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22 GLOBAL WARMING: IS IT REAL AND SHOULD IT BE PART OF A GLOBAL CHANGE PROGRAM?* Stephen H. Schneider The summer of 1988 saw a combination of events that quite literally catapulted the "greenhouse effect" out of the halls of academe and government offices and into the public consciousness. Major drought, intense heat waves, forest fires, a super hurricane, and flooding in Bangladesh--all the kinds of events that had long been forecast as associated with global warming from increasing greenhouse gas build- up--occurred in that year. For months, magazine covers, news broad- casts, and newspapers were dominated by stories of the extreme weather. These were often juxtaposed with stories on the greenhouse effect. It would have been hard for a listener or reader who was not scientifically trained to conclude anything other than that there was an intimate con- nection between the bad weather of 1988 and the buildup of greenhouse gases that had been increasingly discussed by a number of scientists for more than '~ y=~.- . ~ ~ ~ ~ ~~ ~ 1 Was this finally the proof that these academic warnings had indeed come true? On June 23, 1988, James Hansen of the National Aeronautics and Space Administration's Goddard Institute for Space Studies (GISS) stated be- fore the Senate Committee on Energy and Natural Resources that he con- sidered there was a "99 percent' chance that the unusually warm globally averaged temperature records he and a colleague had constructed for the 1980s could not have occurred by chance but rather were the result of the buildup of greenhouse gases.2 He went on to point out that increasing the global temperature would increase the likelihood of extreme heat waves such as those occurring in 1988. He never said that the particular drought of 1988 could have been caused by the greenhouse effect, but only that such events would likely increase as the planet heated up--not a very controversial views and one that has been voiced by many scientists over the past 10 years. *Edited excerpts from testimony given before the Subcommittee on Oceanography and the Great Lakes of the Committee on Merchant Marine and Fisheries, U.S. House of Representatives, May 4, 1989. Any opinions, findings, conclusions, or recommendations expressed in this testimony are those of the author and do not necessarily reflect the views of the National Science Foundation. 209

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210 ASPECTS OF THE DEBATE VISIBLE TO THE PUBLIC Unfortunately, the widespread publicity following the drought in general and that hearing in particular led to the erroneous impression that Hansen and many other climatologists believed that the greenhouse effect was responsible for the drought conditions in 1988. Neither Hansen nor any other qualified atmospheric scientist I am aware of has ever made such a statement. In a recent letter to the New York Times Hansen commented, "as I testified to the Senate during the 1988 heat wave, the greenhouse effect cannot be blamed for a specific drought, but it alters the probabilities. Our climate model, tested by simulations of climate on other planets and past climates on Earth, indicates that the greenhouse effect is now becoming large enough to compete with natural climate variability."4 Hansen did, however, suggest that the warming trend in the 1980s was very likely to have been caused by the greenhouse effect. Therefore, his "99 percent" statement was often confused with the idea that the drought was directly caused by global warming. The latter is obviously absurd, given that the amount of greenhouse gas increase from 1987 to 1988 is very small relative to the major climatic difference between those 2 years. The strongest statement one can make responsibly, given the uncer- tainties, is that increasing the global average temperature could in- crease evaporation stress, thereby slightly altering the odds toward increased drought. However, as is well known by all atmospheric scien- tists, droughts are generally a result of unusual patterns of atmo- spheric circulation, whose causes~are not clear cut and most often are ascribed to unusual temperature patterns in the oceans. Later in 1988 that hypothesis of cause and effect was indeed reaf- firmed by calculations made by Kevin Trenberth and his colleagues at the National Center for Atmospheric Research. Unfortunately, many atmo- spheric scientists were unaware of the exact statements made by Hansen and others who had testified at various hearings during the summer of 1988, and therefore they accepted the widespread perception, reported repeatedly in the media, that such witnesses believed the greenhouse effect and the drought were cause and effect. This misperception caused an angry and understandable counterre- action. For example, the U.S. National Climate Program Office helped to sponsor a workshop on climate trends at the National Academy of Sciences on September 29, 1988. It focused on several difficulties with the observational record used to reconstruct global temperature trends, including the fact that many thermometers have had cities grow around them, which causes an unnatural urban heat island effect. Other errors were noted, such as stations that had moved out to airports or up or down mountains. I will discuss the seriousness of these issues below. While this workshop was taking place, the calculations by Trenberth and his colleagues were announced. These suggested that unusually cool surface temperatures in the equatorial Pacific (actually, the cool water was flanked by warm water) may have been responsible for distorting the jet stream, thereby steering storms up and out of the United States in the spring of 1988 and contributing to the intense drought in the summer. This result was cited frequently in headline after headline that stated,

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211 in effect, "Drought of '88 not caused by greenhouse effect after all." The widespread association that was created from the summer's coverage thereby led to what was, in essence, an artificial debate. In an article published in Science, Trenberth and his colleagues quite rightly argued that "climate simulations indicate that a doubling of carbon dioxide concentrations could increase the frequency of summer droughts over North America. Thus, the greenhouse effect may tilt the balance such that conditions for droughts and heat waves are more likely, but it cannot be blamed for an individual drought."5 That statement is clearly reason- able and quite consistent with those of most other scientists who speak on this issue. The trial by media of the greenhouse effect was thus a nonscientific issue from the very beginning. Nevertheless, many newspaper pieces con- tinue to appear from scientists and others criticizing the "hysteria' being generated by some (usually unnamed) scientists speaking in public forums on this issue. See, for example, S. Fred Singer's "Fact and Fancy on the Greenhouse Effect" (Wall Street Journal, August 30, 1988) and Woods Hole Oceanographic Institution statistician Andrew Solow's "The Greenhouse Effect: Hot Air in Lieu of Evidence" (International Herald Tribune, December 29, 1988~. But the most visible recent such article, I believe, was University of Virginia climatologist Patrick Michaels' article "The Greenhouse Climate of Fear" (Washington Post, January 8, 1989~. Michaels listed a number of issues he believed were improperly reported or underreported in the press, giving examples of "a few recent revelations that somehow got lost with the ozone." He disparaged Hansen's June testimony forecasting that 1988 would be the warmest year on record, even though recent evidence now suggests that indeed it was.6 He went on to refute the greenhouse explanation of drought by citing Trenberth's work to the effect that the drought of 1988 was caused by cold tropical ocean temperatures, something that was never doubted by other scientists. He cited the very recent results of National Oceanic and Atmospheric Administration scientist Tom Karl, who, Michaels said, "arguably knows more about regional climate variation than anyone in the world." Michaels described Karl as saying that the NASA-GISS calcula- tions for warming over the United States were too high by nearly 1F for this century because of the urban heat island effect. Because of this, Michaels continued, "there may have been no this emphasis] global warming to speak of during the last century. Karl's findings surprised none of us who daily toil with the data. But it should be a major shock to those who are using those figures for policy purposes. Is it irre- sponsible to point this out in public? " The results of the Goddard Institute for Space Studies and Climatic Research Unit (CRU) are reproduced here as Figure 22.1. The GISS record shows a warming of about 0.~C in the past 100 years and the CRU record a global warming of about 0.6C over the same period. Both include some of the same thermometers, but the CRU record adds an oceanic data set not included in the GISS record. Both groups were aware of and had attempted to account for the artificial heating in urban areas due to the growth of cities around such thermometers during the past century. The questions, therefore, are how well they were able to make corrections for this urban effect and whether that effect does indeed, as Michaels implied,

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212 ~ 4 0.2 o _ - 0.2 o - o -0 4 2 -0.6 ~ 0.4 fir '_ 0.2 or ~ O c -0.2 0.4 - a) Climatic Research Unit ~ ~ I I I I I I I I I I, .l b) Goddard Institute for Soace Studies It - - -- -- - Annual Mean 5 Year Mean o 8 1900 1920 1940 1960 1980 2000 YEAR FIGURE 22.1 A comparison of the global surface temperature trends of the past 100 years constructed (a) from land and island stations and ocean surface temperature data sets at the Climatic Research Unit and (b) from a similar set of stations (minus the ocean surface temperature data set) at the Goddard Institute for Space Studies. (Sources: P. D. Jones and T. M. L. Wigley, personal communication, 1988; J. Hansen and S. Lebe- deff. 1988. GeophYs. Res. Lett. 15:323.) invalidate the notion that the globe has been warming for the past century. Neither of these groups had easy access to the vast network of rural stations that Karl had at the National Climate Center in Asheville, North Carolina, and that enabled him to check the accuracy of predictions of U.S. temperature trends made by both the GISS and CRU groups. What Tom Karl and Phil Jones found was that the GISS results had overestimated the warming trend in the United States by about 0.3C (an error more than 30 percent lower than that implied by Michaels), but the East Anglia (England) group had only overestimated the warming trend by about 0.15C.7 Recall that the GISS global record suggested a warming of about 0.8G in the past 100 years, and the East Anglia group record a warming of about 0.6C or so.

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213 Therefore, if one makes the very conservative assumption that the entire world record suffers as strong an urban heat island bias as the United States, and thus applies Karl's corrections to the global record (even though they only apply to the United States), then that correction procedure simply reduces Hansen's record and the CRU record to about 0.5C (0.9F) global warming for the past century. This is not con- sistent with the frequent statements in the press that urban biases are likely to eliminate the global warming trend over the past century. Moreover, this is not believed by Karl or any other knowledgeable atmospheric scientists I am aware of. Indeed, Solow has not contested the existence of a global warming trend, only its relevance to the in- creased greenhouse gas burden of the past 100 years. And in an inter- view with Science magazine's Richard Kerr, Karl commented that ''the long-term global warming is something on the order of 0.4C during the past century. Is the turban] bias 0.05C or 0.2C? The chances that it is the same size as the [global] warming are pretty remote. It is a matter of adjusting the rate of rise, not questioning the rise itself"' (Science 243, 603, 1989~. Another example of an overblown debate with little relevance to global warming occurred in mid-January 1989, when a number of Department of Commerce scientists (including Karl) published a U.S. temperature trend record that got widespread press coverage (Weekly Climate Bulletin, No. 89/02, Jan. 14, 1989~. They pointed out that there had been no net warming in the United States over the twentieth century. This, too, was greeted with many headlines that the greenhouse effect had not happened in the United States. Good articles (albeit with questionable headlines) appeared on this subject,8 quoting a number of sources, including the authors of the North American study, to the effect that the United States occupies only a small percentage of the world's area, and that there are almost continent-sized regions that have been warming substantially, as well as continent-sized regions that have been cooling, over the past few decades. Therefore, it is just as foolish to draw global inferences from looking at the small fraction of the earth represented by the United States as it is to predict the outcome of a national election by looking at trends in only one or two states. Nevertheless, an artificial debate has grown over the validity of the greenhouse effect as a result of this publication. SOME ELEMENTS OF THE SCIENTIFIC DEBATE What then, has the debate visible to the public to do with the actual scientific debate? I am afraid the answer is often ''too little." Virtually all atmospheric scientists believe that the greenhouse effect as a scientific proposition is well established and essentially beyond question. The trapping of heat near the earth's surface from the presence of gases such as water vapor, carbon dioxide, methane, and chlorofluorocarbons has been established from literally millions of measurements over the past century. It is also beyond debate that carbon dioxide has increased about 25 percent since the Industrial Revolution, and methane substantially more than that. It is also virtually certain

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214 that the CO2 increases are attributable to human activities, principally the burning of fossil fuels and deforestation, and that methane increases are initially connected with agriculture and land use. There is controversy, but it involves how much trace gases will increase over the next century. Given typical assumptions of growth in fossil fuel use, population, and standards of living into the future, the uncertainties in the greenhouse gas projections are roughly on the order of a factor of 3. Uncertainty in the global climate response to a given increase in trace gases is also typically on the order of a factor of 2 to 3. Taken together, these uncertainties explain why most national and international assessments suggest that a warming ranging from as little as 1C to as much as 10C (1.8 to 18F) is possible by the end of the next century. A few believe that the warming could be even greater or , that a cooling could be triggered, but these views represent a very small segment of scientists. Major assessments put the most probable global temperature increase over the next 50 years or so at several degrees Celsius.9 Less certain than projections of global average temperature increase is the regional distribution of climate changes. Such changes could have major impacts on water resources, agriculture, forests, urban infrastructure, human health, navigation, or coastal planning.l Whether predictions of drying in the midcontinental United States and loss of comparative advantage for U.S. agriculture, which are frequently inferred from climatic models, will be confirmed is simply not known now. Considering the current state of the art, it is doubtful that reliable forecasts of regional climatic changes will be available for another 20 years. That distant date might be accelerated substantially if both present and expanded research efforts were carefully organized, but even then, we cannot guarantee scientific consensus on the reliability of regional predictions of climate change, even in the time frame of a decade. But I believe it is worth a try to make the effort to improve the reliability of regional climate predictions, for such information would help to put evolving policymaking on a firmer factual basis. VALUE JUDGMENTS AND POLICYMAKING Finally, the greatest lack of scientific consensus occurs over whether present uncertainties justify an immediate policy response--which is a value judgment, not an issue resolvable by scientific methods. The policy process is advanced when scientists provide what they are tech- nically competent to offer: estimates of specific consequences of greenhouse gas buildups and their likelihood of occurrence--even if estimates of the latter are based on intuitive judgments of technical experts. Any statements beyond that are the personal opinions of those scientists. Although I believe that scientists, like all citizens, are entitled to opinions on how to deal with those probabilities and conse- quences, we must always be scrupulous to point out that such opinions are personal value judgments.

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215 Whether a certain amount of knowledge justifies urgent action or de- lay depends on whether one fears investing present resources as a hedge against future change more than one fears future change descending with- out some attempt having been made either to slow it or to make invest- ments to adapt to it more effectively. In my value system, the latter is a clearly preferable course, for I believe in insuring against potential catastrophic loss at both the personal and national levels. Waiting for more evidence before acting also represents a value judgment. But the odds of rapid, potentially unprecedented--even catastrophic--climate change are almost certainly in the first decimal place of probability, and I believe the likelihood of a several-degrees- Celsius warming in 50 years is a better-than-even bet. The cost of overreaction is a legitimate issue, but so too is the cost of under- reaction. The more rapidly climate changes evolve, the more difficult it will be for societies to adapt; and it may be impossible for natural ecosys- tems to adapt without substantial dislocations or extinctions of some species. The more rapidly greenhouse gases build up, the more difficult it will be for scientists to forecast the outcome reliably. The less we know about the future, the less easily we will be able to adapt--or even to take advantage of some of the changes that will occur. If we choose to wait for that added degree of certainty before implementing preventive policies, the delay will not be cost-free, for it must occur at the price of forcing living things to adapt to much greater change than what might occur if we were to act today to slow the change or to invest affirma- tively to make our future adaptations easier. Wrangling over a few tenths of a degree in the historic global temperature trends in this hemisphere or that will not change the fact that postponing action Is a basic gamble with our environmental future. Quite simply, the "bottom line" of the evolving greenhouse gas buildup is that we are insulting the environment at a rate greater than our ability to predict the conse- quences and that, under these conditions, surprises are virtually certain. NEED FOR ONGOING SCIENTIFIC INVESTIGATION Continued Observation and Monitoring To establish that the greenhouse effect signal has clearly been detected in the climatic record, we will require another decade or possibly 2 to be sure that the warming of the 1980s (which does appear to be the warmest decade recorded on a global basis) will in fact continue into the 1990s and beyond. The nature of regional climate fluctuations and global trends during the past century is very difficult to establish to a high degree of accuracy because of (1) missing or faulty temperature data as well as (2) incomplete data on other possible competitive causes of climate change, such as changes in the energy output from the sun, human dust, or other pollutants.ll

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216 Model Validation The strongest argument for concern about the rapid buildup of greenhouse gases comes not from the small global climatic trends and large regional climatic noise of the recent past, but rather from the well-validated nature of the physical processes that have long been understood to accompany increases in infrared-radiation-trapping (i.e., greenhouse) gases. Let us consider in more detail the important issue of model valida- tion. Perhaps the most perplexing question about climate models is whether they can ever be trusted enough to provide grounds for altering social policies, such as those governing carbon dioxide emissions. How can models so fraught with uncertainties be verified? There are actually several methods. None of them is sufficient on its own, but together they can provide significant (albeit largely circumstantial) evidence of a model's credibility. The first method is to check the model's ability to simulate today's climate. The seasonal cycle is one good test because the temperature changes involved are large--several times larger, on the average, than the change from an ice age to an interglacial period. General circula- tion models (GCMs) do remarkably well at mapping the seasonal cycle' which strongly suggests they are on the right track. The seasonal test is encouraging as a validation of Mast physics," such as changes in cloudiness. However, it does not indicate how well a model simulates slow processes, such as changes in deep ocean circulation, that may have important long-term effects. A second method of verification is to isolate individual physical components of a model, such as its parameterizations, and test them against either a high-resolution submodel or real data from the field. For example, one can check whether a model's parameterized cloudiness matches the level of cloudiness appropriate to a particular grid box. Or one can test a GCM's grid cloudiness against an isolated mesoscale model. The problem with the former test is that it cannot guarantee that the complex interactions of many individual model components are properly treated. The GCM may be good at predicting average cloudiness but bad at representing cloud feedback. In that case the simulation of the overall climatic response to, say, increased carbon dioxide is likely to be in- accurate. A third method for determining overall, long-term simulation skill is to check a model's ability to reproduce the diverse climates of the ancient earth or even of other planets. Paleoclimatic simulations of the Mesozoic Era, glacial/interglactal cycles, or other extreme past climates help in understanding the convolution of the earth's climate with living things. As verifications of climate models, however, such simulations are also crucial to estimating both the climatic and biological future.12 Overall validation of climatic models thus depends on constant ap- praisal and reappraisal of performance in the above categories. Also important are a model's response to such century-long forcings as the 25 percent increase in carbon dioxide and other trace greenhouse gases Since the Industrial Revolution. Indeed, most climatic models are sensitive enough to predict that warming of at least 1C should have

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217 occurred during the past century. The precise "forecast" of the past 100 years also depends on how a model accounts for such factors as changes in the solar constant, sulfate aerosols, or volcanic dust.13 Indeed, as recent data show, the typical prediction of a 1C warming is broadly consistent but somewhat larger than the amount of warming actually observed. Possible explanations for the discrepancy include the following: 14 (1) state-of-the-art models are too sensitive to increases in trace greenhouse gases by a rough factor of 2; (2) modelers have not properly accounted for such competitive external forcings as volcanic dust or changes in solar energy output--which could have caused fluc- tuations of up to several tenths of a degree Celsius; (3) modelers have not accounted for other external forcings such as regional tropospheric aerosols from agricultural, biological, and industrial activity;] (4) modelers have not properly accounted for internal processes that could lead to stochastic or chaotic behavior; (5) modelers have not properly accounted for the large heat capacity of the oceans taking up some of the heating of the greenhouse effect and delaying, but not ul- timately reducing, warming of the lower atmosphere; (6) both present model forecasts and observed climatic trends could in fact be consistent because models are typically run for equivalent doubling of carbon dioxide, whereas the world has only experienced one-quarter of this in- crease, so that nonlinear processes have been properly modeled and have produced a sensitivity appropriate for a doubling but not for a 25 per- cent increase; and (7) the incomplete and inhomogeneous network of thermometers has underestimated actual global warming during this century. Despite these explanations, the empirical test of model predictions against a century of observations certainly is consistent to a rough factor of 2. This test is reinforced by the good simulation by most climatic models of the seasonal cycle, diverse ancient paleoclimates, hot conditions on Venus, cold conditions on Mars (both well simulated), and the present distribution of climates on earth. When taken together, these verifications provide a strong circumstantial case that the model-' ing of sensitivity of the global surface air temperature to greenhouse gases is probably valid within roughly 2-fold. Another decade or 2 of observations of trends in the earth's climate should produce signal-to- noise ratios sufficiently obvious that most scientists will know whether present estimates of climatic sensitivity to increasing trace gases have been accurate or not. CONCLUDING REMARKS It is my personal belief that we clearly know more than enough to actively pursue actions that both slow the rate of buildup of greenhouse gases and at the same time help solve other societal problems--what has been called the "tie-in strategy." In particular, individuals, firms, and nations should pursue those activities that make general good sense, regardless of whether the greenhouse effect turns out to be much less serious than currently contemplated.16 For example, stepped-up invest- ments in energy production and end-use efficiency, accelerated testing of

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218 nonfossil fuel alternatives, development of more widely climate-adapted crop strains, added flexibility in the management of water systems, and coastal planning to deal with rising sea levels and storm surges would all contribute to solving existing societal problems even if further climate change does not occur. Therefore it seems prudent to consider implementing first those policies that have multiple benefits, including that of buying insurance against the real possibility of large and potentially catastrophic climate change. I hope that recent shrill or irrelevant debates or headlines do not mask the already large scientific consensus that exists over the basic physical phenomenon known as the greenhouse effect, a scientific pro- position over which I have heard virtually no scientific dissent. In any case, regardless of whether society chooses vigorous preven- tion policies as a response to global warming, nearly all analysts agree that some growth in greenhouse gases will continue into the twenty-first century. Therefore a vigorous program of interdisciplinary research on earth systems, what has been called 'global Change," 7 will help society adapt more effectively to the global changes that appear inevitable. NOTES 1. For example, S.H. Schneider and R. Londer, 1984, The Coevolution of Climate and Life (Sierra Club Books, San Francisco), Chapter 8, pp. 294-365; or S.H. Schneider, 1989, Global warming: Scientific reality or political hype? Pp. 53-57 in Global Warming. Hearings before the Subcommittee on Energy and Power of the Committee on Energy and Commerce, U.S. House of Representatives, February 21, 1989 (U.S. Government Printing Office, Washington, D.C.~; S.H. Schneider, 1989, Global Warming: Are We Entering the Greenhouse Century? (Sierra Club Books, San Francisco, 317 pp.~. J.E. Hansen, 1988, prepared statement, The Greenhouse Effect: Impacts on Current Global Temperature and Regional Heat Waves. In Greenhouse Effect and Global Climate Change, hearing before the Committee on Energy and Natural Resources, U.S. Senate, Washington, D.C., June 23, pp. 42-79. For example, L.O. Mearns, R.W. Katz, and S.H. Schneider, 1984, Changes in the probabilities of extreme high temperature events with changes in global mean temperature, J. Clim. and Appl. Meteorol. 23, 1601-1613, pointed out that if temperatures were to increase by only 3F, and nothing else in the climate system changed, then the probability of a heat wave in which 5 or more days in a row in July saw afternoon temperatures greater than 95F would increase in Washington, D.C., from a present probability of around 1 in 6 to a future probability of around 1 in 2. In Des Moines, Iowa, the respective probabilities would change from 1 in 18 to 1 in 5. These changes in the odds on "climatic dice" are what would be expected if only temperature increased by 3F. J. Hansen, letter to the New York Times, January 11, 1989. K.E. Trenberth, G.W. Branstator, and P.A. Arkin, 1988, Origins of the 1988 North American drought, Science 242:1640-1645.

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219 6. For example, in a page 1 story, Philip Shabecoff reports in the February 4, 1989, New York Times that the Climatic Research Unit in East Anglia, England, has concluded that 1988 did, indeed, set a record as the year with the warmest global temperature. P.D. Jones of that unit has confirmed by correspondence the accuracy of this story. T.R. Karl and P.D. Jones, 1989, Urban bias in area-averaged surface air temperature trends, Bulletin of the American Meteorological Society 70:265-270. 8. P. Shabecoff, U.S. Data Since 1895 Fail to Show Warming Trend, New York Times, January 26, 1989, p. 1. 9. See S.H. Schneider, 1989, The greenhouse effect: science and policy, Science 243:771-781, for references to many such assessments. 10. J.B. Smith and D. Tirpak, eds., October 1988, The Potential Effects of a Global Climate Change on the United States: Draft Report to Congress, Vols. I and II (Environmental Protection Agency, Washington, D.C.~. 11. S.H. Schneider, 1989, Science 243:775-776. 12. S.H. Schneider and R. Londer, 1984: The Coevolution of Climate and Life (Sierra Club Books, San Francisco). S.H. Schneider and C. Mass, 1975, Volcanic dust, sunspots, and temperature trends, Science 190:741-746; R.L. Gilliland and S.H. Schneider, 1984, Volcanic, C02, and solar forcing of northern and southern hemisphere surface air temperatures, Nature 310:38-41; J. Hansen, D. Johnson, A. Lacis, S. Lebedeff, P. Lee, D. Rind, and G. Russell, 1981, Climate impact of increasing atmospheric carbon dioxide, Science 213:957-966. S.H. Schneider, 1989, Science 243:775-776. Wigley, T.M.L., 1989, Possible climate change due to SO2-derived cloud condensation nuclei, Nature 339:365-367. 16. See chapter 8 of S.H. Schneider, 1989, Global Warming: Are We Entering the Greenhouse Century? (Sierra Club Books, San Francisco, 317 pp.~. 17. National Research Council, 1988, Toward an Understanding of Global Change: Initial Priorities for U.S. Contributions to the International Geosphere-Biosphere Program (National Academy Press, Washington, D.C.~. 13.

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