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Interpretation of Past Climatic Variability frond PaTeoenviron~ental Inclicators c) INTRO DU CTI O N CHARLES W. STOCKTON University of Arizona The primary objective of this panel is to evaluate the national water supply in light of climatic variability. The following questions then arise: How crucial is knowledge of climatic variability to the design and operation of a water supply system, its reservoirs, and its distribution system? Should we incorporate newly gained knowledge of climatic change into the design and operation of such systems? If so, how much does climate vary? Can the variability be established on the basis of historical rec- ords? What is the probability of climatic change in the next 25 to 50 years? We must first define what we mean by weather and climate. According to the Department of Transportation Select Panel assessing the variability of the climate (Mitchell et al., 1975), the term "weather" refers to the total array of atmospheric conditions varying with time and location on the earth's surface. "Climate" connotes "average weather" but can be viewed from two perspec- tives: 34 1. A purely statistical approach in which climate is the sum of the weather as experienced at a point or over a designated area of the earth for a given period of time. 2. A physical concept that recognizes climate as a basic physical entity and weather as the momentary, transient behavior of the atmosphere attempting to satisfy the re- quirements dictated by the climate for horizontal and vertical transfer of mass, momentum, and energy. There seems to be disagreement even among experts as to the need for understanding climatic variability in rela- tion to water supply. There are those who believe that, since most projects in water resources have an economic life of from 40 to 100 years, and since there appears to have been little or no obvious climatic change over the past 200 years, the chance for natural climatic change in the next 200 years is minimal, and, therefore, the ques- tion is academic. For example, Chin and Yevjevich (1974) purported to show that climatic variation could be re- duced to a deterministic component based on the Milan- kovich theory of astronomical cycles and a simple

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Interpretation of Past Climatic Variabilityfrom Paleoenvironmental Indicators Markovian stochastic component. From this position they went on to state that "since most systems have been built with the economic project life in the range 40 to 100 years, the chances are minimal that the expected natural water supply would be significantly different during these life spans than in the past 200 years." Furthermore, "this question is, however, not crucial for the next several generations of contemporary earth population, but rather is more of an academic interest like many other human concerns with the long-term future." On the other hand, there are those who argue that climatic variability is a part of life on the planet earth and that it is to our advantage to recognize it, understand it, and take it into consideration in our planning processes. For example, Wallis and O'Connell (1973) studied the power of various statistical tests to distinguish small sam- ples taken from Markovian and more persistent generat- ing mechanisms. They concluded that statistical tests based on records of normal hydrologic length would usu- ally lead one to believe that a Markov generating mecha- nism adequately represents hydrologic reality; however, because the tests have no power, this belief, while com- forting, is likely to be erroneous. In a companion study, O'Connell and Wallis (1973) showed that Markov and more persistent generating mechanisms could lead to very different estimates of reservoir firm yield for 50-year design lives even when the generating mechanisms used yielded samples with identical expected values for the mean, variance, and lag-one correlation. They concluded that it was essential that hydrologists and water-resource planners understand the nature of climatic variability and persistence. A similar position was taken by Mitchell et al. (1975), who stated: The climate of the earth is now known beyond any doubt to have been in a more or less continual state of flux. Changeability is an evident characteristic of climate on all reasonable time scales of variation, from that of aeons down to those of millennia and centuries. The lesson of history seems to be that climatic var~a- bility is to be recognized and dealt with as a fundamental quantity of climate, and that it should be potentially perilous for man to assume that the climate of future decades and centuries will be free of similar variability. The issue seems to revolve around the question, "How variable has climate been in the past?" Presumably, if atmospheric behavior is random in time, the definition of climatic variability would be a straightforward exercise in classical statistical sampling theory. One could estimate climatic variability as precisely as desired merely by choosing a long enough averaging interval. The problem here is that, as we go back in time, our data base di- minishes and knowledge of atmospheric variability be- comes less detailed and reliable. However, we do know enough about past climates to establish that long-term atmospheric behavior does not proceed randomly in time. Variations of climate from one geological epoch to another, and from one millennium to another, are clearly 35 too large in amplitude to be explained as random devia- tions from modern averages. How, then, do we study long-term variability? Unfortu- nately, climatic measurements do not extend back much 60 4oo 20 Do 20 4oo 60 6oc 4oo 20 Go 20 40C 6oc 60 100 60 20 0 20 60 t_: ,= ,,_ ,_ 1 ' :.'_ _ _ of. ~ ~ -1750-1759 _ 11 100 1 40 _ 180 _ ~ 180 140 100 60 20 0 20 60 100 140 180 2;~: 180 140 100 60 20 0 2Q 60 100 140 180 i'- =- ~` an,. ~ i . :, ' to . ! ~ He FIGURE 2.1 Growth of the network of surface pressure obser- vations and of the area that can be covered by reliable 10-year average isobars (Lamb, 1969).

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36 beyond the 1900's on any kind of an adequate spatial coverage. Lamb (1969) shows this well in three maps illustrating the growth of the network of surface baromet- ric pressure observations and of the area covered by reliable 10-year-average isobars (Figure 2.11. Obviously, any intensive global or even hemispherical study of climatic variability from instrumental records is limited to the relatively short period 1900 to present. Even on a more local basis, the longest continuous time series of instrumental observations covers less than three cen- turies. Therefore, for longer time scales, climatic variations must be inferred from historical evidence and from the records of various natural phenomena linked in some way to climate. Such paleoclimatic indicators (Table 2.1) dif- fer greatly in the time spans over which they are ap- plicable, in the degree of climatic detail they can provide, in the aspects of climate to which they respond, and in the fidelity of their response. Reconstructions of the paleoclimatic record can lead one to believe that changes of climate, such as those associated with alternating glacial and interglacial stages of the Pleistocene, are smoothly varying functions of time, readily distinguishable from the much more rapid varia- bility of year-to-year changes of atmospheric state. Hence, to give a stable estimate of present-day climate, the aver- aging interval would have to be long enough only to sup- press year-to-year sampling variability but short in com- parison with the duration of a glacial period. Unfortu- nately, the apparent "smoothness" of atmospheric change in the geological past is only an illusion, attributable to the inadequate resolving power of paleoclimatic indi- cators. Most such indicators act to some degree as low- pass filters of the actual climatic chronology. Our more recent experience, based on relatively higher-pass filters such as tree rings, varves, ice-cap stratigraphy, and pollen analysis applicable to postglacial time, suggests that the state of the atmosphere has varied on most, if not all, snorter scales ot time, as well as over the longer geologic time scales. , . a. TRANSFER-FUNCTION ANALYSIS Since about 1960, advances in mathematical and statisti- cal techniques and the availability of high-speed com- puters that enable researchers to handle large amounts of data have for the first time made it possible to quantify climatic parameters derived from secondary sources (Fritts et al., 1971; Webb and Bryson, 19721. Furthermore, these quantified climatic parameters are provided in a form suitable for input into dynamic models of atmo- spheric circulation (CHEAP Project Members, 19761. By providing quantitative data for past conditions, it is be- coming possible to model the dynamics of past circula- tions and to test existing models of the present circulation with regard to their power of explanation (Gates, 19761. At the heart of these quantitative paleoclimatic records is the CHARLES W. STOCKTON concept of transfer-function analysis. Let the matrix X be a defined set of response variables that respond to climate measured over a specified realm of time and space. Let C be a measured set of physical indicators of climate, atmospheric or marine, measured over the same time-space realm and assumed to be caus- ally related to X. Let D be another set of physical parame- ters of the system, independent of the response of X. (D would typically include nonclimatic effects.) Then if D = 0, the system consists of X, C, and a set of climatic responsefunctions Re such that X = Re(C). (1) If D +0, the total response function Rig must be con- sidered, and X = R~(CD). (2) The result is calibration of the climatic signal inherent in the secondary series X, with measured values of the climatic variable or variables of interest. A fundamental problem of quantitative paleoclimatol- ogy is to find a set of transferfunctions ~ such that C can be estimated given X; i.e., C =~(X). (3) Generally, ~ is obtained by direct empirical methods and not by inversion of Re (or R if. The X and C used to derive the transfer function are the calibration data set. The X to which the transfer functions are applied is the climatic reconstruction data set. In the use of any transfer function, the investigator must make several basic decisions. There are fundamental problems concerning the assumptions used in writing any transfer function. Principal among these is the use of"the present as a key to the past." For example, if elements of the biota (in ocean-sediment samples) have evolved since the fossil deposit was formed, the calibration and recon- struction data sets are nonhomogeneous. Another prob- lem is the no-analogue situation, for which fossil values of certain taxa exceed the modern values used to derive the transfer functions. Both problems exist in tree-ring analysis. It is assumed that a tree responds to climatic inputs in a similar fashion throughout its lifespan such that one can make a homogeneous transition between the calibration data set and the reconstruction data set via the transfer function. One can conceive of a no-analogue situation wherein climatic events that have occurred in the past are not present in the calibration data set. As a direct test of any transfer function, the recon- structed data must withstand some sort of validation test to determine the accuracy of estimates of past climate.'$In general, five techniques are currently being used: i. Direct check; In tree-ring work, meteorological rec- ords are used to validate estimates of the reconstructed climate. This usually requires "holding back" some por- tion of the measured record from the calibration process for use in checking reconstructed values.

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Interpretation of Past Climatic Variability from Paleoenvironmental Indicators TABLE 2.1 Characteristics of Paleoclimatic Data Sources (after Kutzbach, 1975) Minimum Usual Continuity Potential Period Open Sampling Dating Variable of Geographical to Study Interval Accuracy Data Source Measured Evidence Coverage (yr) (yr) (yr) Climate Inference Ocean sediments (cores, < 2 cm/ 1000 yr) Ancient soils Marine shorelines Ocean sediments (common deep- sea cores, 2-5 cm/1000 yr) Ocean sediments (common deep- sea cores, 2-5 cm/1000 yr) Ocean sediments (common deep- sea cores, 2-5 cm/1000 yr) Layered ice cores Isotopic compo- sition of plank- tonic fossils; benthic fossils; mineralogic com- position Soil type Episodic Coastal features, reef growth Ash and sand accumulation Fossil plankton composition Continuous Global ocean Episodic Continuous Continuous Isotopic compo- sition of plank- tonic fossils; benthic fossils; mineralogic composition Oxygen-isotope Continuous concentration (long cores) Closed-basin lakes Lake level Episodic Mountain glaciers Terminal positions Episodic Ice sheets Bog or lake sediments Ocean sediments (rare cores, > 10 cm/1000 yr) Layered ice cores Layered lake sediments Tree rings Terminal positions Episodic Pollen-type concen- Continuous "ration, mineral- ogic composition (normal core) Isotopic composition Continuous of planktonic fossils; benthic fossils; mineral- ogic composition Oxygen-isotope con- centration, thick- ness (short cores) Pollen-type concen- tration (annually layered core) Ring width anomaly, density, isotopic composition Written records Phenology, weather Episodic logs, sailing logs, etc. Archeological Varied Episodic records 1,000,000 + 1000 + Lower and mid- 1,000,000 200 +5% latitudes Stable coasts, oceanic islands Global ocean (outside red clay areas) 400,000 +5% 200,000 500 + +5% Global ocean (outside red clay areas) Global ocean (above CaCO3 compensation level) Antarctica; Greenland Lower and midlatitudes 45 S to 70 N Midlatitudes to high latitudes 50 S to 70 N Continuous Antarctica; Greenland Continuous Midlatitude conti- nents Continuous Midlatitudes and high-latitudes continents Global Global 37 +5~o Surface temperature, global ice volume; bottom temperature and bottom-water flux; bottom-water chemistry Temperature, precipita- tion, drainage Sea level, ice volume Wind direction 200,000 500 + 200,000 500 + +5% 100,000 + Variable 50,000 1-100 (variable) 50,000 25,000 (common) 1,000,000 (rare) 10,000+ (common) 200,000 (rare) 10,000+ 200 20 Sea-surface tempera- ture, surface salinity, sea-ice extent Surface temperature, global ice volume; bottom temperature and bottom-water flux; bottom-water chemistry Variable Temperature Evaporation, runoff, precipitation, temper- ature Extent of mountain glaciers Variable Area of ice sheets +~% +5% 10,000 + 1-10 +1-100 10,000 + 1-10 +1-10 1000 1 1 (common) 8000 (rare) 1000 + 1 1 10,000 + Varied Temperature, precipitation, soil moisture Surface temperature, global ice volume; bottom temperature and bottom-water flux; bottom-water chemistry Temperature, accumulation Temperature, precipi- tation, soil moisture Temperature, runoff, precipitation, soil moisture Varied Varied

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38 o 0.1 ~ 0.2 o o.3 In o - E 04 - ~ o.s Q O 0.6 ~ 0.7 a: 0.8 0.9 . _ 1 ~ . ~ ~ ~ ~ ~ ~ 1.0 - 1.5 - 2.0 - 2.5 100 80 60 40 36.2 36.0 35.8 35.6 OBSERVATION: 6180 (%0) CaCO3 (%) FAUNAL INDEX (S) INTERPRETATION: Decreasing global Increasing CaCO3 Decreasing salinity ice volume dissolution 2. Comparison of two or more independently derived biologically based transfer functions: In ocean-sediment analysis this could mean comparing results obtained by using radiolaria to those obtained by using foraminifera. In continental regions, the results obtained by pollen analysis can be compared with those obtained by tree- ring analysis, or results based on tree-ring data from one site can be compared with those from another location. 3. Comparison of isotopic and biologically based esti- mates: Results obtained by isotopic analysis of fossil remains taken from ocean-sediment cores should corre- spond to those derived from species associations. 4. Concordant estimate: Independently derived trans- fer function applied to the same paleoclimatic indicator should produce similar results. Discord can result from the application of two different transfer functions based on one paleoenvironmental group or perhaps from use of one transfer function on more than one group of variables. 5. Synoptic consistency: On an intuitive basis, the spa- tial pattern and absolute range of synoptic maps of recon- structed climate must conform within reasonable varia- tions. Using the high-speed computer and numerical simulation techniques, intuitive evaluations can be made more rigorous and inclusive. In the end, reconstructed climatic data from all sources listed in Table 2.1 must fit together temporally. Another type of problem inherent in the use of transfer functions to derive paleoclimatic information is as- sociated with the mathematical manipulation of the data. Obviously, the researcher wants to choose the transfer- function technique that is most robust against various CHARLES W. STOCKTON FIGURE 2.2 Comparison of paleoen- vironmental indicators of climatic records for the past 1,000,000 years. Roman numer- als indicate inferred climatic periods (mod- if~ed from Figure A.14, U.S. Committee for the Global Atmospheric Research Program, 1975). types of distortion, most precise in terms of error, and most accurate in terms of reconstructed climatic values. Referring to Eq. (3), at least three such problems can be singled out for appraisal. The first problem is the selection and proper applica- tion of appropriate statistical techniques. Generally, some sort of multivariate technique is used. When this is the case, eigenvectors are usually used as a mode of joint behavior classification. A question then arises as to what criteria should be applied for inclusion and whether to use some sort of rotation. Most models currently being used are linear. Is it valid to assume linearity, and, if not, how does the use of a linear model affect the final results? Is it the best policy to utilize transformations? The second problem includes specifying the kinds of variables to be included in X and the space and time to be covered. Under what circumstances does ~ not exist? How does one define the distribution of samples in time and space to be used in the calibration data set? The third problem is the selection of variables and valid estimates of them for inclusion in matrix C. These data must represent a homogeneous reconstruction. For example, what climatic (or environmental) variables are most likely to influence the response and to what degree? Is the relationship linear, and, if not, is it reasonable to assume that the response can be approximated by a linear relationship? Is it wise to use secondary forms of vari- ables such as barometric pressure when it is known that the response is tied directly to such variables as precipi- tation and temperature? Many biological and sedimen- tary monitoring systems show significant lag in their responses to climatic variation. It becomes essential to

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Interpretation of Past Climatic Variability from Paleoenvironmental Indicators assess this effect and to include it in the transfer-function model. RE S ULTS OF S TU DI E S Although the recent attempts to quantify paleoclimate as derived from secondary sources are plagued with prob- lems, the degree of coherence in spatial and temporal variation that is achieved between reconstructions de- rived from different sources by different investigators has been most encouraging. Mitchell et al. (1975) have collected and assembled climatic interpretations based on secondary sources. These results have been used extensively in the rest of this section. LONG-TERM PALEOCLIMATIC INFERENCES (GLOBAL AND HEMISPHERIC SCALE) Major Ice Ages in the Past Billion Years Geological evidence leaves little doubt that, during the past billion years or so, the prevalent condition of macro- scale climate was one of relative warmth as much as 10C warmer than nowand almost total absence of polar ice. This warm condition was, however, punctuated by at least three major ice ages, each around 10 million years long and separated by a few hundreds of millions of years. Beginning roughly 50 million years ago, something appears to have brought about a gradual cooling. This cooling trend culminated, about 2 million years ago, in the arrival of a new major ice age (the Quaternary), char- acterized by a long sequence of perhaps as many as 20 major glacial-interglacial oscillations, which presumably continue to grip the world today. o 15 30 45 60 75 I ~ I ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ 1 1 5 8 11 14 0 30 60 90 11 -38 -32 -28 0.5 ~.2 ~.9 -1.6 FAUNAL INDEX ARBOREAL ICE CORE 6180 IN POLLEN 6180 PLANKTON SHELL FIGURE 2.3 Comparison of paleoenvironmental indicators of climatic records for the past 135,000 years (modified from Figure A.13, U.S. Committee for the Global Atmospheric Research Pro- gram, 1975~. 39 Glacial and Interglacial Stages of the Quaternary Detailed evidence of conditions within the Quaternary shows that periods of glacial extension (glacials) and retraction (interglacials) have alternated in a fairly regular sequence (Kukla, 1961; Broecker and van Donk, 19701. Figure 2.2 shows climatic records for the past million years as deduced from the geological and biological rec- ord. The first section of the figure shows the oxygen- isotope curve for Pacific deep-sea core V28-238, inter- preted as reflecting global ice volume (Shackleton and Opdyke, 19731. The relatively rapid and high-amplitude fluctuations are taken to indicate sudden deglaciations and are designated as the terminations I to VII. The second section shows the calcium carbonate percentage in equatorial Pacific core RC11-209 (Hays et al., 1969~. Low values are taken to indicate periods of rapid dissolu- tion by bottom waters. The third section shows the 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~. Note that each of the three records reflects a climatic fluctuation or "cycle" averaging about 100,000 years. This is particu- larly true during the past 450,000 years. Each cycle started with a short interglacial and ended with an equally short extreme glacial peak. These extremes rep- resent only 20 to 30 percent of the total duration of a typical cycle (length about 100,000 years), and the glacial itself can usually be subdivided into relatively warm interstadials and cooler stadials.* At the peak of the last glacial, 17,000 to 18,000 years ago, an ice sheet 2 km thick covered the northern and middle latitudes of North America as far south as New York, and another sheet in Europe reached as far south as Hamburg, Berlin, and Warsaw. Smaller ice caps and val- ley glaciers covered large areas of the Rockies, Alps, Andes, Hindukush, and many other mountain ranges. Because the volume of ice on the continents was some 50X106 km3 greater than today (Flint, 1971), the oceans stood about 100 m below their present level (Bloom, 1971~. Atmospheric and oceanic circulation, as reconstructed from available surface characteristics, greatly differed from present means (Lamb, 1971~. The mean annual temperatures were lower by about 3C at the equator and 10 to 12C in the midlatitudes of the northern hemisphere. The departure in the global mean was about 2C (C~MAP Project Members, 19761. During glacial maxima, vegetational and faunal zones in temperate regions were displaced to lower altitudes and latitudes as compared with their interglacial loca- * Interstadials are regarded as moderately warm interglacial periods not as extremely warm as present-day conditions. Simi- larly, stadials are moderately cold.

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40 lions. Highly continental climate, dry and with cold win- ters, characterized Europe and Central Asia. As a result, tundra and steppe replaced the pre-existing forests (Fren- zel, 1967~. Greater continentality and desiccation are similarly indicated for parts of Africa and South America (Fairbridge, 1972~. Figure 2.3 shows climatic records for the past 135,000 years: (a) A faunal index reflecting changes in foraminif- eral plankton in a core west of Ireland. The index is an estimate of August sea-surface temperature in degrees Celsius (Sancetta et al., 19731. (b) The percentage of tree pollen accumulated in a Macedonian lake (Van der Hammen et al., 19711. High values indicate warmer and somewhat drier conditions. (c) Oxygen-isotope ratio ex- pressed as PRO in an ice core at Camp Century, Green- land. This is interpreted as indicating changing air tem- peratures over the ice cap (Dansgaard et al., 19711. (d) Oxygen-isotope ratio in skeletons of planktonic foraminifera in a Caribbean core, interpreted as changes in global ice volume. High negative values reflect the melting of ice containing isotonically light oxygen (Emi- liani, 1964~. The interglacial periods were characterized by climate much like that of the recent past, with similar plant and animal distributions. These were periods of retracted ice sheets, high sea levels, and relative warmth. The amplitude of the climatic variations associated with the glacial cycles seems remarkably constant. It is likely that global mean values of atmospheric or oceanic variables differed little between successive interglacials (Emiliani, 1973~. Postglacial Climatic History Although comprehensive evidence is scarce, the earth is apparently now in an interglacial period. It began be- tween 10,000 and 14,000 years ago with general warming accompanied by the decay of the continental ice sheets. The climate of this period is characterized by several marked fluctuations, which appear to have occurred mostly simultaneously in the northern and southern hemispheres. This is especially true (Heusser, 1966) for the drastic variations between 12,000 and 10,000 years before the present (B.P). The limited data currently available suggest that the cool and relatively wet climate of the period was suddenly replaced by a worldwide mild, even warm, period. Even more dramatic was the later catastrophic readvance of the ice masses about 10,800 B.P., which killed entire forests in a period of probably less than a century (Lamb, 1966~. If we disregard some minor fluctuations during the recession of the large continental ice sheets, which dis- appeared completely about 6000 B.C. in Scandinavia and about 4000 B.C. in northern Canada, the climax of the postglacial warming was reached between 5000 and 4000 B.C. This was once more a worldwide phenomenon, where the annual temperatures were 2 to 3C warmer than today. Even in Alaska, Hey CHARLES W. STOCKTON were more than 1C warmer. This period has been de- fined as the "postglacial optimum" or "hypsithermal." Its mild climate, together with the relative dryness in large areas of North America and the Soviet Union, suggests a poleward displacement of the subtropical anticyclonic belt. The arctic sea ice had receded well north of its present position, but there exists no evidence for a com- plete disappearance of its central area north of about 80 latitude. At the same time (and after), the Sahara and the arid parts of the Near East were considerably more humid; this means that in now completely barren, arid areas there was a steppe vegetation produced by occa- sional severe rainstorms. Some evidence also exists for a northward extension of the tropical summer rain belt (Lamb, 19661. During this time, many smaller mountain glaciers completely disappeared, and the snow line was about 300 m higher than today. The sea level gradually rose to its present level but not above it (Shepard and Curray, 19671; after this date it was mainly controlled by the mass budgets of Antarctica and Greenland. A cool episode occurred between 4000 and 3000 B.C., signaled by He re-formation or expansion of mountain glaciers, followed by renewed warming (Figure 2.4~. Cooling and glacial advance took place again between 1400 and 500 B.C. The subsequent warming trend ended before A.D. 600, at least in western North America, where glaciers again advanced, culminating about A.D. 900. The climatic record becomes increasingly detailed and reliable beginning about 1000 years ago, mainly because of He availability of historical accounts in at least the Norm Atlantic sector. The mild conditions of the postgla- cial optimum were nearly reached once more during He early Middle Ages, culminating about A.D. 1200 when ice conditions around Iceland and Greenland were much less severe than today (Figure 2.51. Annual mean tempera- tures in southern Greenland must have been 2 to 4C above present averages (Table 2.2~. Oxygen-isotope ratios in the Greenland icecap (Figure 2.6) confirm the warmer climate there during tliis period. In England (Table 2.2 \ TREE GROWTH FLUCTUATIONS AT UPPER TREELINE A | \` / ~ i Mean /~\ / ~,1: .... 3000 Expansion t Contraction 1 ~ I 1 1 1 1 I ~ I I I I I 1 1 1 : I ~ I I ! 1 1 1 1 ~ ~ I I ~ ~ 1 1 ~ I I I I ~ ~ I 1' 1 2000 1000 B.C. 1 A.D. 1000 1971 IOLOCENE GLACIER F LUCTUATIONS FIGURE 2.4 Periods of low tree growth and glacial advance indicating cool periods in late Holocene time. Tree-ring data are from California; glacial data are mainly from northern hemi- sphere (Demon and Karlen, 1973; LaMarche, 1974).

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Interpretation of Past Climatic Variability from Paleoenvironmental Indicators TABLE 2.2 Average Climatic Conditions over England and Wales (after Mitchell et al., 1975)a Mean Temperatures (C) 41 Annual Annual Summer Winter rain- evapo- Dates (July- (Dec.- fall ration (approx.) Epoch Aug.) Feb.) Annual (mm) (mm) A.D. 1901 - 1950 Recent 15.8 4.2 9.4 932 497 A.D. 155~1700 Little Ice Age 15.3 3.2 8.8 867 467 A.D. 115~1300 Little Optimum 16.3 4.2 10.2 960 517 90(~50 B.C. Subatlantic 15.1 4.7 9.3 96() 979 482 a After Lamb ( 1966). and Figure 2.7), mean annual temperatures were about 1C above recent normals. In contrast to the situation of A.D. 500 to 900, when pronounced cooling of western Norm America is apparently not reflected in the Euro- pean record (Demon and Karlen, 1973), variations of the climate of these parts of the nor~em hemisphere seem remarkably similar during the past 1000 years. Northern hemisphere data suggest that the period after A.D. 1300 was one of widespread change to cooler condi- tions; this period has been termed the historic "Little Ice Age." Glaciers advanced in many parts of the world, but this brief episode cannot be compared with the great glacials lasting several tens of thousands of years. Worldwide glacial recession accompanied global temper- ature rises after 1895. Kutzbach and Bryson (1975) present plots of frequency versus percentage of variation in different climatic rec- ords.-One of these is reproduced as Figure 2.8. From this diagram it is apparent that, based on well-dated tempera- ture records from Central England and Iceland and an isotope record from Greenland, the data in the inter- mediate zone (500 to 1000 years) show considerable per- +0.8 +0.4 4" ~ r, ~ v 0 4 -0.8 11 900 1100 1300 1500 1700 1900 Year (A.D.) FIGURE 2.5 Departure of mean annual temperature in Iceland inferred from extent of sea ice during the past 1000 years. Departures from values for the period of meteorological record (Bargthorsson, 1962, as presented by Bryson, 1974~. sistence and the data at the higher frequencies (10 to 100 years) are nearly random. The authors point out the shortcomings of the study, including We lack of replica- tions from other records and the gap between periods of 500 and 1000 years, where spectral estimates are less reli- able in a statistical sense. They also stress the need for defining details of the climatic spectrum in the inter- mediate range of 500 to 1000 years. Climatic fluctuations at these time scales can have great impact on water resources, yet this is the least known portion of We spectrum. REGIONAL EVALUATION OF PALEO- HYDROLOGIC PHENOMENA It is now quite apparent that, although adequate data may be available for documentation of long-term climatic fluc- tuations on a global or hemispherical scale, the amount available for specific regions can be quite limited. How- ever, recent studies have shown the type of regional hydrologic information that can be obtained from paleo- climatic indicators. Within the framework of the national water demand criteria, we decided to focus on two distinct regionsthe Southwest and the Northeast for concentrated studies. -28 ~ 20e_3~{ :] Tar ; i; ,. . . . . . 700 500 300 100 Age, years before present FIGURE 2.6 Oxygen-isotope ratios in ice from the Camp Cen- tury Core, Greenland. Low values (top of graph) indicate low temperatures. Vertical scale is relative departure of i80 constant compared with a standard (Dansgaard et al., 1971).

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42 ~ 10.5 car . U=J ID 10.0 4-= 9.5 . c: ~ _ - 0.7 E 0.1 r o.o 1 ~ _ ~ ,` ,' N. hemisphere (40 - 70N) t `` (Mitchell, 1961 ) Oaf\ / . - 1 i+30 Q- Central England / c ~ `` ^` ~ ' \Ct' ] +5 ~ :t \__' MEAN ANNUAL TEMPERATURE ~1 TREE GROWTH AT UPPER TREELINE -I _ i , . , , , . , . i 1000 1500 1960 Year (A.D.) FIGURE 2.7 Estimated temperatures in Central England since A.D. 900 compared with tree-ring variations in California indi- eating general cooling in northern hemisphere between A.D. 1300 and 1900 (LaMarche, 1974). These areas are, respectively, one of the fastest growing areas of population and the area of present greatest popu- lation. We have attempted to accumulate long-term in- formation concerning climate for these two regions. Southwestern United States Stockton (1975) and Stockton and {acoby (1976) have attempted to show how long-term total annual runoff ~ 40 a, c, o ~ 30 Q . _ C1 Ad N o 10 wh ite noise ~ CENTRAL ENGLAND. botanical record 11 . ',i ; ; - i ; 1 1 .. . 1 1 ~ / CENTRAL ENGLAND, / historical record / I CE LAND, / / historical record / / CENTRAL ENGLAND / // instrumental record ~ / G R E E N LA N D, ~ 1 80 record ~~\ ~7 ~._~,/f white noise continuum `~ W<~_~ o .01 .02 .03 .04 .05 f ~ (cycles per year) 30 20 P (years) 1000 100 50 FIGURE 2.8 Composite variance spectrum of temperature on time scales of 10 to 103 years derived from instrumental, histori- eal, botanical, and oxygen-isotope records (after Kutzbaeh and Bryson, 1975~. CHARLES W. STOCKTON records can be reconstructed utilizing tree-ring data. Their study provides detailed reconstructions of total annual runoff for several subbasins within the Upper Colorado River Basin. In addition, it suggests that, within a larger basin, the tributary systems can show varying degrees of persistence. For example, Figure 2.9 shows the variance spectra computed for three tributary rivers within the Upper Colorado River Basin. The drainage areas of the three are not greatly different; however, the degree of persistence in the reconstructed records differs substantially. The Green River shows a greater tendency for low-frequency variation than either the Colorado above Cisco or the San Juan at Bluff. It appears that the tendency for long-term climatic persistence may be greater in the Green River Basin than for either the Colorado above Cisco or the San Juan above Bluff. This AU T O S P E C ~ R U M Lags = 56 deg. f reedom = 14 10 o I!) 1.- o .:/ . . . . DECO LO R ~ DO at C I SCO A.... 1~, 1"~\ ' : ~ (\\ I'll ~ '~j I .. I ~ 19.8 76 .: i '_' , I ~ \ ~ ; . . . . . . . . . a' ~ 11~\ ! ! ~ S A N J U A N ,! ~ `; Y""1 . A' . 4.0 2.6 2.0 PERIOD (years) FIGURE 2.9 Comparison of the sample autospeetral functions for the long-term reconstructed runoff records for the Green River at Green River, Utah; the Colorado River at Ciseo, Utah (Colorado mainstem); and the San Juan River at Bluff, Utah (Stockton, 1975~.

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Interpretation of Past Climatic Variability from Paleoenvironmental Indicators UNFILTERED ~ to TIC C air ~ ? o z Ct J IS Z O O ~ T ' 1600 rid I ~ ~ ~ . 1800 1900 f ILTEREO ' I ' ' am'- 1 '' ' ' ' ~ ' ' ' ' 1 ' ' ' ' ~ ' ' ' ' 1 ' ' ' ~ ~ 1700 1800 1900 GREEtd RlvER AT GREEN Rl\/ER. UTAH 3'0 c :: ~~j ~~` \!~: ~ `~U'-t . . , 1600 1 700 1800 UNFI LTERE D FILTERED ~ . ~ . . 1 900 ~ lo ~ o ~ to ~5 ~ ~ ~ ~ ~ _ ~ 1600 ' 1 - 1700 1800 1900 COLORADO RIVER At LEE FERRY. ARIZONA FIGURE 2.10 Reconstructed hydrographs for total annual runoff for the Green River at Green River, Utah, and the Colorado River at Lee Ferry, Arizona. In each case, the lower graph is the same data but with Me high-frequency components (those with a frequency greater than 10 years) removed (Stockton, 1975). suggests the need for evaluation of paleoclimatic varia- tion in rather limited areas. The long-term hydrograph for the Upper Colorado River Basin as a whole does not exhibit the degree of long-term persistence that is found in the Green River reconstruction (Figure 2.10~. However, analysis of the persistence does show that the series is significantly different from random and is best modeled by a mixed autoregressive-moving average scheme. In addition, the reconstructed hydrograph shows that the early part of the twentieth century was characterized by a period of anomalously high sustained flow, the longest in the entire 450-year reconstruction. The gauged record alone would not reveal this fact, so anyone depending solely on the gauged record would obtain inflated estimates of mean annual flow and variance. The reconstructed hydrograph is consistent with the secular variation shown in tempera- ture trends as illustrated by LaMarche (1974) (see Figure 2.7~. Also, the degree of persistence seems to be of the same magnitude as that suggested by Kutzbach and Bry- son (1975) for records of similar length (Figure 2.8~. LaMarche (1973) studied tree-line changes in the White Mountains of east-central California and found 43 that, between A.D. 1300 and 1600, abrupt climatic change resulted in lowering of the timberline some 70 m. He attributed this to an apparent climatic change to much colder summers or to fairly cold summers and drier springs, autumns, and winters. Judging from later work, this condition apparently lasted at least up to the early 1900's. This example serves to illustrate two points. First, that apparently climatic change can occur over a rela- tively short time period (hundreds of years) and, second, that there is additional evidence for the large flow anomalies as reconstructed in the hydrograph for the Upper Colorado River (Figure 2.10~. From the foregoing evidence, it appears that within the southwestern United States climatic change has occurred during the past 500 or so years, that it has occurred over a fairly short time span, and that it has been reflected in the annual runoff, at least for the Upper Colorado River Basin. Northeastern United States The only currently existing detailed regional analysis using historical data for the northeastern United States is

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44 FIGURE 2.11 Annual temperatures and annual precipitation totals for the eastern seaboard of the United States for the period 173~1967a representative, reconstructed synthetic series centered on Philadelphia (after Landsberg et al., 1968~. ~ 45.0 ', 44 n CHARLES W. STOCKTON 58r 5'S 55 be 54 52 51 50 49 15 - 114 1740 1760 1780 1800 1820 1840 1860 1880 1900 1920 1940 1960 Year 47.0 46.C 13 1 1 .. 10 120 _ 115 38.0 _ 37.OI ~ 1 1 t 1_ ~ 1 1 1730 1750 1775 1790 1810 by Landsberg et al. (1968~. They used historical climatic records along the eastern seaboard, centered on Philadel- phia, and a least-squares technique to reconstruct provi- sionally a 230-year record of temperature and precipita- tion (Figure 2.11~. They noted a trend in the annual temperature data principally "caused by the lack of cold years since the turn of the 20th century." The trend is also confirmed by a variance spectrum analysis. No mention is made of the annual precipitation series, but Figure 2.11 illustrates the anomalous wet period extending from about 1830 to 1880. Although considerable paleoclimatic work has been done in the northeastern section of the United States, much of it is still qualitative. Many of the results are in the form of pollen data and tree-ring data. Using pollen data, Webb and Bryson (1972) presented quantification of July mean temperature, summer precipi- tation, and precipitation minus potential evaporation for an area in north-central United States. This investigation was later expanded by Bryson (1974) to cover a larger area. These reconstructions cover the past 15,000 years and show an extreme temperature drop (as much as 8C) at about 10,000 years B.P. Blasing (1975) shows reconstruction of climatic types on a national scale and indicates that certain climatic anomaly patterns have been more prevalent during the previous two centuries than in this one. However, his conclusions are based on reconstructions of large-scale ~ ~ _- 1830 1850 1870 1890 1910 1930 1950 19 70 Year atmospheric circulation patterns, derived from tree-ring data in western North America. CONCLUSIONS The methods of quantitative paleoclimatology enable us to increase our knowledge of many details of climatic history. By increasing our knowledge of past climate, we gain a valuable perspective to our view of climate of the present and future. There exist, at present, isolated time series that indeed suggest important climatic changes at all time scales. However, the job of transforming this information into spatial maps so that we can study patterns of change with adequate spatial detail is just beginning. Until these maps exist, we cannot accurately characterize periods as warm or cool, wet or dry except at specific locations. This will not occur without concentrated research efforts and con- siderable support in the future. RE FE RE NC E S Blasing, T. J. (1975). Methods for analyzing climatic variations in the North Pacific sector and western North America for the last few centuries. Ph.D. dissertation, University of Wisconsin, Madison.

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Interpretation of Past Climatic Variability from Paleoenvironmental Indicators Bloom, A. L. (1971). Glacial-eustatic and isostatic controls of sea level since the last glaciation, in The Late Cenozoic Glacial Ages, K. K. Turekian, ea., Yale U. Press, New Haven, Conn., pp. 355-379. Broecker, W. S., and J. van Donk (1970). Insolation changes, ice volume, and the ~80 record in deep sea cores, Rev. Geophys. Space Phys. 8, 169. Bryson, R. A. (1974). A perspective on climatic change, Science 184, 753. Chin, W. Q., and V. Yevjevich (1974). Almost-periodic, stochastic process of long-term climatic changes, Colorado State U. Hy- drology Papers, No. 65. CHEAP Project Members (1976). The surface ofthe ice-age earth, Science 191, 1131. Dansgaard, W., S. T. Johnsen, H. B. Clausen, and C. C. Langway, Jr. (1971). The Late Cenozoic Glacial Ages, K. K. Turekian, ea., Yale U. Press, New Haven, Conn., pp. 267-306. Denton, G. H., and W. Karlen (1973~. Holocene climatic changes, their pattern and possible cause, Quaternary Res. 3, 155. Emiliani, C. (1964~. Paleotemperature analysis of Caribbean cores P6304-8 and P6304-9 and a generalized temperature curve for the past 465,000 years, J. Geol 74, 109. Emiliani, C. (1973~. Quaternary hypsithermals, Quaternary Res. 3, 270. Fairbridge, R. W. (1972). Climatology of a glacial cycle, Quater- nary Res. 2, 283. Flint, R. F. (1971~. Glacial and Quaternary Geology, John Wiley and Sons, New York. Frenzel, B. (1967). Die Klimaschwanhungen des Eiszeitalters, Brunswick. Fritts, H. C., T. J. Biasing, B. P. Hayden, and J. E. Kutzbach (1971~. Multivariate techniques for specifying tree-growth and climate relationships and for reconstructing anomalies in paleoclimates, J. Appl. Meteorol. 10, 845. Gates, W. L. (1976). Modeling the ice-age climate, Science 191, 1138. Hays, J. D., T. Saito, N. D. Opdyke, and L. H. Burckle (1969). Pliocene-Pleistocene sediments of the equatorial Pacific, their paleomagnetic, biostratigraphic and climatic record, Geol. Soc. Am. Bull. 80, 1481. Heusser, C. J. (1966~. Polar hemispheric correlation: palynologi- cal evidence from Chile and the Pacific Northwest of America, in World Climate,from 8000 to 0 B.C., J. S. Sawyer, ea., Royal Meteorol. Soc., London, pp. 124-141. Imbrie, J., J. van Donk, and N. G. Kipp (1973~. Paleoclimatic investigation of a late Pleistocene Caribbean deep-sea core: comparison of isotopic and faunal methods, Quaternary Res. 3, 10. Kukla, J. (1961). Quaternary sedimentation cycle, Survey of Czechoslovak Quaternary, Institut Geologiczny, Prace, 34, pp. 145-154, Warszawa. Kutzbach, J. E. (1975). Diagnostic studies of past climates, in The Physical Basis of Climate and Climate Modeling, CARP Publi- cation Series No.16, WMO, Geneva, Switzerland, pp. 119-126. Kutzbach, J. E., and R. A. Bryson (1975~. Variance spectrum of Holocene climatic fluctuations in the North Atlantic sector, in Proceedings of the wMo/IAMAP symposium on long-term climatic fluctuations, WMO, No. 421, pp. 97-104. LaMarche, V. C. (19731. Holocene climatic variations inferred 45 from beeline fluctuations in the White Mountains, California, Quaternary Res. 3, 623. LaMarche, V. C., Jr. (1974~. Paleoclimatic inferences from long tree-ring records, Science 183, 1043. Lamb, H. H. (1966~. The Changing Climate, Methuen, London. Lamb, H. H. (1969~. Climatic fluctuations, in World Survey of Climatology 2~5), 17~249. Lamb, H. H. (1971~. Climates and circulation regimes developed over the northern hemisphere during and since the last ice age, Paleogeog. Paleoclimatol. Paleoecol. 9, 125. Landsberg, H. E., C. S. Yu, and L. Huang (1968~. Preliminary reconstruction of a long time series of climatic data for the eastern United States, Univ. Maryland Inst. Fluid Dynamics, Appl. Math. Tech. Note BN-571, 30 pp. Mitchell, S. M., V. C. LaMarche, E. S. Epstein, P. R. Julian, M. F. Meir, and B. J. Kukla (1975~. Chap.2, Variability of the climate of the natural troposphere, in Department of Transportation Climatic Impact Assessment Program, Monograph 4. O Connell, P. E., and J. R. Wallis (1973~. Choice of generating mechanism in synthetic hydrology with inadequate data, Int. Assoc. Hydrol. Sci., Madrid Symp., June. Prell, W. L. (1974~. Late Pleistocene faunal and temperature patterns of the Columbia Basin, Caribbean Sea, Geol. Soc. Am. Special Paper. Sancetta, C., J. Imbrie, and N. G. Kipp (1973). Climatic record of the past 130,000 years in North Atlantic deep sea core V23-82: correlation with the terrestrial record, Quarternary Res. 3, 110. Shackleton, N. J., and N. D. Opdyke (1973). Oxygen isotope and paleomagnetic stratigraphy of equatorial Pacific core V28-238: oxygen isotope temperatures and ice volumes on a 105 and 106 year scale, Quaternary Res. 3, 39. Shepard, F. P., and J. R. Curray (1967). Carbon-14 determina- tions of sea level changes in stable areas, in Progress in Oceanography, Pergamon Press, New York. Stockton, C. W. (1975~. Long-term streamflow reconstruction in the upper Colorado River basin using tree rings, in Colorado River Basin Modeling Studies, proceedings of a seminar held at Utah State University, Logan, Utah, July 1~18, 1975, C. G. Clyde, D. H. Falkenborg, and J. P. Riley, eds., Utah Water Research Laboratory, College of Engineering, Utah State U., Logan, Utah, pp. 401~41. Stockton, C. W., and G. C. Jacoby (1976). Long-term surface water supply and streamflow levels in the upper Colorado River basin, Lake Powell Research Project Bull. (in press). U.S. Committee for the Global Atmospheric Research Program (1975). Understanding Climatic Change: A Program for Ac- tion, National Academy of Sciences, Washington, D.C. Van der Hammen, T., T. A. Wijmstra, and W. H. Zagwijn (1971~. 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