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Water Supply and the Future Climate 1 STEPHEN H. SCHNEIDER and RICHARD L. TEMKIN National Center for Atmospheric Research WHAT IS CLIMATIC CHANGE? This chapter addresses a variety of questions related to issues of understanding the climate and how it might change and, additionally, how climate is related to the water supply. Many conflicting aspects of climate have recently been discussed in the news. One hears about approaching ice ages on the one hand and the melting of the ice caps on the other with both natural and human- induced postulated causes. For example, Reid Bryson (in Alexander, 1974) has said that "there is very important climate change going on right now, and if the trend continues, will affect the whole human occupation of the earthlike a half billion people starving." On the other hand, former U.S. Secretary of Agriculture Earl Butz has been quoted to the effect that such statements are at best without scientific bases and are at worst apocalyptic nonsense. Obviously, there is much confusion on the issue. Although the following discussion may add little to reduce the uncertainties, it is an attempt to show the range of arguments used, with a further attempt to tie the issues to questions of water supply in order that one may 25 gain a feeling for the types of uncertainties that decision makers may have to face in the future. Figure 1.1 shows some long-term temperature records extracted from ocean-sediment cores over the last 700,000 years (Emiliani, 1972~. There remains considerable dispute about the mag- nitude of the temperature fluctuations, which are on the order of 5C, but the figure does, nonetheless, show an important point: over this several-hundred-thousand-year period there were fairly large excursions in temperature, with the cold and warm periods known as glacials and interglacials, respectively, these being separated in time by some 10,000 to 100,000 years. From a statistician's point of view, this record might appear to resemble a stationary time series. However, from the perspective of a human lifetime, or for that matter all of human history, the record contains dramatic climatic changes. Hence one may raise the question: "What is climatic change?" In reply, it should be stated that climate is a time average of the instantaneous state of atmosphere (i.e., the weather events) and that the weather itself is unpredictable in detail past a few weeks (see CARP, 1975, for a review).

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26 - 304 25- c~ _ 20- 0 100 200 300 400 TODAY ~ TIM E ( X 1000 YEARS AGO) FIGURE 1.1 Long-term temperature trends extracted from ocean sediment cores (after Emiliani, 1972). Some people, in fact, believe it to be unpredictable in practice after only a few days, but it is theoretically possible that some skill of weather prediction exists up to the period of a few weeks. The atmosphere scrambles itself to a point where there is practically no recognition of its initial condition after some two to four weeks. Therefore, any climatic average that one takes that is longer than that predictability period is in essence av- eraging a fluctuating time series of unpredictable weather events. Does this mean that there can be no climatic predictability a month or a season or a decade ahead? The answer is clearly "yes" for weather but maybe "no" for climate, since there is no established theoretical reason why a time average of weather must also be unpredicta- ble. From this average one would not be able to predict, for example, on what day in the next month and where a storm is going to take place; but one might be able to predict a few months into the future the number of storms that may pass through a given region in a given time. This latter possibility still remains theoretically conceivable with above-zero probability. In 1974, at the Global Atmospheric Research Program Conference on Climate Modeling (held in Stockholm), a large number of experts attempted to set up a plan for future climate research (GARP, 1975~. On the first day of the conference, about a dozen modelers, who were mainly interested in discussing questions about parame- terizing their models and deciding what physical theories and observations were needed for their verification, de- cided that in the first half hour they would dispose of the issue of a definition of climate. Hours later they were still working on this definition. Essentially, the definition began from the obvious fact that climate is just a time average of weather. However, this being rather imprecise implied the need for some subdefinitions, e.g., the "climatic time series," which was defined as the time series of some fluctuating climatic parameter, say, rain- fall. A "climatic sample" was also defined. It is the length of time over which one computes statistics for that time series. The "climate" is simply the statistical properties of the climatic time series taken over a specified sample period. When one takes two climatic samples and finds that they are different, "climatic change" is further de- fined. One still needs to investigate the statistical signifi- cance of those differences. That is, one must ask: Does the STEPHEN H. SCHNEIDER and RICHARD L. TEMKIN difference in the climate of the two samples occur be- cause of unpredictable fluctuating components, e.g., the daily weather, or because of changes in the long-time statistics (or an ensemble mean) traceable to some varying physical forcing mechanism, e.g., usually a boundary forc- ing factor such as a volcanic dust veil? From the above it is clear that in studying climatic change the length of the averaging period and the size of the averaging region are quite important. In fact, the physical processes that are most influential on the short time scales could well be different from those that operate on the long ones. Thus, a result of the Stockholm meeting (as seen from the participants' lengthy attempt to define climate) is that anyone referring to "climate" should be certain that He sample length is more than three weeks (which is more than the period of weather predictability) and that extreme care should be used in specifying the averaging period and statistical procedure. It is important that precision be used in the specification of the defini- tion of climate. In the remainder of this paper, samples and averages will be considered for much shorter terms than ice ages and for periods longer than three weeks. Figure 1.2 shows that the climate fluctuates on all time scales (U.S. Committee for the Global Atmospheric Re- GENERALIZED TRENDS IN GLOBAL CLIMATE Air Temperature COED WARM 1960 ' ' <' 1920 _ 880 - ~~N O .2 ~ .6 to C 900 COED WARM 1900 _ ~ ~500 _~i 1300 _ ~ 1100 - (EASTERN- - ~EUROPE- (a)THE LASTIOO YEARS I ~1.5C I (b)THE LAST 1,000 YEARS Mid-Latitude Air Temperature COLD WARM - ~ o In ~ 10 o 15 In C] 20 . In 0 25 I 30 . LEGE N D 1. Thermal Maximum of 1940s 2. Little Ice Age 3. Cold Interval 4. Present Interglacial (Holocene) 5. Last Previous Inter- glacial (Eemian) s COLD WARM o CO In LU o .: IOC In 0 12e 2e 5C 7e 150 \ 1 1 1 1 ~ 10C ~ 10C (c) THE LAST 10,000 YEARS (d ) THE LAST 100,000 YEARS FIGURE 1.2 Generalized trends in global climate represented by approximate surface-temperature patterns that prevailed over a variety of time scales for the past lSO,OOO years.

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Water Supply and the Future Climate search Program, 19751. In the past 1000 years, tempera- ture has fluctuated on the order of 1~/2C. This may seem to be of minimal magnitude, but, from Figure 1.1, it is recalled that the magnitude of the glacial to interglacial oscillation was only about 5C globally, although actually many times this at the high latitudes (Matthews et al., 1971~. The cool period in Figure 1.2(b) between about 1400 and 1800 is known as the Little Ice Age in Europe and shows a decrease in temperature of only about 1C. A change of this magnitude for food crops can be put in perspective when one notes that with a couple of degrees movement northward in latitude through the central part of the United States one would find that a 10-day drop in growing season is matched roughly by a 1C drop in average temperature. The implication here is that a large-scale areal change in temperature of some 1C is not necessarily trivial for people. Finally, the recent record EFigure liars shows a hemispheric warming of about half a degree to 1940, followed by a cooling. Again a problem exists with this record in that it is really an estimate of a hemispheric average. Even though it is based on instru- mental observations taken at many points, there still re- main large areas left uncounted or uncovered by mea- surements. When referring to it as a hemispheric average, sampling error bars on the order of tenths of a degree should probably be placed on this record. It is clear then that difficulties exist even in pinning down past climatic changes to a very high degree of accuracy. It has also been mentioned that human societies are vulnerable to small changes in the climatic system and that a hemispheric temperature change of only a few degrees, or even perhaps tenths of a degree, can be important. PHYSICAL FACTORS CAUSING CLIMATIC CHANGE Figure 1.3 (from Schneider and Mesirow, 1976) contains a very simple overview of the mechanisms that drive our climate. The straight arrows represent the energy coming from the sun, of which annually about 30 percent is re- flected (the reflectivity is called the earth's albedo). The wavy arrows represent the outgoing planetary infrared radiation. It is well known that the tropics are warm because they receive more heat from the sun than they emit as infrared radiation, and the poles are cold because of a greater average solar zenith angle and the presence of more highly reflective ice- and snow-covered surfaces. Thus in a simple picture the warm air rises in the tropics and moves poleward. As the air rises, it takes with it the fast rotational speed of the equator, which provides the momentum that creates the westerly winds in the mid- latitudes. Because the circulation is driven by the equator-to-pole temperature difference, the system is much more vigorous in the winter hemisphere than in the summer hemisphere. This equator-to-pole temperature difference driven cir- culation is quite important, since a variation in global 27 Terrestrial Infrared Radiation Reflected Sunlight ~ ~ < Ode \ Incoming Sunlight Winds . ~ ~>4 ,,, ~ \ Ode Winds -Equator Tropics- \ ~~: / my\ Terrestrial Infrared Radiation Reflected Sunlight FIGURE 1.3 Schematic illustration of how the major weather systems of the earn are driven by the unequal heating between the equator and the poles. The tropics intercept a much larger fraction of Me incoming solar energy than do the polar zones, thus giving rise to the motions that regulate We climate. mean temperature is usually amplified at the poles. For example, from Figure 3.6 of Matthews et al. (1971), it is seen that the major recent temperature change (in the northern hemisphere at least) occurred in the higher latitudes. Some theoretical studies (e.g., Manabe and Wetherald, 1975) also suggest that a change in global temperature from changed surface heating will be amplified at the poles and thus will result in a change in the equator-to-pole temperature gradient. Since the circu- lation systems depend on this gradient, a seemingly small change in global temperature may thus latitudinally shift the average location of the circulation systems. Based on both some theoretical considerations and observations, Bryson (1974) argues that if the north polar region cools more than the equatorial region, the Asian and African monsoon belts get compressed, that is, they would move slightly equatorward, and the midlatitude baroclinic zones, the westerly belts, would also move toward the equator. Comparable evidence (e.g., Kellogg, 1977) exists suggesting from reconstructions of climatic warm periods that the opposite occurs when the poles warm more than the equator. This hypothesis is beginning to appear to have some theoretical support, since some re- cent numerical modeling experiments also show a tem- perature gradient monsoon effect. For example, Gates (1976) and Williams et al. (1974) have shown with numeri- cal simulations that during the last (Wisconsin) ice age, the monsoon rains were weakened in their simulations. Nevertheless, the connection between circulation re- gimes and equator-to-pole surface-temperature gradient is still based on fragmentary evidence and thus remains somewhat controversial and qualitative.

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28 The point to be made here is that marginal, seemingly small changes in zonal or hemispheric means could in turn, cause large changes in local regions that are near boundaries of different circulation regimes. But even if one of these shifts did occur, it would not neces- sarily mean that the "end of the world" has come. If the globe warmed up a degree or two and if the U.S. corn belt then moved from the Iowa area northward by a few hundred kilometers, the result might be but a small per- turbation from a global evolutionary point of view to world food supplies and the earth's carrying capacity. However, in the present world situation in which people are locked into national boundaries, and there is little global food reserve, such marginal shifts could be serious (see the discussion in Schneider and Mesirow, 19761. Since crops are usually planted based on the pre-existing climatic conditions and their expected continuance, slow and gradual changes can be anticipated before crops are planted, thus perhaps avoiding a crisis. One of the main points to make here is that if changes should come quickly, a catastrophe might well result. What causes the climate to change? Obviously, the output of the sun is very much implicated. Figure 1.4 (Thompson, 1973) is a plot of the double sunspot cycle. Since there are not negative sunspots, the part below the X axis is just the second half of the double sunspot cycle when the magnetic polarity of the spots has reversed. The figure further shows that there have been droughts in the U.S. high plains that correspond to alternate minima in the double sunspot cycle. A question arises as to whether a new drought will be beginning in the late 1970's, since the sun has recently passed through another minimum. Problems, however, arise as to why there may be a good correlation here. One could ask: Why do something like droughts in the U.S. plains correlate with sunspots and not some other atmospheric variable represented by a long-term continuous record? These droughts occur, apparently, in different parts of the plains, and they move around. Nor is their occurrence On lL id o A an J 43: Z 1740 1760 1780 1800 1820 1840 1860 1880 1900 1920 1940 1960 1980 YEAR l Solar Cycle And Drought In Nebraska Drought Periods H 1 H H I I H H H ? 1 1 1 1 1 1 1 1 1 1 1 FIGURE 1.4 A plot of the double sunspot cycle versus drought in Nebraska. The graph suggests that droughts in the U.S. Great Plains tend to occur in a 22-year cycle centered near the minima of that cycle. If this relationship holds, then the next such drought is "due" in the last half of the 1970's (after Thompson, 1973). STEPHEN H. SCHNEIDER and RICHARD L. TEMKIN precisely timed with alternate sunspot minima. But, why do they occur in the plains? Others have looked for periodicities to match with the sunspot cycles, of which there are thousands of possibilities. The problem is this: without a physical connection one can get oneself in statistical trouble. Figure 1.5 (Stetson, 1937) shows an example of the sort of work that has been done in this area. Here, the sunspot number is correlated with the Dow tones stock market average, and the quality of wine vintage, or number of automobiles. In Figure 1.6 (Stetson, 1937) there is an even more fascinating "correlation" sunspot number and rabbit population. This type of corre- lation attempt may be disturbing to us as scientists; but perhaps vegetation is affected by sunspots. If one has a physical theory, then one has some confidence in such statistical correlations that is, one could take a time series of the variability of the sunspot cycle and look around for geophysical phenomena like wine harvests or ant 1924 1926 1928 1930 1932 1934 <,, 500 ~t 450 _ (I 400 _ O 350 _ C) 300 _ A: 250 _ of 200 _ I 150 _ 100 10O Pn~ 40 20 O _ h 1925 1927 1929 1931 1933 .... ..... 1925 1 927 1931 1933 1935 1937 FIGURE 1.5 Plots of the Dow Jones stock market averages, wine vintages, and automobiles compared with the number of sunspots (after Stetson, 1937).

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Water Supply and the Future Climate 70 60 50 40 30 20 In A: J o 11 o z c/) z O J J 1 I< J e - ... ... 1923 1925 1927 1929 100 An an on O- 1935 1 937 1925 1927 1929 1931 1933 1935 1937 FIGURE 1.6 Plots of business, rabbit population, building con- tracts, and tree growth compared with the number of sunspots (after Stetson, 1937). droughts, and then one might find some fit. But if one looked at a hundred of these possibilities and found one with 99 percent confidence, then one should question this kind of correlation on both statistical and physical grounds. On the other hand, there may be something physical there, nonetheless. But one must be extremely cautious. The fact is, of course, that sunspots are a visible man- ifestation of change on the element that provides us with our primary energy source. Could there also be a change in the "solar constant" with sunspots? Nobody has mea- sured the solar constant to better than 1-2 percent. Yet present models indicate that a change of this magnitude 29 would be sufficient to cause the kind of climatic changes seen in Figure 1.2. Eddy (1976), in his recent analysis of historical records, has shown that the sun may have lost its spots from about 1640 to 1700, which corresponds with the fairly consistent and strong temperature drop on most of the paleoclimatogical records for this time. Whether or not this is indicative of change in the solar constant is still an open question (see Schneider and Mass, 1975~. Again, why are the droughts seemingly localized near Nebraska, and why is the cycle not reflected in many other global records? Some have invoked physical mechanisms other than solar constant variation with sunspots (e.g., Dickinson, 1975), namely, changes in the magnetic field of the earth, which modulate galactic cosmic rays, which create parti- cles that form in the stratosphere, which, in turn, might make cirrus clouds, which can perturb radiation fluxes in the climatic system with enough energy to explain some terrestrial climate fluctuations. The real problem is that in the absence of either adequate measurement or adequate theory, one is basically left with the statistical correla- tions; and in the absence of theory, a fight is quite likely. At least at present there are some people seriously look- ing at the theoretical aspects of a solar-climate relation- ship. Another possible cause of climatic change with sup- porting evidence is volcanic eruptions. Figure 1.7 (Ellis and Pueschel, 1971) shows the apparent transmittance of solar direct beam radiation at Manna Loa in Hawaii at the 3000-m height observatory. In 1963, there was about a 2 percent drop in the direct solar radiation reaching the observatory, although 75 percent of this attenuated direct beam would still reach the surface because of scattering in the forward direction. This decrease occurred im- mediately following the Agung volcanic eruption. Note that roughly a half percent decrease in total solar energy reaching that station occurred and lasted for several years following that eruption. If model calculations are right, a decrease in solar radiation of that magnitude might drop the earth's global temperature on the order of several tenths of a degree. Volcanism is thus another potential mechanism of climate change. On short time scales, and maybe even longer ones, there is always the possibility of internally caused cli- matic changes. The atmosphere could be viewed as a very fast oscillating device connected to the oceans by some spring, with the oceans being an even larger mass con- nected by a bigger spring to another mass, which are the glaciers. This whole system is an oscillatory one. By analogy, there is difficulty in determining whether some of the observed climatic changes are due just to the internal vibrations of this system, i.e., the redistribution of energy among the main reservoirs the atmosphere, the oceans, and the glaciers or are, in fact, due to exter- nal pushes by variations on the sun or by volcanoes. A problem lies in separating out the external factors the volcanoes, the sun, or even carbon dioxide from human activitiesfrom the internal redistributions. Based on

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30 IL An He 93 en ~ Z Z ~ cry 92 llJ ~ Q By _ 111 1 1 1 1 1 1 1 1 1 1 1 1 94 ~ ~ to - LIT 1-1I[14=~ II I S~. , . . ~ ~ ~ ~ ~ It I I f I t I I ~ I I 1958 1959 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 T I ME (YEARS) 91 90- essentially intuition, one might speculate that the shorter-time-scale fluctuations are internal, but the long- term changes are externally caused. The problem is that no quantitative theory of climate exists to end the intui- tive speculations. CLIMATIC MODELS AS ESTIMATORS OF CLIMATIC CHANGE There is one other important "externality" in the system, and that is people. Technically, this is not the traditional economic usage of the term "externality" (i.e., external economies or diseconomies). Since people are part of the biota, which are in our definition part of the climatic system, people are not truly an externality to the climatic system. However, in an important sense the social cost of many peoples' activities (e.g., those that release carbon dioxide to the atmosphere) does represent a situation where the producers (or users), who benefit from the activity that generates, say, this carbon dioxide, do not pay for the costs at the time of energy usage. Instead, the costs will be "externalized" to future generations. Thus, the analogy to the economic meaning of externality is valid. Figure 1.8(a) is a graph from a projection made in 1971 (Machta, 1971) of the carbon dioxide concentration in the atmosphere. Burning of fossil fuels puts carbon dioxide into the atmosphere, of which roughly a half to three fourths stays in the air, with the remaining fraction taken up by the oceans and biosphere. It also appears rea- sonably certain that an increase in carbon dioxide would affect the radiation balance of the earth in such a way as to warm the earth's surface. But quantitatively, how much of a heating might occur is another unanswered question. One has to go to a model to make such estimates because Here are no available historical data to perform an actuar- ial study. There is no analogy in geological history (re- cent at least) from which one can obtain a scaling factor for the climatic response to anthropogenic physical labo- ratory experiments (which in fact do not really exist in this case). It is probably true that these theoretical models of the STEPHEN H. SCHNEIDER and RICHARD L. TEMKIN FIGURE 1.7 Plot of apparent atmospheric transmittance at Mauna Loa Observatory. Note the years of major volcanic eruptions (after Ellis and Pueschel, 1971). CO2 radiation effect have some observational justifica- tion. After all, one can predict the surface temperatures and the vertical temperature profiles of the CO2 atmo- spheres of Mars and Venus, albeit not perfectly, but Hey still agree somewhat with the observations of those planets. This gives some indication that the models are not two orders of magnitude off. However, the issue as to He quantitative accuracy of He models remains. Models show that the increase in temperature from about 1900 to 1975 should have been on the order of a few tenths of a degree globally because of the increased carbon dioxide (see Figure 1.81. Therefore, one could argue that the carbon dioxide theory has been proved wrong by the fact Hat the northern hemisphere, at least after 1945, has cooled even though CO2 increased exponentially. How- 390 1 T ~ ~ i I Mode! Calculation Of Atmospheric CO2 Frarn rnmh'~clinn Of Fr`.R;! FIIaI. (a) , I , r I ~ 1 Mods' Coicuiation Of Atmospheric CO2 _ From Combustion Of FOSRjI Fuels 380 ~ 370 o , 360 350 of o ~ 340 in i 330 cat o c, 3~0 he 300 290 _ 280 ~ 1 1 1 1 1 1 1 1 1 1 1 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000 / . _ /~ ~ MCL O LOO Observed j YEAR YEAR (b) FIGURE 1.8 Projections of atmospheric carbon dioxide con- centration from fossil fuels calculated by models of the carbon dioxide cycle. Projection (a) shows an early estimate of 375 parts per million (ppm) CO2 concentration by the year 2000, whereas an updated model (b) predicts a CO2 concentration of near 390 ppm. Projection (a) is from Machta (1971), and projection (b) is from Machta and Telegadas (1974).

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Water Supply and the Future Climate ever, since the fluctuations in global temperature that have occurred naturally before humans could have had any real impact have been on the order of l/2 C, or perhaps even larger, the cooling does not eliminate the CO2 warm- ing argument. Therefore, when a model predicts a change on the order of a few tenths of a degree, it is impossible to distinguish this carbon dioxide temperature "signal" from the natural climatic "noise." Thus it cannot be said categorically that the theory is incorrect. However, car- bon dioxide in the atmosphere is increasing exponen- tially. According to most projections, the next 10 percent increase would occur in about two decades with the sub- sequent 10 percent increase in about one decade. As Broecker (1975) indicated, one may then certainly expect the CO2 effect to exceed the climatic noise level; that is, if the modeling predictions are correct, the signal will become dramatically detectable very quickly (he projects sometime after 1980~. A test for the verification of these models is for them to show a high degree of faithful reproduction of atmo- spheric variability. Yet, on the other hand, a test of these models for sensitivity (to CO2 increase, e.g.) is very dif- ficult to devise. One is left with the terrible dilemma that in order to verify this kind of climate change (i.e., a detectable, significant climatic change from CO2 possibly as early as a generation away) one uses a tool that itself is not entirely verifiable. Perhaps the only way, in a sense, to eliminate this problem is to have the atmosphere itself "perform the experiment" and verify the models. Aside from the environmental and social risks of such a happen- ing, this implies that a model is substantially complete or at least contains the predominant physics, chemistry, and other mechanisms. This example of CO2 uncertainties is typical of a large class of climatic problems at present. FOOD-CLIMATE CONSIDERATIONS Consider at this point some of the food-climate issues. Figure 1.9 shows a record of corn yield in Missouri (Decker, 1974~. The solid line is an average trend over the last 70 years; the dots are the individual yields, that is, the number of bushels per acre for the state's area, and the squares represent the yields in the drought years. One notices that a tremendous increase in yield occurred after about 1940, and this is particularly noticeable in the 1950's, 1960's, and early 1970's. There is no question that this increase was caused by technology, that is, crop strains had been developed that could have higher yields and that were especially responsive to fertilizer applica- tions or the use of pesticides. It is also seen on this figure that the relative percentage of yield variability (especially during the dust bowl period and the last drought in the 1950's) was very high relative to the variability in crop yield that has occurred recently (195~1973~. Technology has been given credit by some for both of the above improvements. But McQuigg et al. ~ 1973) showed that the period from 1956 through about 1973 was 31 120 100 O 80 _ 9 - c~ lo' 60 _ > 40 20 _- Years without major drought Years with major drought _ ~ ~ ._ ~ _ ;. ~ ~~e ~. ~ . I , I ~1 , 1 , 1 1900 1920 1940 1960 1980 YEAR FIGURE 1.9 Solid line is the trend in corn yield per acre in Missouri (after Decker, 1974). The circles represent individual yearly yields for years without major drought, and the squares are for years with drought. Note that since 1956, not only have yields increased significantly but variability in yields from one year to the next has been reduced. also an extremely unusual one weatherwise. As Figure 1.10 (Gilman, 1974) shows, that period differs from many other periods in climatic history. For example, the sum- mer rainfall and summer temperature in the five major wheat states in the United States during the 1930's "dust bowl era" were below normal and above normal, respec- tively, and this combination is bad for crops. One should note that there still is much noise in the system including another drought in the 1950's (coincidentally 20-some years away from the previous one). But there is an unobtrusive looking 16-year period from 1957 to 1973, which may be called the "high-yield era" and shows (with one excep- tion) all years with normal or above-normal rainfall and normal or below-normal temperature, which is in fact abnormally good for crops. However, there is no physical reason to expect that this abnormal trend will continue. When one plans for food at least, one cannot look only at a period of 15 years; one needs to look at periods longer than that in consideration of the future (see the discussion in Chapter 4 of Schneider and Mesirow, 1976).

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32 ABOVE: NORMAL] BELOW] ABOVE1 NORMAL: BELOW] FIVE "WHEAT BELT" STATES (Oklahoma, Kansas, Nebraska, S. Dakota and N. Dakota) DUST BOWL ERA HIGH YIELD ERA 1' ~ k .1 I , I , I r I ~ l T I 1~ r r T I SUMMER RAINFALL . B~ nit n ~~} new , ~d ~ mu ~ ~ Lit L{' SUMMER TEMPERATURE 1 , I , I , I , I , I , I , I 00 1910 1920 1930 1940 1950 1960 1970 YEAR FIGURE 1.10 Seventy-five-year record of summer average tem- perature and rainfall in the five major wheat-producing states of the United States, compiled by Donald Gilman of the National Weather Service and showing the 10-year drought period (i.e., high temperatures and low rainfall) of the dust bowl era and the 15-year recent high-yield era (i.e., above average rainfall and below-average temperature). RELATION TO WATER-SUPPLY ISSUES Two conclusions are now applicable to the water-supply issue. The first suggests that one should accumulate data over a fairly long period in order to obtain some actuarial frequencies of the kinds of fluctuations that one might expect. Perhaps this would be centuries for reliable statis- tics, but it is at least 20 years. The second conclusion is that even though a time series may look stationary if viewed from "far enough back," a closer view might reveal that over a much shorter and more recent climatic sample period changes could be occurring because of human effects. Furthermore, these changes could be rather substantial in the next 20 to 50 years and may well change not only climatic means but also the frequencies of the shorter-term fluctuations. In essence, the real mes- sage here is that if we must contend with climatic uncer- tainty due to the natural fluctuations, that uncertainty will probably be greater in the future because there is a good possibility that the climatic system's boundary conditions are also changingperhaps from human activities. There should now be added one other point. In the only perfect forecast ever made by Joseph in the Book of Genesis he warned of seven years of feast and seven years of famine and, of course, proposed the solution of prudence in the face of environmental variability namely, a food reserve. Now, whether what Schneider and NIesirow (1976) have called "The Genesis Strategy" for food reserves also applies by analogy for water-supply planning as part of the solution to an expectation of climatic uncertainty or whether planning should rather be to make society less vulnerable to the kinds of fluctua- tions that have occurred in the past few centuries are STEPHEN H. SCHNEIDER and RICHARD L. TEMKIN issues that need to be debated further. The one thing that is very clear is that climatic variability has been the rule in the past, but now there are additional unknowns pro- duced by people, so it certainly would be prudent to expect considerable variability in future climate. FINDINGS AND RECOMMENDATIONS 1. The climate varies on all time scales, and the mean- ing of climatic change depends on the defining period for the climatic average. 2. The climate of the past century or two is not neces- sarily typical of the climate over the past few thousand years, particularly on a regional scale. 3. Theory is yet unable to predict the future climate, so an actuarial analysis of recent past records, while not guaranteed valid, is probably the best quantitative way to estimate the range of future climatic variability. However, there is considerable numerical modeling evidence to the effect that human activities, i.e., production of CO2, could detectibly change the "equilibrium" climate by as early as A.D. 2000 and that such a disruption could also change the patterns of climatic variability as well as climatic means. 4. World food supplies are very dependent on climatic stability, and world food supplies and needs are currently in a precarious balance that depends on large food trans- fers and stable supplies of fertilizer, water, and seeds. Population growth in the face of unstable or fluctuating food supplies, i.e., the classical Malthusian problem, may change agricultural demand for water in the future. At least, the implications of this tight margin for reserves of both food and water need to be examined. 5. The implications of these long-term growth (de- mand) and needs (supply) projections should be examined in the context of hedging, and the choice must be clarified as a value judgment fundamentally contrast- ing short-range economic benefits versus long-range catastrophic risks. REFERENCES Alexander, T. (1974). Ominous changes in the world's weather, Fortune 89, 90. Broecker, W. S. (1975). Climatic change: Are we on the brink of a pronounced global warming? Science 189, 460. Bryson, R. A. (1974). A perspective on climatic change, Science 184, 753. Decker, W. (1974). The climatic impact of variability in world food production. Prepared for the 1973 Annual Meeting of the American Association for the Advancement of Science, San Francisco. Reprinted in Am. Biol. Teacher 36, 534. Dickinson, R. E. (1975). Solar variability and the lower atmo- sphere, Bull. Am. Meteorol. Soc. 56, 1240. Eddy, J. A. (1976). The Maunder minimum, Science 192, 1189. Ellis, H. T., and R. F. Pueschel (1971~. 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