Chappell, 1970). Chronology of curves (a) to (d) controlied by 1:C dates (solid circles) and by stratigraphic correlation with dated horizons (open circles). Curve (e) is controlied by 1âC dates for the portion younger than 20,000 years and by uranium growth methods for the older segments.
158 UNDERSTANDING CLIMATIC CHANGE years and was followed abruptly by a cold interval of substantial glacial growth lasting several thousand years. The interval between this post- Eemian event (c. 115,000 years ago) and the most recent glacial maxi- mum 18,000 years ago was characterized by marked fluctuations superimposed on a generally declining temperature. An intense glacial event about 75,000 years ago is sometimes used to separate the interval into an older and generally nonglacial regime and a more recent glacial one. A remarkable feature of the climatic record of the past 150,000 years is that both the present and the Eemian interglacials began with an abrupt termination of an intensely cold, fully glacial interval. Be- cause these catastrophic episodes of deglaciation have left such a strong imprint on the climatic record, they have been named (in order of increasing age) termination I and termination II (see Figure A.14 and Broecker and van Donk, 1970). The Last 1,000,000 Years For at least the last 1,000,000 years the earthâs climate has been char- acterized by an alternation of glacial and interglacial episodes, marked in the northern hemisphere by the waxing and waning of continental ice sheets and in both hemispheres by periods of rising and falling temperatures. How clearly these fluctuations are stamped on the various proxy data records is shown in Figure A.14. The most prominent features of the isotope curve shown here are seven terminations, mark- ing a transition from full glacial to full interglacial conditions. All but one (termination III) of these changes are relatively rapid monotonic swings and provide an objective basis for defining a climatic âcycleâ for at least the last 700,000 years. As shown in Figure A.14, these same fluctuations can be recognized in diverse and widely distributed records, including the chemical composition of Pacific sediments, fossil plankton in the Caribbean, and the soil types in central Europe. These âcycles,â identified as A to E by Kukla (1970), are found in each of the records shown in Figure A.14 and may be grouped into a climatic âregimeâ covering the last 450,000 years (designated a). The earlier records (regime 8) show higher-frequency fluctuations with less co- herence among the various proxy climatic recorders. The Last 100,000,000 Years Although continuous and detailed records are lacking for these earlier times, at least the broad outline of this period of climatic history may
APPENDIX A 159 5 ISOTOPIC COMPOSITION OF CHEMICAL COMPOSITION TAXONOMIC COMPOSITION CENTRAL EUROPEAN 3 OF EQUATORIAL PACHIC W204, RECORD : g i (CORE v2e-236) (CORE RCII-208) (CORE vVi2-122) (Red Hi, Grae, Crechesiovelia eo â0 =6â-BCO â8S o oT a > a - AP i b o3 = lg? - ry 2 C c c 2 3 3 5 D â t. 3 ume 1). Aa > E e +.4 8 v _ Fist [F â +s s 6 9g g*1 ba] q B [a ra vi \ *8 q 7t nt Yj) @ +7 & Â¥ Bt = co) ot rT? + + fend pen â + + + TT T -10 «6-8 3=6.-20 0S -285 SS 100ââ«é] eo «0 %.%2 30 MB ME ;} 2 468 Observation 80" (%e) Ceco, (%) FAUNAL INDEX (3) SO TYPE tnterpretetion: OECREASING GLOBAL âINCREASING CoCOs DECREASING SALINITY TEMPERATE CLMATE (CE VOLUME OfSSOLUTION â_____â~» â_______â» âââââ= oe FIGURE A.14 Climatic records of the last 1,000,000 years. (a) Oxygen-isotope curve in Pa- cific deep-sea core V28â-238, interpreted as reflecting global ice volume (Shackleton and Opdyke, 1973). The relatively rapid and high-amplitude fluctuations are taken to indicate sudden deglaciations and are designated as the terminations | to Vil. (b) Calcium carbonate percentage in equatorial Pacific core RC11â209 (Hays et al., 1969). Low values are taken to indicate periods of rapid dissolution by bottom waters. (c) Faunal index reflecting changing composition of Caribbean foraminiferal plankton, calibrated as an estimate of sea-surface salinity in parts per thousand (imbrie et al., 1973). Glacial periods are marked by the influx of plankton preferring higher-salinity waters (Prell, 1974). (d) Sequence of soil types accumulating at Brno, Czechoslovakia (Kukia, 1970). Type 1 is a wind-biown loess with a fossil fauna of cold-resistant snaiis or gley soils indicating extremely cold conditions; type 2 includes pellet sands and other hillwash deposits, partly interbedded with loess; type 3 includes brownearth and tschernosem soils; type 4 includes parabrownearth (lessivé) soils; type 5 are soils of temperate savannas, including brownlehms, rubified brownlehms, and rublified lessivés with large, hollow carbonate con- cretions. The duration of each soil type at this locality is plotted in proportion to the maximum thickness observed. Note that each record shown here reflects a climatic fluctuation or ââcycleâ averaging about 100,000 years. This Is particulariy true during the last 450,000 years (climatic regime a). Chronology of the curves Is obtained by linear Interpolation between indicated control points, shown by solid circles. be discerned. From the climatic point of view, perhaps the most strik- ing aspect of world geography at the beginning of this interval was the essentially meridional configuration of the continents and shallow ocean ridges, which must have prevented a circumpolar ocean current in either hemisphere. In the south this barrier was formed by South America and Antarctica (which lay in approximately their present
160 UNDERSTANDING CLIMATIC CHANGE latitudinal positions); by Australia (then a north-eastward extension of Antarctica); and by the narrow and relatively shallow ancestral Indian Ocean (Dietz and Holden, 1970). About 50,000,000 years ago the AntarcticaâAustralian passage began to open (Kennett et al., 1973), and as Australia moved northeastward, the Indian Ocean widened and deepened. Both paleontological and sedimentary evidence suggest that about 30,000,000 years ago the Antarctic circumpolar current system was first established. This must be considered a pivotal event in the climatic history of the past 100,000,000 years, and when the evi- dence of global plate movements is complete, it may well be possible to account for much of the secular climatic changes of this period as a response to the changing boundary conditions imposed by the distribu- tion of land and ocean. During the last part of the Mesozoic era (from 100,000,000 to 65,000,000 years ago) global climate was in general substantially warmer than it is today, and the polar regions were without ice caps. About 55,000,000 years ago numerous geologic records (Addicott, 1970; Flint, 1971) make it clear that global climate began a long cooling trend known as the Cenozoic climatic decline (see Figure A.15). Evidence from the marine record indicates that about 35,000,000 years ago Antarctic waters underwent a substantial cooling (Douglas and Savin, 1973; Shackleton and Kennett, 1974a, 1974b). There is direct evidence that ice reached the edge of the continent in the Ross Sea area some 25,000,000 years ago; and during the Oligocene epoch, roughly 35,000,000 to 25,000,000 years ago, global climate was generally quite cool (Moore, 1972). During early Miocene time (20,000,000 to 15,000,000 years ago) evidence from low and middle latitudes indicates a warmer climate, but isotopic evidence and faunal data indicate that this warming did not affect high southern latitudes. About 10,000,000 years ago there is widespread evidence of further cooling, substantial growth of Antarctic ice (Shackleton and Kennett, 1974a, 1974b), and growth of mountain glaciers in the northern hemisphere (Denton et al., 1971). For general descriptive purposes the present glacial age may be defined as beginning at this time. Indirect evidence from marine sediments indicates that about 5,000,000 years ago the already substantial ice sheets on Antarctica underwent rapid growth and quickly attained essentially their present volume (Shackle- ton and Kennett, 1974a, 1974b). This evidence is generally consistent with direct records from the Antarctic continent, which show that between 7,000,000 and 10,000,000 years ago a large ice sheet existed in West Antarctica, and that by about 4,000,000 years ago the ice sheet in East Antarctica had developed to essentially its present volume
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162 UNDERSTANDING CLIMATIC CHANGE (Denton et al., 1971; Mayewski, 1973). The present Antarctica ice mass is equivalent to about 59 m of sea level. Although the behavior of the smaller, West Antarctic ice sheet is incompletely known, the available evidence indicates that it has under- gone considerable fluctuation, and that its variations are roughly syn- chronous with the northern hemisphere glacialâinterglacial cycle. This may be due to the fact that while the East Antarctic ice sheet is solidly grounded on the continent, much of the West Antarctic ice mass is grounded on islands or on the sea floor and could therefore be sig- nificantly influenced by sea-level variations due to glacial changes in the northern hemisphere. Such continental ice sheets first appeared in the northern hemisphere about 3,000,000 years ago, occupying lands adjacent to the North Atlantic Ocean (Berggren, 1972b), and during at least the last million years the ice cover on the Arctic Ocean was never less than it is today (Hunkins et al., 1971). The Last 1,000,000,000 Years Our knowledge of the climatic events over this time range consists principally of evidence of glaciations as preserved in the geological record. This may be seen in perspective with that for the more recent periods discussed above in Figure A.15. The present glacial age is seen to be at least the third time that the planet has suffered widespread continental glaciation. The Permo-Carboniferous glacial age (about 300,000,000 years ago) occurred at a time when the earthâs land masses were joined in a single supercontinent (Pangaea). The area of this continent was distributed in roughly equal proportions between the hemispheres, with a concentration of land in the midlatitudes (Dietz and Holden, 1970). Glaciated portions of Pangaea included parts of what are now South America, Africa, India, Australia, and Antarctica. One or more earlier glacial ages are known from late Precambrian times (about 600,000,000 years ago), from the indications of glacia- tion in deposits now widely scattered over the globe, including Green- land, Scandinavia, central Africa, Australia, and eastern Asia (Holmes, 1965). Although other glacial ages may have occurred besides those recog- nized in Figure A.15, none has left such a clear and widespread imprint on the geological record (Steiner and Grillmair, 1973). While the evi- dence is far from complete, it may be that each of the earthâs major glacial agesâincluding the present oneâresulted from crustal move- ments that permitted the development of sharp thermal gradients over a continental land mass that includes a pole of rotation. To establish
APPENDIX A 163 this or other hypotheses of long-period climatic changes, however, will require the assembly of a much more complete geological record and the performance of appropriate climate modeling experiments. GEOGRAPHIC PATTERNS OF CLIMATIC CHANGE While the chronology of certain features of climatic change may be revealed by the analysis of instrumental and paleoclimatic data at in- dividual sites, the geographic pattern of these changes is an equally important characteristic. From what we know of the behavior of the (present) atmosphere, it would be remarkable if there were not a defi- nite spatial structure to the variations on all climatic times scales. The search for these patterns requires synoptic data for the various climatic elements, and this is presently available only from the records of modern observations and from a few marine proxy sources. Structure Revealed by Observational Data The task of describing the spatial and temporal structure of climatic variations from the observations of the instrumental era is far from complete. Most studies have therefore focused primarily on local or regional climatic changes. Lamb and Johnson (1961, 1966) have made comprehensive analyses of intrahemispheric and interhemispheric cli- matic indices, and the statistical structure of these circulation variations has been studied by Willett (1967), Wagner (1971), Iudin (1967), Brier (1968), and Kutzbach (1970). Such analyses, especially of hemispheric pressure data, reveal that the year-to-year and decade-to- decade variations have a spatial structure that may be associated with amplitude and phase changes of the long planetary waves in the atmosphere. The essentially two-dimensional character of climate is masked in studies of zonally averaged parameters, although these may be use- ful for other purposes. An example of the importance of both zonal and nonzonal spatial variability of the atmospheric circulation is provided by the first eigenvector pattern (or empirical orthogonal function) of hemispheric pressure for January, shown in Figure A.16, as well as by the patterns of pressure, temperature, and rainfall variabil- ity shown in Figures A.17, A.18, and A.19. These data suggest an association between the changes in the monthly average intensity and position of the Aleutian and Icelandic lows. For example, during the first two decades of this century there has been a tendency for decreased intensity and westward extension of the Aleutian low, coupled with an
164 UNDERSTANDING CLIMATIC CHANGE io Y Se of AGO CT. 01 Bae ao , ; 7 . / oN . pe we {> . \ AP \ ° ° ââ oe â a: | j Se ° f 2 5 . © . , : : ® ny 7 | . \ s FIGURE A.16_ The first eigenvector of northern hemisphere sea-level pressure, based on the individual mean January maps for 1899-1969 (Kutzbach, 1970). This spatial function accounts for 22 percent of the total inter-January variance. increased intensity and northeastward shift of the Icelandic low. Lamb (1966) and Namias (1970) have described important regional changes in temperature and precipitation associated with these circulation changes. The opposite tendency has prevailed since the mid-1950âs, and Lamb (1966), Winstanley (1973), and Bryson (1974) have described the possible relationships between the changing midlatitude circulation patterns of the 1960âs, the equatorial shift of the subtropical highs, and the increasing frequency of droughts along the southern fringes of the monsoon lands of the northern hemisphere (see Figure A.8). These changes appear to reflect an equatorward extension of the westerly wave regime and a contraction of the Hadley circulation,
APPENDIX A 165 FIGURE A.17 Standard deviation (mbar) of monthiy mean pressure at sea level, 1951- 1966 (Lamb, 1972). (a) December, northern hemisphere; (b) July, southern hemisphere.
166 UNDERSTANDING CLIMATIC CHANGE p ; J *s si December FIGURE A.18 Standard deviation (°C) of monthiy mean surface air temperature in the northern hemisphere, 1900-1950 (Lamb, 1972). (a) July; (b) December.
APPENDIX A 167 40 ty Jy l =< °o meres . F t. | es oO 20 ef = Y o | â- os 100 40 20 0 °F 0 = 20â = ep Rainfall variability The figures denote percentage departures from normal is Under 10 10-15 15-20 20-25 25-30 30-40 Over 40% _JCIEj ZZee oe FIGURE A.19 The variability of mean annual rainfali for the worid (adapted from Petterssen, 1969). although much further analysis is clearly required to confirm such a conjecture. Interhemispheric relationships of climatic indices have been (and remain) less amenable to study because of the general lack of observa- tions from the southern hemisphere. Observations are sufficient, how- ever, to show that the circulation in the southern hemisphere is some- what stronger and steadier than that in the northern hemisphere. Whether this results in the southern hemisphere circulation leading that in the northern hemisphere, or whether variable features in the equatorial circulation influence both hemispheres similarly, is not presently known (Bjerknes, 1969b; Fletcher, 1969; Lamb, 1969; Namias, 1972a, 1972b). It is likely that interhemispheric relationships of one sort or another are important for the understanding of climatic variations, and that our ability to describe them will require the availability of much more comprehensive data than now exist from the southern hemisphere, the equatorial region, and the oceanic and polar regions of the northern hemisphere. The present accumulation of upper-air data, especially in the north- ern hemisphere since the early 1950âs, however, has permitted a beginning of the study of the three-dimensional spatial and temporal variability of the general circulation. A foundation of basic statistics is
168 UNDERSTANDING CLIMATIC CHANGE provided by calculations of the means and variances of standard meteorological variables (see, for example, Crutcher and Meserve, 1970; Taljaard et al., 1969) and by atlases of energy budgets (Budyko, 1963). The covariance structure of circulation patterns at 700 mbar in the northern hemisphere is treated by OâConnor (1969), and other aspects of the tropospheric circulation have been considered by Gommel (1963) and Wahl (1972). The most comprehensive analysis of atmospheric circulation statistics, however, is that based on the period 1958-1963 as undertaken by Oort and Rasmusson (1971). While this work docu- ments the monthly, seasonal, and annual variations of many features of the observed general circulation (in the northern hemisphere), it does not directly address many of the variables of primary climatic interest. Using the same data set, however, Starr and Oort (1973) have reported an unmistakable downward trend of the mean air temperature in the northern hemisphere of 0.6°C over the five-year interval shown in Figure A.20. Diagnostic studies of this type represent great investments of time and effort but are essential steps toward the monitoring of climate and an assessment of the mechanisms of climatic variation. A complete description of climatic changes from instrumental records must also include studies of the momentum and energy budgets of the atmosphere and oceans and their variability with time over many years and decades. While this must remain largely a task for the future, several efforts have established the existence of significant interannual variations in the atmosphere. Krueger et al. (1965) have discussed the May April April April April April 1958 1959 1960 1961 1962 1963 TTUUEUUTUVE Terre rr yyy repr resryrvr er ye rererervery ry r rrr ry erry 0.47 - ©, 0.2F 4 a 7 o 0 2 r + o â0.2F â 3BOT _oab :J a 4 L ! = | 1 1959 1960 1961 1962 1963 FIGURE A.20 Monthly mean mass-averaged values of the northern hemisphere tempera- ture for the period May 1958 to April 1963 (Starr and Oort, 1973). The consecutive monthly averages are plotted on the scale marked at the top; the bottom scale shows the beginning of each caiendar year.
APPENDIX A 169 interannual variations of available potential energy, and Kung and Soong (1969) have described the fluctuations of the atmospheric kinetic energy budget. As noted previously, the interannual variations of pole- ward angular momentum and energy fluxes has been studied compre- hensively by Oort and Rasmusson (1971). A measure of this variability is shown in Figure A.21 and amounts to about +30 percent of the mean transports. ol 149 + © sanwee 4 B i or eg S23 Ww âg i7 (a) g 00 + + a we g a TT x, * * 64~ =a Pr o 4 G 6 o â 3 a 20 Latitude °F x z 2 © snrese ] 4 8 -~ + ww 3b OY o (b) @Hw 6 wv YU rn) aN eg © mei ww Latitude FIGURE A.21 Interannual variability of poleward eddy transports for the month of January as shown by five years data (Oort and Rasmusson, 1971). The solid curve is the 5-year mean for 1959-1963. (a) Angular momentum transport; (b) sensible heat transport.
170 UNDERSTANDING CLIMATIC CHANGE The unique global potential of satellite-based measurements has been exploited by Vonder Haar and Suomi (1971), who have sum- marized the satellite measurements of planetary albedo and of the planetary radiation budget for the five years 1962 to 1966. They found large interannual variations in the zonally averaged equator-to-pole gradient of the net radiation as shown in Figure A.22. This forcing function can now be monitored routinely by meteorological satellites and opens the door to more detailed studies of atmospheric energetics than heretofore possible (Winston, 1969). Vonder Haar and Oort (1973) have combined satellite measurements of the earthâs radiation budget with atmospheric energy transport calculations to produce a new estimate of the poleward energy transport by the northern hemisphere N.H. 0.3 _~ RK ~ J ât 0.2 ~ , NET RADIATION GRADIENT (cal em72 min7") / K / 0.1 = a SS SS OS SS A a CE ee Winter Spring Summer Fall Winter âââ S.H. LL. AN 0.3 F- 0.2 [ 7 asf Y b 0 FIGURE A.22 Seasonal (dashed lines) and interannual variation (verticai bars) of the equator-to-pole gradient of net radiation as measured from sateliites (Vonder Haar and Suomi, 1971). (a) Northern hemisphere; (b) southern hemisphere.
APPENDIX A 171 oceans. They find that the oceanic heat transport averages about 40 per- cent of the total in the 0-70 °N latitude band and accounts for more than half at many latitudes. Another example of the use of satellite- derived measurements of climatic indices is given by Kukla and Kukla (1974). Their measurements of the interannual changes in the area of snow and ice cover in the northern hemisphere are shown in Figure A.23 and reveal year-to-year fluctuations of the order of a few percent. Note, however, the relatively large change during 1971 and the subse- quent maintenance of extensive snow and ice coverage and an associated increase of the reflected solar radiation. Time variations of the surface energy budget on a global scale are icon | | prt Co! cat day") (10° ton?) 33 +- 4 * +17 as 4 1 36 + 18 as 37 + 38 + 19 + Ty T 20 + T 1969 1970 =âSs«971 1972 1973 FIGURE A.23 Twelve-month running means of snow and ice cover in the northern hemisphere (upper curve) and the estimated reflected solar radiation disregarding variations of cioudiness (lower curve), as reported by Kukla and Kukla (1974). The averages are plotted on terminai dates, with the years marking January 1.
172 UNDERSTANDING CLIMATIC CHANGE not available from direct observations and must be inferred from the conventional measurements of temperature, humidity, cloudiness, wind, and radiation. Fletcher (1969) has drawn attention to the variations in the energy budget of polar regions as a function of variable sea-ice con- ditions, while Sawyer (1964) has noted the possible role of fluctuations in the surface energy budget as a cause of interannual variations of the general circulation itself. A number of observational studies of large-scale interaction between the ocean and the atmosphere have illustrated the complexity and im- portance of this mechanism; see, for example, Weyl (1968) and Lamb and Ratcliffe (1972). Bjerknes (1969b) has considered the response of the North Pacific westerlies to anomalies of equatorial sea-surface temperature and variations in the Hadley circulation, while Namias (1969, 1972b) has described positive feedback relationships between large-scale patterns of ocean-surface temperature in midlatitudes and the circulation of the overlying atmosphere. Such modes of atmosphereâ ocean coupling may be important parts of climatic fluctuations and must be given further study. In summary, we may say that observational data at the earthâs sur- face show that during the period 1900 to 1940 the northern hemisphere as a whole warmed, although some areas (mainly the Atlantic sector of the Arctic and northern Siberia) warmed far more than the global average, some areas became colder, and others showed little measurable change (Mitchell, 1963). In the time since 1940, an overall cooling has occurred but is again characterized by a geographical structure; cooling since 1958 has occurred in the subtropical arid regions and in the Arctic (Starr and Oort, 1973). There is also some evidence that the northern hemisphere oceans are cooling (Namias, 1972b), although the oceanic data base necessary to confirm this has not yet been assembled. Structure Revealed by Paleoclimatography Most of the work done to date on climatic change beyond the time frame encompassed by meteorological observations represents a study of time series taken at specific sites. This lack of synoptic data on the longer-range climatic changes is a serious handicap to the portrayal and understanding of the mechanisms involved. In order to underscore these points, and to encourage further research, we present here ex- amples of the few proxy data that have been assembled to reveal a spatial structure of climatic change.
APPENDIX A 173 Distribution of Ice Sheets The continental margins of the northern hemisphere ice sheets at their maximum extension during the last million years are clearly marked by the debris deposits in terminal moraines, while the extent of sea ice is recorded by features preserved in marine sediments. Figure A.24 120 100 80 60 VO"âS \ IS0K, IGOK IBO- iGo,r i4oh 40 l ji \ I20 lOO 80 60 FIGURE A.24 Maximum extent of northern hemisphere ice cover during the present glacial age (modified after Fiint, 1971). Continental ice sheets, indicated by the dotted area, are B, Barents Sea; S, Scandinavian; G, Greeniand; L, Laurentide; C, Cordilieran. Sea ice is indicated by the cross-hatched pattern. The boundary mapped is the southern- most extent of the ice margin that occurred in any sector during the iast million years. The iast giacial maximum, about 18,000 years ago, occupied about 90 percent of the area shown here (see aiso Tabie A.2).
174 UNDERSTANDING CLIMATIC CHANGE shows the distribution of maximum ice cover, and Table A.2 gives statistics of the areas of the individual continental ice sheets. In North America the ice extended as far south as 40 °N and spanned the entire width of the continent, while in Europe the ice sheet extended only to about 50 °N. Note that large regions in eastern Siberia were unglaciated. Sea-Surface Temperature Patterns The north-south migration of polar waters in the North Atlantic in response to major cycles of glaciation is shown in Figure A.25. During glacial maxima these waters were found as far south as 40 °N, well beyond the present extent of polar waters. A synoptic analysis of the ocean surface temperatures of 18,000 years ago (at about the time of the last glacial maximum) is shown in Figure A.26. These temperature estimates have been derived by multivariate statistical techniques applied to planktonic organisms as preserved in about 100 deep-sea cores in scattered locations across the North Atlantic (McIntyre et al., 1974). The most striking feature of this glacial-age map is the extensive south- ward displacement of the 10 to 14°C water, while the warmer water was found in nearly its present position. In parts of the Sargasso Sea the glacial-age ocean was, if anything, slightly warmer than it is today. Because the atmosphere receives much of its heat from the sea, such estimates of sea-surface temperature are likely to be important in de- veloping a satisfactory reconstruction of past climates, and it is there- fore important to consider their reliability. Berger (1971), for ex- ample, has suggested that carbonate dissolution on the sea bed may distort the taxonomic composition of the fossil fauna on which such paleotemperature estimates are based. Kipp (1974), on the other hand, shows that when the statistical transfer functions are calibrated on ma- terials that incorporate the dissolution effects, an unbiased estimate of such parameters as the sea-surface temperature can be obtained. The temperature reconstruction in Figure A.26(b) is based on the statistics of the foraminiferal fauna distribution and encompasses 91 percent of the variance of the data (McIntyre, 1974). The 80 percent confidence interval of each of the cores is +1.8°C (Kipp, 1974). Shackleton and Opdyke (1973), using a revised isotopic method based on the differ- ence between 1°O values in benthic and planktonic species, have pro- vided an independent confirmation of the sea-surface temperature estimates of Imbrie et al. (1973) for a portion of the glacial-age Caribbean. Other reconstructions of paleo-ocean surface temperatures have been based on data from radiolaria, coccoliths, and foraminifera; and al-
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APPENDIX A 177 though some discrepancies are revealed where independent data are available, the derived ocean temperatures show considerable spatial coherence (McIntyre, 1974). Such estimates of past sea-surface tem- perature will prove useful in climatic simulations with numerical general circulation models (see Appendix B). Patterns of Vegetation Change Figure A.27 illustrates the use of fossil pollen data to record the changes in vegetation accompanying the deglaciation of eastern North America during the interval 11,000 to 9000 years ago. At the beginning of this time, pine species occupied sites in the southeastern Appalachians, but as the ice retreated, the pine moved farther north and west to colonize newly uncovered areas. A relatively complete chronology of the retreat of the Laurentide ice sheet itself is given by radiocarbon dating (Bryson et al., 1969). Patterns of Aridity For only four desert areas in the world do we have enough information to plot aridity as a function of time, and even in these areas the record extends back only a few tens of thousands of years. As shown in Figure A.28, the data suggest a degree of synchroneity between the two African regions and the Great Basin, while the records from the Middle East are quite different. None of the data from closed-basin lakes show significant correlation with the glacial record, and we are clearly a long way from understanding the response of arid regions to glacial cycles. More generally, insufficient research has been devoted to the role of desert regions in the processes responsible for the climate of the earth. Patterns of Tree-Ring Growth Changes of thickness of the growth rings added by trees each year reflect environmental change in a complex way. By appropriate calibra- tion, such data may be made to furnish significant climatic information for the past several hundred to several thousand years. Studies of many tree-ring series over a wide geographic area can, moreover, provide accurately dated synoptic evidence of regional climatic patterns (Fritts, 1965). Fritts et al. (1971) have demonstrated the feasibility of reconstruct- ing the anomalies of sea-level pressure and temperature from the spatial patterns of tree growth over western North America. Examples
178 UNDERSTANDING CLIMATIC CHANGE (a) (b) 95 85 75 Rieas ens lien niente tie ire caieni nin na inion 7 50 < 45 â Cc) 40 90 80 70 FIGURE A.27 The distribution of pine pollen at selected times dur- ing the deglaciation of eastern North America (Bernabo et ai., 1974). Contours are lines of poilen frequency, expressed as a percent of total pollen. Control points representing radiocarbon dated cores are indicated by the open circles. The approximate margins of the Laurentide ice sheet are indicated by the stippled pattern (after Bryson et al., 1969).
APPENDIX A 179 LARGE LARGE GREAT BASIN WESTERN wn USA | Wy ! DEAD < \L / SEA z / â / @ SMALL ! 1 1 ! ââ1 ! SMALL Oo 10 20 300 40 50 O 10 20 30 40 50 CG LARGE LARGE wn ° AFRICAN LAKE oO RIFT u VALLE Y CHAD 3 < lJ a < f SMALL ' L 4 ! a ! i SMALL O 10 20 30 40 500 10 20 30 38640 50 THOUSANDS OF YEARS AGO FIGURE A.28 Variations in the size of closed-basin lakes, as indicated by the degree of aridity found from radiocarbon dating of shoreiines and bottom sediments. Higher rainfali and iower evaporation may be inferred at the times of iarger water surfaces. (a) From Broecker and Kaufman (1965); (b) from Kaufman (1971); (c) from Butzer et al. (1972); (d) from Servant et al. (1969). of such synoptic maps based on average decadal growth are given in Figure A.29. Although such reconstructions show considerable varia- tion in the year-to-year climatic states, the inferred variations in the intensity of Icelandic and Aleutian lows, for example, are similar to those described in the modern record (Kutzbach, 1970). The develop- ment of an expanded network of tree-ring sites could significantly broaden our knowledge of the patterns of climatic fluctuations over the past several centuries. SUMMARY OF THE CLIMATIC RECORD In this survey of past climates, the characteristic time and spatial structures of climatic variations have been discussed as though there were sufficient data to document large regions of the globe. This is true only for the more recent parts of the instrumental period, as there are large gaps in the presently available historical and proxy climatic records. With these limitations in mind, it is nevertheless useful to summarize the general characteristics of the climatic record:
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APPENDIX A 181 1. The last postglacial thermal maximum was reached about 6000 years ago, and climates since then have undergone a gradual cooling. This trend has been interrupted by three shorter periods of more marked cooling, simliar to the so-called Little Ice Age of A.D. 1430â1850, each followed by a temperature recovery. The well-documented warming trend of global climate beginning in the 1880âs and continuing until the 1940âs is a continuation of the warming trend that terminated the Little Ice Age. Since the 1940âs, mean temperatures have declined and are now nearly halfway back to the 1880 levels. 2. Climatic changes during the past 20,000 years are as severe as any that occurred during the past million years. At the last glacial maxi- mum, extensive areas of the northern hemisphere were covered with continental ice sheets, sea level dropped about 85 m, and sea-surface temperatures in the North Atlantic fell by as much as 10°C. At northern midlatitude sites not far from the glacial margins (locations now occupied by major cities and extensive agricultural activity), air temperatures fell markedly, drastic changes occurred in the precipita- tion patterns, and wholesale migrations of animal and plant communi- ties took place. 3. The present interglacial intervalâwhich has now lasted for about 10,000 yearsârepresents a climatic regime that is relatively rare during the past million years, most of which has been occupied by colder, glacial regimes. Only during about 8 percent of the past 700,000 years has the earth experienced climates as warm as or warmer than the present. 4. The penultimate interglacial age began about 125,000 years ago and lasted for approximately 10,000 years. Similar interglacial agesâ each lasting 10,000+ 2000 years and each followed by a glacial maxi- mumâhave occurred on the average every 100,000 years during at least the past half million years. During this period fluctuations of the northern hemisphere ice sheets caused sea-level variations of the order of 100 m. In contrast, the East Antarctic ice sheet has apparently varied little since reaching its present size about 5 million years ago, while the West Antarctic ice sheet appears to have been disintegrating for many thousands of years. 5. About 65 million years ago global climates were substantially warmer than today, and subsequent changes may be viewed as part of a very long-period cooling trend. For even earlier times, the proxy climatic evidence becomes increasingly fragmentary. The best docu- mented records suggest two previous extensive glaciations, occurring about 300 million and 600 million years ago.
182 UNDERSTANDING CLIMATIC CHANGE FUTURE CLIMATE: SOME INFERENCES FROM PAST BEHAVIOR The overall picture of past climatic changes described in this survey suggests the existence of a hierarchy of fluctuations that stand out above the âwhite noiseâ or random fluctuations presumed to exist on all time scales. In addition to the dominant period of about 100,000 years, there are apparent quasi-periodic fluctuations with time scales of about 2500 years and shorter-period fluctuations on the order of 100â400 years. Each of these explains progressively less of the total variance but may nevertheless be climatically significant. No periodic component of climatic change on the order of decades has yet been clearly estab- lished, although significant excursions of climate are observed to occur in anomalous groups of years. In view of the limited resolving power of most climatic indicators, especially those for the relatively remote geological past, it is difficult to establish whether the apparent fluctuations are quasi-periodic or whether they more nearly represent what are basically random Markov- ian âred-noiseâ variations. In the case of the longer-period variations (of 100,000-year and 20,000-year periods), there is circumstantial evi- dence to suggest that these may have been induced in some manner by the secular variations of the earthâs orbital elements, which are known to alter the seasonal and latitudinal distribution of solar radiation re- ceived at the top of the atmosphere. In other cases, the observed varia- tions have yet to be convincingly related to any external climatic control. The mere existence of such variations does not necessarily mean that changes in the external boundary conditions are involved, however. The internal dynamics of the climatic system itself may well be the origin of some of these features. Whether forced or not, climatic behavior of this type deserves careful study, as the conclusions reached bear directly upon the problem of inferring the future climate. The prediction of climate is clearly an enormously complex prob- lem. Although we have no useful skill inpredicting weather beyond a few_weeks into the future, we have a compelling need to predict the â climate for years, decades, and even centuries ahead. Not only do we have to take into account the complex year-to-year changes possibly induced by the internal dynamics of the climatic system, and the likely continuation of the (yet unexplained) quasi-periodic and episodic fluctuations of the last few thousand years discussed above, but also the changes induced by possibly even less predictable factors such as the aerosols added to the atmosphere by volcanic eruptions and by man himself (Mitchell, 1973a, 1973b). These questions lie at the heart of
APPENDIX A 183 the problem of climatic variation and are given consideration elsewhere in this report. In the face of these uncertainties,any projection o limate carries a great risk. Nevertheless, we may speculate about the possible course of global climate in the decades and centuries immediately ahead by making certain assumptions about the character of the major fluctuations noted in the climatic record. In the following paragraphs we attempt to draw together these considerations into an overall assess- ment of the probable direction and magnitude of present-day climatic change, taking into account the risk of a major future change associated with the seemingly inevitable onset of the next glacial period. Potential Contribution of Sinusoidal Fluctuations of Various Time Scales to the Rate of Change of Present-Day Climate Estimates of the amplitudes of all the principal climatic fluctuations identified in this report are listed in Table A.3 (where they have been made consistent with the data presented in Figure A.2 and are expressed in terms of the total range of temperature between maxima and minima). On the assumption that all of these fluctuations can be approximated by quasi-periodic sine waves, the ratio of the amplitude (A) to the period (P) of each fluctuation becomes proportional to the maximum contribution of that fluctuation to the rate of change of climate. By considering also the phase of each fluctuation, as inferred from the paleoclimatic record, the contribution of each fluctuation to the present- day rate of change can be estimated (see Table A.3). Estimation of the phase of each sinusoidal fluctuation (indicated by the estimated dates of the last temperature maximum in Table A.3) permits an assessment of the sign and magnitude of the contribution of each fluctuation to the total rate of change of globally average tempera- ture in the present epoch. The sum of these individual contributions (â0.015°C/yr) agrees reasonably well with the observed rate of change of â0.01°C/yr during the past two decades, as determined from analyses of surface climatological databy Reitan (1971) and by Budyko (1969). It should be noted that this trend is dominated by the shortest fluctuations, and especially by the fluctuations of the order of 100 years (see Figure A.6). The estimated maximum rate of change associated with all time scales of climatic fluctuation shown in Figure A.2 is plotted as a con- tinuous function of wavelength in Figure A.30. The family of curves also shown in this figure indicates the relationship between maximum
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APPENDIX A 185 1 1 _ p, 71.0 7, 5 5 2 | â â 10 1 ° & â S ? oO 8Oo za Z x O + O + 10 2 & = _ q < = aa. r â 1073 F w O- = x > 410% © E E < < uu = a - if 00001 1 l I { 10° 100,000 10,000 1,000 100 10 1 PERIOD OF FLUCTUATION (years) FIGURE A.30 Relative (maximum) rate of change of climate contributed by climatic fluctuations, as a function of characteristic wavelength. The family of parallel curves shows the expected relationship in Markovian ââred noiseâ as characterized by the serial correlation coefficient at a lag of one year. The dashed line is a conservative estimate of actual climate as inferred from the data of Figure A.2 and from additional data on the shorter-period fluctuations from Kutzbach and Bryson (1974). The dotted curve shows the modifications to be expected if the principal fluctuations identified in Table A.3 were actually quasi-periodic. rate of change and wavelength in Markovian âred noise,â for various degrees of ârednessâ characterized by the value of the serial correla- tion coefficient at a time lag of one year (Gilman et al., 1963). By com- parison with these curves, it is suggested that the observed shorter- period climatic fluctuations (i.e., fluctuations of the order of 100 to 200 years) are not clearly distinguishable from random fluctuations, whereas the longer-period fluctuations (especially those with periods of 20,000 years or more) may be appreciably larger in amplitude than would be expected in random noise. The contributions of the longer- period fluctuations to present-day climatic change are seen nonetheless
186 UNDERSTANDING CLIMATIC CHANGE to be relatively small. Should the longer-period fluctuations be non- sinusoidal (or episodic) in form, rates of change perhaps ten times larger than the magnitudes shown in Figure A.30 could be possible. Even such rates, however, would contribute little over and above the normal interannual variability of present-day global climate, and the cumulative change of climate associated with the longer-period fluctua- tions would remain relatively small until several centuries had elapsed. Despite its simplistic view of climatic change, this exercise is an instructive one in that it demonstrates how difficult it would be for longer-period sinusoidal fluctuations to contribute substantially to the changes of climate taking place in the twentieth century. If the longer- period fluctuations are those that primarily determine the course of the glacialâinterglacial succession of global climate, it would seem that the transition to the next glacial periodâeven if it has already commencedâwill require many centuries to accumulate to a drastic shift from present climatic conditions. In assessing such projections, however, we must keep in mind that our ability to anticipate the locally important synoptic pattern of climatic variations is limited. The work of Mitchell (1963), for example, has shown that while the northern hemisphere average air temperatures rose only about 0.2°C during the period 1900 to 1940, there were many areas that deviated markedly from this hemispheric average trend. Parts of the eastern United States, for example, exhibited a 1.0°C rise in average temperature (5 times the hemispheric average), parts of Scandinavia and Mexico showed temperature increases of 2.0°C (10 times the hemispheric average), while in Spitsbergen the warming was 5°C (25 times the hemispheric average). The correspond- ing data on other climatic elements are sparse but may be expected to exhibit comparable or even greater spatial variance. Likelihood of a Major Deterioration of Global Climate in the Years Ahead As noted above, the longer-period climatic fluctuations seem to be associated with larger amplitudes of change than those consistent with Markovian âred-noiseâ behavior. The same cannot be said, however, of the shorter-period fluctuations. For the moment let us suppose that all the fluctuations described in this report are actually random fluctua- tions, in the sense that transitions between successive maxima and minima may occur at random (Poisson-distributed) intervals of time rather than at more or less regular intervals. The probability that one or more transitions of a fluctuation will occur in an arbitrarily specified
APPENDIX A 187 length of time may then be calculated from the negative binomial distri- bution. Following this approach, we can assess the risk of encountering a change of climate in the years ahead as rapid as the maximum rate of change otherwise associated with sinusoidal climatic fluctuations on each of the characteristic time scales noted above. Such a measure of risk, for time intervals between 1 year and 1000 years into the future, can be inferred by interpolation between the curves of transition prob- ability in Figure A.31. The proper interpretation of this figure will be apparent from the following examples: [pea _â. oad = | L C COT âa #4 i ae ~S e-" Le _" a â a YY PROBABILITY 0.1 TRANSITION 0.0) _ oO = o _ LE & a = oeâ - 0.00! Ls Ait ty Agiil ee oe oe iO 100 1000 WAITING TIME (YEARS) FIGURE A.31 Probability of onset of climatic transitions analogous to the changes between maxima and minima in climatic fluctuations of arbitrarily selected characteristic wavelengths (interior numbers, in years), as a function of elapsed time after present. Dashed curves denote probability of one transition; solid curves denote that of one or more transitions. Based on the assumption that intervals between transitions are strictly random (Poisson distributed).
188 UNDERSTANDING CLIMATIC CHANGE 1. The curve labeled 100,000 in the figure indicates the probability of a major transition of climate (in either direction) that is normally associated with climatic fluctuations on the time scale of 100,000 years (a change of global average temperature of up to perhaps 8°C in a total time interval of 50,000 years or less). The curve indicates that if successive transitions of this kind recur at random time intervals as assumed here, the onset (or termination) of such_a transition will occur in the next â100 -years with a a pprobability of about 0. 002 and in the next 1000 y years with a probability of about 0.02. 2. The dashed curve labeled 100 in the figure indicates the prob- ability of one transition of climate (in either direction) that is normally associated with climatic fluctuations on the time scale of 100 years (a change of up to perhaps 0.5°C in a total time interval of about 50 years or less). Such a transition is indicated to have a probability of about 0.02 of occurring in the next year, a probability of about 0.16 of occur- ring in the next 10 years, and a probability of about 0.35 of occurring in the next 50 years. The solid line labeled 100 in the figure indicates the probability of one or more transitions of the same kind, which rises from about 0.2 in the next 10 years to about 0.8 in the next 100 years. If it can be assumed that the typical duration of such a transition (when it occurs) is not less than four or five decades, and that only one such transition can occur at the same time, then the dashed curve would be the appropriate guide for estimating such probabilities in the next few decades. Otherwise, the solid curve would be a more appropriate guide. When Figures A.30 and A.31 are considered together, it is suggested that whether climatic fluctuations are or are not quasi-periodic, those that are most relevant to the course of global climate in the years and decades immediately ahead are the s shorter-period (historical ) fluctua- tions and not the longer-period (glacial) âfluctuations. Even if the phase of the longer-period changes is such as to contribute to a cooling of present-day climate, the contribution of such fluctuations to the rate of change of present-day climate would seem to be swamped by the much larger contributions of the shorter-period (if more ephemeral) historical fluctuations. We must remember, however, that this analysis assumes a simple model of climatic change in which climatic fluctua- tions of various periods are independent and therefore additive. The paleoclimatic record presented here does not preclude the possibility that relatively sudden climatic changes could arise through interactions between fluctuations of different periods. One may still ask the question: When will the present interglacial end? Few paleoclimatologists would dispute that the prominent warm _
APPENDIX A 189 periods (or interglacials that have followed each of the terminations of the major glaciations have had durations of 10,000 + 2000 years. In each case, a âperiod of considerably colder climate has followed im- ea Te ee mediately after the interglacial interval. Since_about 10 ,000 years hahas elapsed since the onset of the present period of prominent\ warmth,, thee question naturally arises as to whether we are indeed on the brink âof a period of colder climate. Kukla and Matthews (1972) have already called attention to such a possibility. There seems little doubt that the present period of unusual warmth will eventually give way to a time ; of colder climate, but there is no consensus with regard to either the magnitude or rapidity of the transition. The onset of this climatic de- cline could be several thousand years in the future, although there is a finite probability that a serious worldwide cooling could_ befall the earth within the next hundred years. _ What is the nature of the climatic changes accompanying the end of a period of interglacial warmth? From studies of sediments and soils, Kukla finds that major changes in vegetation occurred at the end of the previous interglacial (Figure A.14). The deciduous forests that covered areas during the major glaciations were replaced by sparse shrubs, and dust blew freely about. The climate was considerably more continental than at present, and the agricultural productivity would have been marginal at best. The stratification of fossil pollen deposits in eastern Macedonia (Figure A.13) also clearly shows a marked change in vege- tative cover between interglacial warmth and the following cold periods. The oakâpine forest that existed in the area gave way to a steppe shrub, and grass was the dominant plant cover. Other evidence from deep-sea cores reveals a substantial change in the surface water temperature in the North Atlantic between interglacial and glacial periods (Figure A.13), and the marine sediment data show that the magnitude of the characteristically abrupt glacial cooling was approximately half the total glacial to interglacial change itself. The question remains unresolved. If the end of the interglacial is ed episodic in character, we are moving toward a rather sudden climatic change of unknown timing, although as each 100 years passes, we have a perhaps a 5 percent greater chance of encountering its onset. If, on the other hand, these changes are more sinusoidal in character, then the climate should decline gradually over a period of thousands of years. These are the limits that we can presently place on the nature of this transition from the evidence contained in the paleoclimatic record. These climatic projections, however, could be replaced with quite different future climatic scenarios due to âmanâs inadvertent interference _ with the otherwise natural variation (Mitchell, 1973a). This aspect of
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