The global warming debate: Science or politics?
Stephen H. Schneider Narional Center for Amsoheric Resehrch Boulder, CO 80307
To predict the result of some event in nature, researchers commonly design and perform an experiment. But what if the issues are very complex or the scale of the experiment unmanageably large? Forecasting the effect of human pollution on climate poses just such a dilemma, for this uncontrolled experiment is now being performed on “Laboratory Earth.” How then can we be anticipatory, if no meaningful physical experiment can be performed? Although nothing can provide certain answers, we can turn to a surrogate lab-not a room with test tubes and Bunsen burners, but a small box with transistors and microchips. We can build mathematical models of the Earth and perform our “experiments” in computers.
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Mathematical models translate conceptual ideas into quantitative statements. Models are usually not faithful simulators of the full complexity of r e ality, of course, but they can tell us the logical consequences of explicit sets of plausible assumptions. To me, that certainly is a big step beyond pure concep tion. To put it more crudely, modeling is a major advance over “hand-waving.” Let me first point out that not all knowledgeable scientists are in agreement as to the probability that such changes will occur In fact, people who have followed the very noisy, often polemical debate in the medii recently might get the impression (a false impression, in my opinion) that there are but two radically opposed schools of thought about global warming. One is that climatic changes will be so severe, so sudden, and so certain that major species extinctions will intensify, sealevel rise will create tens of millions of environmental refugees, millions to perhaps billions of people will starve, and ecosystems will be devastated. The second school of thought argues that there is nothing but uncertainty about global warming, that no evidence from the 20th century supports what the modelers have predicted, and that the people who encourage change are just “environmental extremists”; thus there is no need for management response to an event that is improbable, and in no case should any such responses interfere with the free market and bankrupt the nation (e.g., see the cover story of Forbes magazine [I]).
Unfortunately, although such a highly charged and polarized debate make3 entertaining opinion page reading or viewing for the ratings-domi~ t e dmedia, it is a poor reflection of the actual scientific debate or the broad consensus within the scientific community. In my opinion, forecasts of “the end of the world” or “nothing to worry about” are the two least likely cases, with ahnost any scenario in between being more probable. Figure 1 shows a projection of global warming possibilities into the 21st cen-
“Models can tea us the l o g i d consequences of sets of phsible aFsIUnpti0lts.J~ tury drawn by a consensus group of scientists that was convened by the well-established International Council of ScientificUnions. It shows warming from a very moderate 0.5 “C (0.9 OF) up to a catastrophic 5 “C (9 or greater warming before the end of the next c e n w . I do not hesitate to call the latter extreme catastrophic because that is the magnitude of warming that occurred between about 15,000 years ago and 5000 years ago, the end of the last ice age to our present interglacial e p och. It took nature some 5000-10,000 years to accomplish that warming, and it was accompanied by a 100 m (330 ft)
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or so rise in sea level, thousands-ofkilometers migration of forest species, radically altered habitats, species extinctions, species evolution, and other major environmental changes. Critics of immediate policy responses to global warming are quick to point out the many uncertainties that could reduce the average projections made by c l i t e models (such as the middle line on Figure 1). Indeed, most climate modelers include similar caveats in their papers, and many resent the critics who imply that they alone responsibly point out these uncertainties to the public, and that the modelers somehow deliberately suppress uncertainties in order to overstate the issue (3, 4). Many critiques (e.g., 5) some how forget to stress that the sword of uncertainty has two edges: that is, uncertainties in physical or biological processes, which make it possible for the present generation of models to overestimate future warming effects, are just as likely to have caused the models to underestimate change. Public policy makers face the dilemma of deciding what actions to take, even though no one knows in detail what will happen. In my opinion the scientiiic community will not be able to provide definitive information during the next decade or perhaps two about the precise timing and magnitude of century-long climate changes, especially if research efforts remain at current levels. Public policy makers will have to address how much information is enough to act on and what kinds of measures can be taken to deal with the plausible range of environmental changes. Unfortunately, the probability of such changes cannot be estimated by definitive analytical methods. Rather, we will have to rely on the intuition of experts, which is why a highly confusing and polarized media debate can paralyze anticipatory management. Fortunately, making such scientific judgments is the purpose of d e liberative bodies such as the U S . National Academy of Sciences or the International Council of Scientific Unions. The NAS regularly convenes a spectrum of experts to provide the best estimates of the probabilities of various scenarios of change. These people can deliberate to a considerableextent away h m the confusion of the noisy medii debates in which extreme opposites are typically pitted. Half a dozen such assessments during the past 10 years have all reaffirmed the plausibility of unprecedented climate change building into the next 50-100 years. Arguments of critics Let me briefly summark the arguments of critics, some of whom chalEnvlron. Scl. Technol., Vol. 24, NO.4,1990 453
lenge these assessments. Manv critics cogend that the warming trenh of the past century of about 0.5 “C (e.g., s) is suspect because the thermometer record is not very reliable (e.g., 7). Of course, the scientists who produce such records say the same thing (8), but many aren’t certain whether the needed corrections will reduce the trend or continue it. Moreover, some critics say that certain data have been ignored, such as the ocean temperature data collected by schooners in Victorian or pre-Victorian times, because some of those records suggest that the 1850s may have seen ocean temperatures nearly as warm as the present (9, 10). The reason that those kinds of pre- 1890 ocean temperature data are typically discounted in most assessments is that they were collected over only a few percent of the Earth’s surface. In addition, the measurements simply are not reliable. One conclusion that seems reasonable to infer from the available temperature records is that a warming of some 0.5 “C (0.9 OF) has occurred globally during the past 100 years. If greenhouse gas pollution were the only cause of that warming trend, then this would be broadly consistent with the middle of the lower half of the projected range of warming made by climate models (Figure 1). Does this mean that nature has already told us that future global warming will be half of what most models typically project? Unfortunately, we have been accurately measuring the energy output of the sun from space only during the past 10 years or so; therefore, we have little knowledge of the precise quantitative nature of this or other factors that could have influenced the temperature trends this century. Without such factors being accounted for precisely, even accurate temperature data during the past century couldn’t tell us very much about how the Earth has responded to the pollution injected since preindustrial times. Furthermore, although some critics have suggested that a century-long heating up of the sun of a few tenths of a percent could account for the 20th century warming of the Earth (e.g., 5), these critics often forget to mention that it is equally likely, since it was essentially unmeasured, that the sun could have cooled down by that amount, thereby damping any greenhouse effect that otherwise would have been in the record, and thus fooling us into thinking that the global warming from pollution to date is half of what we actually had. Quite simply, the critics can’t have it both ways. What we don’t know could increase or decrease our estimates. 434
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Use of commter models Finally, when advocates of concern about the prospects of global warming-and I am unabashedly one of them (11)-stand before groups such as congressional committees, their concerns are not based solely on speculative theory. Rather, they rely on the fact that the models used to foreshadow the future have already been validated to a considerable degree, although not to the full satisfaction of any responsible scientist, For example, we know from observations of nature (a point often neglected by the critics of global warming) that the last ice age, which was about 5 “C (9 OF) colder than the present era, also had carbon dioxide levels about 25% less than the preindustrial values. Methane, another potent greenhouse gas, also was reduced by a factor of nearly 2 relative to preindustrial values. Ice cores in Antarctica contain gas bubbles that are more than 150,000 years old, and these can be used to measure any changes in the composition of the atmosphere. Such measurements have shown that the previous interglacial age, some 125,000130,000 years ago, had C 0 2 and methane levels comparable to those in the present interglacial age. The nearly simultaneous changes in these greenhouse gases and in planetary temperature over geological epochs is roughly what one would expect based on projections from today’s generation of computer models. However, we still cannot assert that this greenhouse gastemperature coincidence is proof that our models are quantitatively correct, because other factors were operating during the ice age-interglacial cycles. The best we can say is that the evidence is strong but circumstantial.
Science and the public debate One misperception about the scientific discussions is that the bulk of the scientific community is in constant controversy and disarray regarding the basic science of the greenhouse effect. The greenhouse effect, the heat-trapping properties of the atmosphere and its gases and particles, is well understood and well validated. Indeed, it is as good a theory as there is in the atmospheric sciences. It explains, for example, the very hot conditions under the thick atmosphere of Venus and the very cold conditions under the thin, weak “greenhouse” of Mars. It explains the thousands of laboratory observations of the transfer of radiant energy through various gases, the millions of aircraft and balloon observations of the Earth’s temperature structure and its radiative fluxes, and the literally billions and bil-
lions of satellite observations of the same quantities. Moreover, A. Raval and. V. Ramanathan (12) recently used satellite observations to study the water vapor-greenhouse feedback mechanism, a process that is central to most models’ estimates of some 3 =t1.5 “C equilibrium warming from doubling C02. They conclude that “The greenhouse effect is found to increase significantly with sea surface temperature. The rate of increase gives compelling evidence for the positive feedback between surface temperature, water vapour and the greenhouse effect; the magnitude of the feedback is consistent with that predicted by climate models” (12). In other words, the heat-trapping capacity of the atmosphere is well understood and well measured on Earth, and much of the sometimes polemical debate in the media over the greenhouse effect has little basis in reality. This empirical confirmation of the natural greenhouse effect, which is consistent with the greenhouse effect of climate models, stands in stark contrast to the theoretical postulates of Lindzen (10) that negative temperature-water vapor feedback processes in parts of the tropics will reduce present model estimates of global warming by a factor of 6 . It is well known that the 25% increase in carbon dioxide that has been documented since the industrial revolution, the 100% increase in methane since the industrial revolution, and the introduction of man-made chemicals such as chlorofluorocarbons (also responsible for stratospheric ozone depletion) since the 1950s should have trapped between one and two extra watts of radiant energy over every square meter of Earth. That part is well accepted by most climatological specialists. However, what is less well accepted is how to translate that one or two watts of heating into X degrees of temperature change, because this involves assumptions about how that heating will be distributed among surface temperature rises, evaporation increases, cloudiness changes, ice changes, and so forth. The uncertainty factor of 2 to 3 in global temperature rise projections, as cited in typical NAS reports, reflects a legitimate estimate of uncertainty held in most of the scientific community. Indeed, recent modifications by the British Meteorological Office of their climate model to attempt to mimic the effects of cloud droplets halved its sensitivity to doubled C 0 2 , but it is still well within the often-cited 1.5-4.5 “C range. However, the authors of the study wisely pointed out that “although the revised cloud scheme is more de-
tailed, it i s not necessarily more accurate than the less sophisticated scheme” (13). I have never seen this forthright and important caveat quoted by any of the global-warming critics who cite the British work as a reason to lessen our concern. Finally, as stated earlier, predictionof the detailed regional distribution of climatic anomalies-that is, where and when it will be wetter and drier, how many floods might occur in the spring in California, or the number of forest fires in Wyoming in August-is simply highly speculative, although some plausible scenarios can be given (seebox). Although climate models are far from fully verified for future simulations, the seasonal and p a l e o c l i t i c simulations are strong evidence that state-of-the-art c l i t i c models already have considerable potential. A very re-
cent and careful statistical study has reaffirmed the model-suggested l i between global surface temperature trends i d measured C@ trends over the past 30 years (13,although a decade or so more probably will be required to detect the anticipated global warming signal with a high deyee of An awareness of just what confidence. models are and what they can and cannot do i s probably the best we can ask of the public and its representatives. Then the tough policy problem will be how to apply society’s values in facing the future, given the possible outcomes that climate models foreteu. The global-warming debate, then, is both science and politics. However, the public must understand that there are vastly greater disagreements over what to do about the prospect of global warming (i.e., a political value issue)
Large Stratospheric cooling (virtually certain). Reduced Ozone conce trations in the moer stratosohere will lead to reduced absorution solar ultraviolet iadiation and therefore less heating. Increases In t stratospheric concentration of carbon dioxide and other radiatively a five trace gases will increase the radiation of heat from the strat sphere. The combination of decreased heating will lead to a major lowering of temperatures in the upper stratospher Global mean surface warming (very probable). For a doubling atmospheric carbon dioxide (or its radiative equlvalent from all t greenhouse gases), the long-term global mean pected to be In the range of 1.5 to 4.5 OC. The most significant uncertainty arises from the effects of clouds. Of course, the actual rate of warming during the next century will be governed by the growth rate of greenhouse gases, natural fluctuations in the climate System, and t detailed response of the slowly responding parts of the climate syste (i.e., Oceans and glacial ice). Global mean precipitation Increase (very probable). Increased heat. ing of the surface will lead to increased evaporation and therefore preater global mean Drecbitation. Desuite this increase In global av i g e prGipitation, some individual regions might well experience d creases in rainfall. Reductlon of sea ice (very probable). As the climate warms, total s ice is expected to be reduced. Polar winter surface warming (very probable). As the sea Ice boundary is shifted poleward, the models predict a dramatically enhanced surface warming in winter polar regions. The greater fraction of open water and thinner sea ice will probably lead to warmlng of the polar surface air by as much as 3 times the global mean warming. Summer continental dryness warmlng (likely In the long term). Several studies have predicted a marked longterm drying of the so11 mola lure over some mid-latitude interior continental reglons during summer. This dryness Is mainly caused by an earlier termination of snowmelt and rainy periods and an earliet onset of the springtosummer reduction of soil wetness. Of course, these simulations of long-term equlllb rium conditions may not offer a reliable guide to trends during the next tew decades of changing atmospheric composition and changing climate. Highhtitude precipltation increase (probable). AS the ciimate warms, the increased poleward penetration of warm, moist air should increase the average annual precipitation in high latitudes. Rise In global mean sea level (probable). A rise in mean sea level ia generally expected due to thermal expansion of sea water in the warmer future climate. Far less certain is the contribution due to melt-
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than over the precise probability (Le., a scientific debate) that unprecedented climate change i s being built into the 21st century climate. ’lb me. the latter is at least & even bet. The National Center for Amospheric Researchi s sponsored by the National Science Foundation. Any opinions, findings, conclusions, or recommendations expressed in this article are those of the author and do not narssarily reilect the views of the National Science F d m .
References (I) Brookes, W. T Forbes Dec. 25, 1989, pp. 96-102. (2) Jaeger, 1. Developing Policies for Responding to Climatic Change: A Summary of’ the Discussions and Rccommen&ions of rhL Workshops Held in Villneh 28 September to 2 October 1987; World Meteorological Organization: Geneva, Switzerland, April 1988: WCIP-I WMOl TD-No.225. (3) “Loads of Media Coverage,” Ddnoit News, Editorial, No”. 22, 1989. (4) Schneidcr, S. H. “News Plays Fast and Loose With the Facts,” Detroit News, Dec. 5 , 1989, editorial page. (5) Scienrijc Perspectives on the Greenhouse Problem; George C. Marshall Institute: Washington, DC,1989. (6) Wigley, T.M.L., Ed. Scienrije Assessment of Climate Change: World Metcorological Organization Intergovernmental Panel on Climate Change Working Group 1 (Section 8): Geneva, Switzerland, in press. (7) Ellsaesser, H.W. Amos. Environ. 1984, 18,431-34.
(8) Karl, T. R.: Jones, p. D. Buil. Am. Meteorol. SOC. 1989, 70, 265-70. (9) Newell, R. E. et al. Tcehnol. Rev. 1989, 92,80.
(IO) Lindzen, R. Bull. Am. Meteoroi. Soc.. in press.
(11) Schneider, S. €Global I. Warming: Are We Enrering the Greenhouse Cenrury? Si-
erra Club: San Francisw, 1989.
(12) Raval, A.; Ramanathan, V. Nature 1989,
342,758. (13) Mitchell, J.F.B.; Senior, C. A,: Ingram, W. 1. Nature 1989,341, 132-34. (14) National Academy of Sciences; Currem Issues in Atmospheric Change: National Academy Press: Washington, DC,1987. (15) Kuo, C.; Lindberg, C.; Thomson, D.I. Nature 1990,343.708-13.
Stephen € Sdvlridor, I . a kading climatologist m d environmental policy analyst, is head of the Interdisciplinary Climte Syst e m at the Na!ioM[ Center for Atmospheric Research in Boulder. CO. He is the editor of the interdisciplinaryjournal Climate Change, author of Global Warming and seveml other books, and has contributed to over 100 scientific publications on climate theory, modeling, model validation, and the implications of climatic change. Environ. Sci. Technol., Voi. 24, No.4. IS90 435
