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4 Effects on Climate 4.1 EFFECTS OF CARBON DIOXIDE Joseph Smagorinsky 4.l.l Excerpts from "Charney" and "Smagorinsky* Reports From the beginning of our Committee's work it was clear that at least one aspect of the C02 issue would require continuing attention: the linkage between increases in atmospheric C02 and changes in climate. This question had been addressed in l979 by a panel chaired by the late Jule Charney (Climate Research Board, l979), and I was asked to lead a similar group to take a second look in the light of subsequent research results. Our report, Carbon Dioxide and Climate: A Second Assessment (C02/Climate Review Panel, l982), was to its authors both reassuring and disappointing. On one hand, we found no reasons to dispute the judgments of the Charney group: increased carbon dioxide can poten- tially produce climate changes sufficiently significant to merit con- cern. On the other, we continued to find large uncertainties in the timing, magnitude, character, and spatial distribution of these changes. At this writing, the results of our study and its predecessor still represent in my view a sober, balanced, and responsible consensus on the climatic implications of increased CO2. Thus, the summarized conclusions of our study (and the earlier Charney report) are repro- duced below. However, in the year that has elapsed since their formulation, the scientific issues have been further illuminated by research, and I will append in Section 4.l.2 some comments as an epilogue to the C02/Climate Review Panel report. 266
267 Summary and Conclusions We have examined the principal attempts to simulate the effects of increased atmospheric C02 on climate. In doing so, we have limited our considerations to the direct climatic effects of steadily rising atmospheric concentrations of CO2 and have assumed a rate of C02 increase that would lead to a doubling of airborne concentrations by some time in the first half of the twenty-first century. As indicated in Chapter 2 of this report, such a rate is consistent with observations of C02 increases in the recent past and with projections of its future sources and sinks. However, we have not examined anew the many uncertainties in these projections, such as their implicit assumptions with regard to the workings of the world economy and the role of the biosphere in the carbon cycle. These impose an uncertainty beyond that arising from our necessarily imperfect knowledge of the manifold and complex climatic system of the earth. When it is assumed that the CO2 content of the atmosphere is doubled and statistical thermal equilibrium is achieved, the more realistic of the modeling efforts predict a global surface warming of between 2°C and 3.5°C, with greater increases at high latitudes. This range reflects both uncertainties in physical understanding and inaccuracies arising from the need to reduce the mathematical problem to one that can be handled by even the fastest avail- able electronic computers. It is significant, however, that none of the model calculations predicts negligible warming. The primary effect of an increase of CO2 is to cause more absorption of thermal radiation from the earth's surface and thus to increase the air tem- perature in the troposphere. A strong positive feedback mechanism is the accompanying increase of moisture, which is an even more powerful absorber From Carbon Dioxide and Climate: A Scientific Assessment, Report of an Ad Hoc Study Group on Carbon Dioxide and Climate, National Research Council, l979.
268 2 CARBON DIOXIDE AND CLIMATE: A SCIENTIFIC ASSESSMENT of terrestrial radiation. We have examined with care all known negative feed- back mechanisms, such as increase in low or middle cloud amount, and have concluded that the oversimplifications and inaccuracies in the models are not likely to have vitiated the principal conclusion that there will be appreciable warming. The known negative feedback mechanisms can reduce the warming, but they do not appear to be so strong as the positive moisture feedback. We estimate the most probable global warming for a doubling of C02 to be near 3°C with a probable error of ± 1.5°C. Our estimate is based primarily on our review of a series of calculations with three-dimensional models of the global atmospheric circulation, which is summarized in Chapter 4. We have also re- viewed simpler models that appear to contain the main physical factors. These give qualitatively similar results. One of the major uncertainties has to do with the transfer of the increased heat into the oceans. It is well known that the oceans are a thermal regulator, warming the air in winter and cooling it in summer. The standard assumption has been that, while heat is transferred rapidly into a relatively thin, well- mixed surface layer of the ocean (averaging about 70 m in depth), the trans- fer into the deeper waters is so slow that the atmospheric temperature reaches effective equilibrium with the mixed layer in a decade or so. It seems to us quite possible that the capacity of the deeper oceans to absorb heat has been seriously underestimated, especially that of the intermediate waters of the subtropical gyres lying below the mixed layer and above the main thermo- cline. If this is so, warming will proceed at a slower rate until these inter- mediate waters are brought to a temperature at which they can no longer absorb heat. Our estimates of the rates of vertical exchange of mass between the mixed and intermediate layers and the volumes of water involved give a delay of the order of decades in the time at which thermal equilibrium will be reached. This delay implies that the actual warming at any given time will be appre- ciably less than that calculated on the assumption that thermal equilibrium is reached quickly. One consequence may be that perceptible temperature changes may not become apparent nearly so soon as has been anticipated. We may not be given a warning until the CO2 loading is such that an appreciable climate change is inevitable. The equilibrium warming will eventually occur; it will merely have been postponed. The warming will be accompanied by shifts in the geographical distribu- tions of the various climatic elements such as temperature, rainfall, evapora- tion, and soil moisture. The evidence is that the variations in these anomalies with latitude, longitude, and season will be at least as great as the globally averaged changes themselves, and it would be misleading to predict regional climatic changes on the basis of global or zonal averages alone. Unfortunately, only gross globally and zonally averaged features of the present climate can
269 Summary and Conclusions 3 now be reasonably well simulated. At present, we cannot simulate accurately the details of regional climate and thus cannot predict the locations and intensities of regional climate changes with confidence. This situation may be expected to improve gradually as greater scientific understanding is acquired and faster computers are built. To summarize, we have tried but have been unable to find any overlooked or underestimated physical effects that could reduce the currently estimated global warmings due to a doubling of atmospheric C02 to negligible propor- tions or reverse them altogether. However, we believe it quite possible that the capacity of the intermediate waters of the oceans to absorb heat could delay the estimated warming by several decades. It appears that the warming will eventually occur, and the associated regional climatic changes so impor- tant to the assessment of socioeconomic consequences may well be signifi- cant, but unfortunately the latter cannot yet be adequately projected.
270 Summary of Conclusions and Recommendations For over a century, concern has been expressed that increases in atmospheric carbon dioxide (CO2) concentration could affect global climate by changing the heat balance of the atmosphere and Earth. Observations reveal steadily increasing concentrations of CO2, and experiments with numerical climate models indicate that continued increase would eventually produce significant climatic change. Comprehensive assessment of the issue will require projection of future CO2 emissions and study of the disposition of this excess carbon in the atmosphere, ocean, and biota; the effect on climate; and the implications for human welfare. This study focuses on one aspect, estimation of the effect on climate of assumed future increases in atmospheric CO2. Conclusions are drawn principally from present-day numerical models of the climate system. To address the significant role of the oceans, the study also makes use of observations of the distributions of anthropogenic tracers other than CO2. The rapid scientific developments in these areas suggest that periodic reassessments will be warranted. The starting point for the study was a similar 1979 review by a Climate Research Board panel chaired by the late Jule G. Charney. The present study has not found any new results that necessitate substantial revision of the conclusions of the Charney report. SIMPLIFIED CLIMATE MODELS AND EMPIRICAL STUDIES Numerical models of the climate system are the primary tools for investigating human impact on climate. Simplified models permit economically feasible From Carbon Dioxide and Climate: A Second Assessment, Report of the CO2/Climate Review Panel, National Research Council, National Academy Press, l982.
27l 2 CARBON D|OXlDE AND CLlMATE: A SECOND ASSESSMENT analyses over a wide range of conditions. Although they can provide only limited information on local or regional effects, simplified models are valuable for focusing and interpreting studies performed with more complete and realistic models. The sensitivity of global-mean temperature to increased atmospheric CO2 estimated from simplified models is generally consistent with that estimated from more complete models. The effects of increased CO2 are usually stated in terms of surface temperature, and models of the energy balance at the surface are often employed for their estimation. However, changes in atmospheric CO2 actually affect the energy balance of the entire climate system. Because of the strong coupling between the surface and the atmosphere, global-mean surface warming is driven by radiative heating of the entire surface-atmosphere system, not only by the direct radiative heating at the surface. Theoretical and empirical studies of the climatic effects of increased CO2 must properly account for all significant processes involved, notably changes in the tropospheric energy budget and the effects of ocean storage and atmospheric and oceanic transport of heat. For example, studies of the isolated surface energy balance or local observational studies of the transient response to short-term radiative changes can result in misleading conclusions. Otherwise, such studies can grossly underestimate or, in some instances, overestimate the long-term equilibrium warming to be expected from increased CO,. Surface energy balance approaches and empirical studies are fully consistent with comprehensive climate models employed for CO2 sensitivity studies, provided that the globally connected energy storage and transport processes in the entire climate system are fully accounted for on the appropriate time scales. Indeed, empirical approaches to estimating climatic sensitivityâ particularly those employing satellite radiation budget measurementsâshould be encouraged. ROLE OF THE OCEANS The heat capacity of the upper ocean is potentially great enough to slow down substantially the response of climate to increasing atmospheric CO2. The upper ocean will affect both the detection of CO2-induced climatic changes and the assessment of their likely social implications. The thermal time constant of the atmosphere coupled to the wind-mixed layer of the ocean is only 2-3 years. The thermal time constant of the atmosphere coupled to the upper 500 m of the ocean is roughly 10 times greater, or 20-30 years. On a time scale of a few decades, the deep water below 500 m can act as a sink of heat, slowing the rise of surface temperature. However, tracer data indicate that the globally averaged mixing rate into the deep ocean appears
272 Summary of Conclusions and Recommendations 3 to be loo slow for it to be of dominant importance on a global scale for time scales less than 100 years. The lagging ocean thermal response may cause important regional differences in climatic response to increasing C02. The response in areas downwind from major oceans will certainly be different from that in the interior of major continents, and a significantly slower response to increasing CO2 might be expected in the southern hemisphere. The role of the ocean in time-dependent climatic response deserves special attention in future modeling studies, stressing the regional nature of oceanic thermal inertia and atmo- spheric energy-transfer mechanisms. Progress in understanding the ocean's role must be based on a broad program of research: continued observations of density distributions, tracers, heat fluxes, and ocean currents; quantitative elucidation of the mixing processes potentially involved; substantial theoretical effort; and development of models adequate to reproduce the relative magnitudes of a variety of competing effects. The problems are difficult, and complete success is unlikely to come quickly. Meanwhile, partially substantiated assumptions like those asserted here are likely to remain an integral part of any assessment. In planning the oceanographic field experiments in connection with the World Climate Research Program, particular attention should be paid to improving estimates of mixing time scales in the main thermocline. Present knowledge of the interaction of sea-ice formation and deep-water formation is still rudimentary, and it will be difficult to say even qualitatively what role sea ice will play in high-latitude response and deep-water formation until the climatic factors that control the areal extent of polar pack ice in the northern and southern hemispheres are known. Field experiments are required to gain fundamental observational data concerning these processes. CLOUD EFFECTS Cloud amounts, heights, optical properties, and structure may be influenced by CO2-induced climatic changes. In view of the uncertainties in our knowledge of cloud parameters and the crudeness of cloud-prediction schemes in existing climate models, it is premature to draw conclusions regarding the influence of clouds on climate sensitivity to increased CO2. Empirical approaches, including satellite-observed radiation budget data, are an impor- tant means of studying the cloudiness-radiation problem, and they should be pursued. Simplified climate models indicate that lowering of albedo owing to decreased areal extent of snow and ice contributes substantially to CO2 warming at high latitudes. However, more complex models suggest that
273 4 CARBON DIOXIDE AND CLIMATE: A SECOND ASSESSMENT increases in low-level stratus cloud cover may at least partially offset this decrease in albedo. In view of the great oversimplification in the calculation of clouds in climate models, these inferences must be considered tentative. OTHER TRACE GASES Although the radiative effects of trace gases (nitrous oxide, methane, ozone, and chlorofluoromethanes) are in most instances additive, their concentrations can be chemically coupled. The climatic effects of alterations in the con- centrations of trace gases can be substantial. Since trace-gas abundances might change significantly in the future because of anthropogenic emissions or as a consequence of CO2-initiated climatic changes, // is important to monitor the most radiatively significant trace gases. ATMOSPHERIC AEROSOLS Atmospheric aerosols are a potentially significant source of climate varia- bility, but their effects depend on their composition, size, and vertical-global distributions. Stratospheric aerosols consisting mainly of aqueous sulfuric acid droplets, which persist for a few years following major volcanic eruptions, can produce a substantial, but temporary, reduction in global surface temperature and can explain much of the observed natural climatic variability. While stratospheric aerosols may contribute to the infrared greenhouse effect, their net influence appears to be surface cooling. The climatic effect of tropospheric aerosolsâsulfates, marine salts, and wind-blown dustâis much less certain, in part because of inadequate observations and understanding of the optical properties. Although anthro- pogenic aerosols are particularly noticeable in regions near and downwind of their sources, there does not appear to have been a significant long-term increase in the aerosol level in remote regions of the globe other than possibly the Arctic. The climatic impact of changes in anthropogenic aerosols, if they occur, cannot currently be determined. One cannot even conclude that possible future anthropogenic changes in aerosol loading would produce worldwide heating or cooling, although carbon-containing Arctic aerosol definitely causes local atmospheric heating. Increased tropospheric aerosols could also influence cloud optical properties and thus modify cloudiness- radiation feedback. This possibility requires further study.
274 Summary of Conclusions and Recommendations 5 THE LAND SURFACE Land-surface processes also influence climate, and the treatment of surface albedo and evapotranspiration in climate models influences the behavior of climate models. Land-surface processes largely depend on vegetation coverage and may interact with climatic changes in ways that are as yet poorly understood. VALIDATION OF CLIMATE MODELS Mathematical-physical models, whether in a highly simplified form or as elaborate formulations of the behavior and interactions of the global atmo- sphere, ocean, cryosphere, and biomass, are generally considered to be the most powerful tools yet devised for the study of climate. Our confidence in them comes from tests of the correctness of the models' representation of the physical processes and from comparisons of the models' responses to known seasonal variations. Because decisions of immense social and economic importance may be made on the basis of model experiments, it is important that a comprehensive climate-model validation effort be pursued, including the assembly of a wide variety of observational data specifically for model validation and the development of a validation methodology. Validation of climate models involves a hierarchy of tests, including checks on the internal behavior of subsystems of the model. The parameters used in comprehensive climate models are explicitly derived, as much as possible, from comparisons with observations and/or are derived from known physical principles. Arbitrary adjustment or tuning of climate models is therefore greatly limited. The primary method for validating a climate model is to determine how well the model-simulated climate compares with observations. Comparisons of simulated time means of a number of climatic variables with observations show that modern climate models provide a reasonably satisfactory simulation of the present large-scale global climate and its average seasonal changes. More complete validation of models depends on assembly of suitable data, comparison of higher-order statistics, confirmation of the models' represen- tation of physical processes, and verification of ice models. One test of climate theory can be obtained from empirical examination of other planets that in effect provide an ensemble of experiments over a variety of conditions. Observed surface temperatures of Mars, Earth, and Venus confirm the existence, nature, and magnitude of the greenhouse effect. Laboratory experiments on the behavior of differentially heated rotating fluids have provided insights into the hydrodynamics of the atmosphere and ocean circulations and can contribute to our understanding of processes such
275 6 CARBON DlOXlDE AND CLlMATE: A SECOND ASSESSMENT as small-scale turbulence and mixing. However, they cannot simulate adequately the most important physical processes involved in climatic change. Improvement of our confidence in the ability of climate models to assess the climatic impacts of increased CO2 will require development of model validation methods, including determination of the models' statistical prop- erties; assembly of standardized data for validation; development of obser- vations to validate representations of physical processes; standardization of sensitivity tests; development of physical-dynamical and phenomenological diagnostic techniques focusing on changes specifically attributable to increased CO2; and use of information from planetary atmospheres, laboratory exper- iments, and especially contemporary and past climates (see below). PREDICTIONS AND SCENARIOS A primary objective of climate-model development is to enable prediction of the response of the climate system to internal or external changes such as increases in atmospheric CO2. Predictions consist of estimates of the probability of future climatic conditions and unavoidably involve many uncertainties. Model-derived estimates of globally averaged temperature changes, and perhaps changes averaged along latitude circles, appear to have some predictive reliability for a prescribed CO2 perturbation. On the other hand, estimates with greater detail and including other important variables, e.g., windiness, soil moisture, cloudiness, solar insolation, are not yet sufficiently reliable. Nevertheless, internally consistent and detailed specifications of hypothetical climatic conditions over space and timeâ "scenarios"âmay be quite useful research tools for analysis of social responses and sensitivities to climatic changes. INFERENCES FROM CLIMATE MODELS While present models are not sufficiently realistic to provide reliable predictions in the detail desired for assessment of most impacts, they can still suggest scales and ranges of temporal and spatial variations that can be incorporated into scenarios of possible climatic change. Mathematical models of climate of a wide range of complexity have been used to estimate changes in the equilibrium climate that would result from an increase in atmospheric CO2. The main statistically significant conclusions of these studies may be summarized as follows: 1. The 1979 Charney report estimated the equilibrium global surface warming from a doubling of CO? to be ' 'near 3°C with a probable error of
276 Summary of Conclusions and Recommendations 7 ± / .5°C.'' No substantial revision of this conclusion is warranted at this time. 2. Both radiative-convective and general-circulation models indicate a cooling of the stratosphere with relatively small latitudinal variation. 3. The global-mean rates of both evaporation and precipitation are projected to increase. 4. Increases in surface air temperature would vary significantly with latitude and over the seasons: (a) Warming would be 2-3 times as great over the polar regions as over the tropics; warming would be significantly greater over the Arctic than over the Antarctic. (b) Temperature increases would have large seasonal variations over the Arctic, with minimum warming in summer and maximum warming in winter. In lower latitudes (equatorward of 45° latitude) the warming has smaller seasonal variation. 5. Some qualitative inferences on hydrological changes averaged around latitude circles may be drawn from model simulations: (a) Annual-mean runoff increases over polar and surrounding regions. (b) Snowmelt arrives earlier and snowfall begins later. (c) Summer soil moisture decreases in middle and high latitudes of the northern hemisphere. (d) The coverage and thickness of sea ice over the Arctic and circum- Antarctic oceans decrease. Improvement in the quality and resolution of geographical estimates of climatic change will require increased computational resolution in the mathematical models employed, improvement in the representation of the multitude of participating physical processes, better understanding of airflow over and around mountains, and extended time integration of climate models. It is clear, however, that local climate has a much larger temporal variability than climate averaged along latitude circles or over the globe. OBSERVATIONAL STUDIES OF CONTEMPORARY AND PAST CLIMATES Observational studies play an important role in three areas: (1) the formulation of ideas and models of how climate operates, (2) the general validation of theories and models, and (3) the construction of climate scenarios. Studies based on contemporary climatic data have provided a useful starting point for diagnosis of climatic processes that may prove to be relevant to the CO2 problem. The results of the Global Weather Experiment are now being
277 8 CARBON DIOXIDE AND CLIMATE: A SECOND ASSESSMENT analyzed and will provide a unique data base for model calibration and validation studies. Further analyses and diagnostic studies based on contem- porary climatic data sets, particularly the Global Weather Experiment data set, should be encouraged. However, scenarios based on contemporary data sets do not yet provide a firm basis for climatic assessment of possible CO2- induced climatic changes, nor should they be considered adequate at present for validation of CO2 sensitivity studies with climate models. Studies of past climatic data are leading to important advances in climate theory. For example, the large climatic changes between glacial and inter- glacial periods are being linked with relatively small changes in solar radiation due to variations in the Earth's orbit. If confirmed, these studies will improve our understanding of the sensitivity of climate to small changes in the Earth's radiation budget. A large multidisciplinary effort will be required to acquire the requisite data and carry out the analysis, and such work should be encouraged. Studies of past climate are also potentially valuable because they deal with large changes of the climate system, including the atmosphere, oceans, and cryosphere; because they can reveal regional patterns of climate change; and because there is knowledge of the changes in forcing (now including changes both in atmospheric CO2 concentrations and in solar radiation) that are apparently driving the system. 4.l.2 Epilogue Despite the relatively brief interval between the Charney report and our Panel's study, a considerable volume of additional work had been carried out. The virtually exponential growth in scientific research in the C02/climate area reflects increasing consciousness of the issue's social and scientific importance, burgeoning interest in the more general problems of climate, the development of an active
278 community of scientists and institutions concerned with the problem, andâby no means leastâcontinued strong support by funding agencies, notably the U.S. Department of Energy. The excellent reviews assembled by Clark (l982) and Reck and Hummel (l982) and conducted by the U.S. Department of Energy (l983) make a further detailed compendium of research unnecessary at this time. Their content, however, demon- strates clearly the health of this area of research and the need for periodic critical reviews. The consciously conservative assessments conducted by National Research Council groups revealed continued progress in basic understanding of the climate system. However, our understanding of the local and regional details of man-made climate changes is advancing only slowly. This frustratingly slow advance reflects no lack of talent, effort, or resources but rather the inherent difficulty of the task. Although our pace is slow, it is forward and provides us with increasingly clear views of future climate. Our Panel considered a scenario of increasing CO2 concentrations generally consistent with that postulated by an international assess- ment group in l980, i.e., a slow growth leading to a concentration of about 450 ppm by about 2025. This scenario lies within the range of uncertainty suggested by the later studies of Nordhaus and Yohe (this volume, Chapter 2, Section 2.l) and Machta (this volume, Chapter 3, Section 3.5) presented in this report and is consistent with their estimates of a doubling of airborne CO2 toward the end of the twenty- first century. As Machta describes below in Section 4.2, the growing concentrations of other radiatively active trace gases considerably complicate the problem. Tropospheric ozone, methane, nitrous oxide, and the chlorofluoromethanes also interact with thermal radiation and can produce significant additional perturbations to the Earth's heat budget. Thorough discussions have been presented by Chamberlain et al. (l982), Ramanathan (l982), and Hansen et al. (l982). While projection of the future concentrations of these gases is even more problematical than for C02, it does not seem improbable that the perturbation of the heat budget due to these trace gases could approach the magnitude of the perturbation due to C02 alone. If a doubling of atmospheric concentration of C02 is attained in the latter part of the next cen- tury, a concomitant rise in concentrations of other greenhouse gases would imply that the climatic equivalent of a doubling would be reached much sooner. As Revelle and Suess (l957) observed, by changing the atmosphere's composition mankind is conducting a great and unprecedented geophysical experiment. Since we have no laboratory analog of this experiment, we must attempt to predict its outcome by recourse to some modelânatural or analytical. In view of the complexity of the global climate system, with its myriad possibilities for unexpected and counterintuitive feed- backs and responses, the most desirable model would be the Earth itself. Indeed, Budyko and Yefimova (l98l) attempted to relate paleoclimatic reconstructions to estimated contemporary CO2 concentrations and have reached conclusions consistent with those drawn from numerical models. Others (e.g., Kellogg, l977) have cited previous warm periods in the Earth's history as possible guides to the regional pattern of climatic changes in a CO2-warmed Earth. Both approaches are hampered by
279 inadequacies in dataâparticularly in chronologyâand problems in causality. For example, glacial-interglacial oscillations seem to be paced by orbital changes, and in these cases CO2 changes may be responses to rather than instigators of the associated climate changes. Similarly, the warm climate of the early Holocene, often cited as a model of a warmer Earth, has been plausibly explained (Kutzbach and Otto-Bliesner, l982) in terms of changes in the Earth's orbit, although close study shows a complex time sequence of climate changes. Thus the search for a historical analog to CO2-induced climatic change is strewn with pitfalls. In the absence of satisfactory natural models, we must turn to mathematical models based on the most reliable physical and empirical relationships that we can muster. For example, Idso (l980, l982) employed an empirically calibrated linear model relating radiant energy absorbed at the Earth's surface and surface air temperature, but our Panel found his analysis incomplete and misleading. More complete models treat the entire atmospheric column and calculate numerically the exchanges of sensible, latent, and radiative energy between layers and with space. Extension to two and three dimensions permits the energy balance and climate of the globe to be simulated with consid- erable realism and allows for interaction of climate with the oceans and with changes in surface characteristics and snow/ice cover. The results of a number of model simulations of contemporary climate and the climate corresponding to increased C02 were reviewed by our Panel. Further comprehensive reviews have been made by Schlesinger (l983a,b), Reck (l982), and Budyko et al. (l982). In common with our Panel, these generally take as a convenient index the calculated equilibrium change in globally averaged surface air temperature for doubled (or sometimes quadrupled) CO2. The values from studies with comprehensive models and realistic boundary conditions lie within the range suggested by the Charney Panel in l979 and which our Panel found no grounds for changing. In this connection, one must recall that this range represents the best judgments of the two panels based on a small sample of inhomogeneous butâbecause of common physical assumptionsâ not independent numerical experiments. Continued research will, I hope, result in a sharpening of these estimates. Two recent simulations with comprehensive general-circulation models at the National Center for Atmospheric Research (Washington and Meehl, l983) and the NASA Goddard Institute for Space Studies (Hansen, l983) have yielded results also within or very near this range. The global mean temperature index permits us to compare in general terms the potential magnitude and rate of climatic changes due to CC^ with natural changes of the past. As suggested above, global mean surface temperature in the later decades of the next century may be l.5 to 4.5°C warmer than today. At the lower end of this range, the change is comparable in magnitude with the difference between the cold decades of "the Little Ice Age" or the early Holocene warm period and today. The higher end approaches in magnitude the difference between the last glacial maximum and today and enters a climatic range with which mankind has had little experience. Rates of change due to CO2 are projected to be a few tenths of a degree Celsius per decade. As discussed by the
280 Ad Hoc Panel on the Present Interglacial (l974) and by Clark et al. (l982), such rates are for short periods comparable with the rapid changes observed in some regions in the earlier part of this century or at the onset of the Little Ice Age. If sustained for a century or more, the most recent parallel is with the retreat of the Wisconsin glaciation. While convenient and reasonably unambiguous, estimates of projected changes in equilibrium global mean temperature are not very useful. Parameters of climate other than temperatureâprecipitation, storm tracks, cloud cover, for exampleâand the frequencies of extreme events are at least as important in determining the real impact of climatic change. For example, tropical storm formation seems to be related to sea-surface temperature. Although hurricanes are not explicitly treated at present in general-circulation models, one might infer that warmer ocean temperatures would increase their penetration into midlatitudes; such inferences and the topic of climatic extremes deserve careful investigation. In general, the impacts of changing climate would be felt most acutely in terms of local changes that can be expected to vary widely across the Earth's surface. Moreover, as our Panel noted, the pace of change will be slowed and its evolution complicated by the ocean's buffering effects. Here recent measurements of transient tracers in the ocean (cf. Brewer, this volume, Chapter 3, Section 3.2) have tended to confirm the notions of the ocean's circulation and thermal capacity expressed in our report. Thus, understanding of the transient and local responses of climate to increasing C02 is far more relevant to our concerns than the ultimate globally averaged change in equilibrium temperature. Despite the general agreement on the overall magnitude of the C02 effect, our understanding of these regional and temporal effects is only rudimentary. The Panel identified a few changes in zonally averaged quantities that appeared to have some statistical reliability. However, as noted by Manabe et al. (l98l) and exhibited in the comparisons of Schlesinger (l982), more detailed local and regional responses are smothered in a sea of noise. Assessment of the truly relevant aspects of man-made climatic change has only just begun. Climate changes over the United States and over other major agricul- tural regions of the world are naturally of great interest. Unfortu- nately, there is at present little that can be responsibly asserted about the details of such changes. In the Panel's report, we pointed out a few relevant inferences that seemed to be emerging from some experiments, notably a tendency toward summer dryness in midlatitudes (e.g., Manabe et al., l98l; Hayashi, l982). These suggest, for example, some expansion of steppe and desert climates in the latitudes of the United States with increased CO2. This inference is consistent with paleoclimatic data on warmer periods, althoughâas noted aboveâthe analogies are by no means precise. As we gain more confidence in the regional details of climate simulations, analyses of climate model results in terms of climate categorizations tuned to the needs of impact analysis will become useful. Climate models must be markedly improved and much more analysis of the implications of their results must be done before we will be able
28l to place useful confidence in their detailed results. The most chal- lenging, and perhaps the most intractable, problem is cloudiness. It is easy to show that small changes in cloudiness can alter the Earth's heat budget as much or more than the expected changes in CO2 concen- trations. Models with different formulations for cloudiness show great differences in global and regional climate sensitivity, even if their simulations of contemporary climate and of globally averaged changes are comparable. Our Panel concluded that "One should not trust model prediction schemes until they produce meaningful simulations of observed seasonal cloud cover and the seasonal radiation components." I believe that this reservation still stands and presents the outstanding unsolved problem in climate modeling. The parameterizations of boundary-layer convective processesâparticularly in the tropicsâand radiation trans- port also embody significant uncertainties (Kandel, l98l; Luther, l982). Despite our reservations about climate models we have no choice but to use them if we wish to assess the possibilities for changed climate in a changed atmosphere. We can shore up our confidence by conscien- tiously validating models through comparison with nature. The three- dimensional general-circulation models, in particular, can be closely compared with the real world. Indeed, as summarized for example by Gates (l982), the models simulate reasonably well the principal features of today's mean climate, the annual march of the seasons, and even the markedly different climates of the distant past. Of course, a most reassuring validation would be the unequivocal detection of the CO2-induced climate changes that the models predict to be currently taking place. This problem is discussed at length in Chapter 5 by Weller et al., who also conclude from empirical studies that the real sensitivity of the climate system to CO2 increases is in the lower part of the range indicated by models. Quantitative estimates of the uncertainties in model predictions would be extremely useful. Some crude notion may be gained from studying the range of results obtained by different investigators employing different models and methodologies. Indeed, such results are the primary source for the uncertainty estimates proposed in the Charney report and left unchanged by our Panel. Attempts at more rigorous analysis are also being made (Hall et al., l982). However, it must be recognized that all modelers incorporate similar ensembles of variables and physical processes and employ fundamentally similar algorithms (Schneider and Dickinson, l974). One must always admit the possibility of some overlooked or underestimated feedback, e.g., cloudiness, that would markedly change the results. Careful probabilistic analysis of model simulations (e.g., Hayashi, l982) can more clearly separate statistically significant conclusions from meaningless noise; the conclusions are usually discouraging, but the sparse scraps of sig- nificance that remain become even more precious. Finally, one may attempt to delimit the uncertainties attributable to various possible sources of sensitivity through numerical experimentation. Unsuspectedâ and possibly largerâsources of error may lurk in the wings. Neverthe- less, we can hardly expect policymakers to lend credence to our predictions of climatic changes until we can demonstrate that changes are actually taking place (see Chapter 5) and to some degree quantify
282 the credibility of our forecasts. Thus, the development of objective confidence limits for climate sensitivity estimates (e.g., Katz, l982) is an important task for climate modelers. Our Panel discussed at length some dissenting inferences of the magnitude of CC^'s effect on climate. I believe that our report fairly assessed these assertions and put them in proper perspective with respect to other research. In summary, the conclusions of our study appear to remain valid. Man-made increases of CO2 and other trace gases in the atmosphere may be reasonably expected to change climate significantly within the lifetimes of a large fraction of the world's inhabitants who are alive today. (According to Ausubel and Stoto, l98l, 40% of the current population will still be alive 50 years hence.) The change will be large and rapid; it will be greater in global terms than any natural climate changes that civilized man has yet experienced, although, as Schelling observes in Chapter 9 of this report, far less than the climate changes mankind has voluntarily undertaken through migration. We have some general notions of how the climate change will be dis- tributed across the face of the Earth. In particular, there are indications of dryer and hotter summers for some already overheated and underwatered regions of our country, but these are as yet a very uncertain basis for decision making. There is a good prospect that further research can slowly sharpen and validate our predictive tools to give us more useful answers (see CO2/Climate Review Panel, l982, pp. 48-50). References Ad Hoc Panel on the Present Interglacial (l974). Report. Interdepart- mental Committee for Atmospheric Sciences, Federal Council for Science and Technology. ICAS l8B-FY75. Washington, D.C. Ausubel, J. H., and M. A. Stoto (l98l). A Note on the Population Fifty Years Hence. International Institute for Applied Systems Analysis, Laxenburg, Austria. Budyko, M. I., and N. A. Yefimova (l98l). Impact of carbon dioxide on climate. Meteorol. Hydrol. 2:5-l7. Budyko, M. I., K. Ya. Vinnikov, and N. A. Yefimova (l982). The dependence of the air temperature and precipitation on carbon dioxide concentration in the atmosphere. Meteorol. Hydrol. 4:5-l3. Chamberlain, J. W., M. M. Foley, G. J. MacDonald, and M. A. Ruderman (l982). Climatic effect of trace constituents. In Carbon Dioxide Review: l982, W. C. Clark, ed. Oxford U. Press, New York. Clark, W. C., ed. (l982). Carbon Dioxide Review: l982. Oxford U. Press, New York, 469 pp. Clark, W. C., K. H. Cook, G. Marland, A. M. Weinberg, R. M. Rotty, P. R. Bell, L. J. Allison, and C. L. Cooper (l982). The carbon dioxide question: a perspective for l982. In W. C. Clark, ed. (l982), pp. 3-43. Climate Research Board (l979) . Carbon Dioxide and Climate: A Scientific Assessment. National Academy of Sciences, Washington, D.C.
283 CO2/Climate Review Panel (l982). Carbon Dioxide and Climate: A Second Assessment. National Research Council, National Academy Press, 72 pp. Gates, W. L. (l982). Paleoclimatic modelingâa review with reference to problems and prospects for the pre-Pleistocene. In Climate in Earth History. Geophysics Study Committee, National Academy of Sciences, Washington, D.C., pp. 26-4l. Geophysical Fluid Dynamics Laboratory (l982). Activities FY80-Plans FY8l. Environmental Research Laboratories, National Oceanic and Atmospheric Administration. Hall, M. C. G., D. G. Cauci, and M. E. Schlesinger (l982). Sensitivity analysis of a radiative-convective model by the adjoint method. J. Atmos. Sci. 39:2038-2050. Hansen, J. E., A. Lacis, and S. A. Lebedeff (l982). Commentary. In W. C. Clark, ed. (l982), pp. 284-289. Hansen, J. E. (l983). Climate model sensitivities to changed solar irradiance and CO2. In Climate Processes and Climate Sensitivity, Maurice Ewing Series, Vol. 4. American Geophysical Union, Washington, D.C. Hayashi, Y. (l982). Confidence intervals of a climatic signal. J-. Atmos. Sci. 39:l895-l905. Idso, S. B. (l980). The climatological significance of a doubling of the earth's atmospheric carbon dioxide concentration. Science 207:l462-l463. Idso, S. B. (1982a). A surface air temperature response function for earth's atmosphere. Boundary Layer Meteorol. 22:227-232. Idso, S. B. (l982b). Carbon Dioxide: Friend or Foe? IBR Press, Ternpe, Ariz., 92 pp. Kandel, R. S. (l98l). Surface temperature sensitivity to increased atmospheric CO2. Nature 293:634-636. Katz, R. W. (l982). Statistical evaluation of climate experiments with general circulation models: a parametric time series modeling approach. J. Atmos. Sci. 39:l446-l455. Kellogg, W. W. (l977). Effects of Human Activities on Global Climate. Tech. Note No. l56 (WMO No. 486). World Meteorological Organization, Geneva, 47 pp. Kutzbach, J. E., and B. L. Otto-Bliesner (l982). The sensitivity of African-Asian Monsoon climate to orbital parameter changes for 9000 years BP in a low-resolution general circulation model. J. Atmos. Sci. 39:ll77-ll88. Luther, F. M. (l982). Radiative effects of a CO2 increase: results of a model comparison. In Proceedings: Carbon Dioxide Research Conference: Carbon Dioxide, Science and Consensus. U.S. Dept. of Energy, CONF-820970, III-l77-III-l93. Manabe, S., R. T. Wetherald, and R. S. Stouffer (l98l). Summer dryness due to an increase of atmospheric CO2 concentration. Climatic Change 3:347-386. Ramanathan, V. (l982). Commentary. In W. C. Clark, ed. (l982), pp. 278-283. Reck, R. A. (l982). Introduction to the Proceedings of the Workshop. In Reck and Hummel (l982).
284 Reck, R. A., and V. R. Hummel (l982). Interpretation of Climate and Photochemical Models, Ozone and Temperature Measurements. AIP Conference Proceedings No. 82. American Institute of Physics, New York, 308 pp. Revelle, R., and H. E. Suess (l957). Carbon dioxide exchange between atmosphere and ocean and the question of an increase of atmospheric C02 during the past decades. Tellus 9:l8-27. Schlesinger, M. E. (l983a). Simulating CO2-induced climatic change with mathematical climate models: capabilities, limitations, and prospects. In Proceedings, Conference on Carbon Dioxide, Climate, and Consensus, Coolfont, Virginia. U.S. Dept. of Energy, Washington, D.C. Schlesinger, M. E. (l983b). A review of climate models and their simulation of C02-induced warming. Intern. J. Environ. Studies 20.: l03-ll4. Schneider, S. H., and R. E. Dickinson (l974). Climate modeling. Rev. Geophys. and Space Phys. l2:447-493. U.S. Department of Energy (l983). Proceedings: Carbon Dioxide Research Conference: Carbon Dioxide, Science, and Consensus. CONF-820970, February l983. Washington, W. M., and 6. A. Meehl (l982). A summary of recent NCAR general circulation experiments on climatic effects of doubled and quadrupled amounts of C02. In Proceedings, Conference on Carbon Dioxide, Climate, and Consensus, Coolfont, Virginia. U.S. Dept. of Energy, Washington, D.C. Washington, W. M., and G. A. Meehl (l983). General circulation model experiments on the climatic effects due to a doubling and quadrupling of carbon dioxide concentrations. J. Geophys. Res., in press.
285 4.2 EFFECTS OF NON-CO, GREENHOUSE GASES Lester Machta C02 is not the only gas or geophysical property capable of modifying the future climate. While a discussion of every potential climate modifier lies beyond this report, certain of them offer sufficient similarities to CO2 (e.g., they are also "greenhouse" gases) that a brief survey is justified. Non-C02 greenhouse gases (and other climate modifiers) bear on the C02 issue in several important ways: (l) they can enhance the climate changes expected from rising atmo- spheric C02 and hence confuse the expected CO2 changes; (2) the past climate responses to other forcing functions can aid in the interpretation of C02 warming; but (3) perhaps most important, predictions of the future climate require that all potential factors be taken into account. In the past few decades the remarkable increase in interest in atmospheric chemistry along with improved technology have made it possible to measure changes with time of the concentration of many constituents of air not formerly possible. Despite this new capability, there may still be other greenhouse gases beyond those noted below that are not yet measured. Chlorofluorocarbons. This class of gases originates from industrial activities and has been emitted to the atmosphere during the past 50 years. They are increasing in the atmosphere approximately as expected from their growth in emissions. CFC-ll, CFC-l2, and CFC-22, the three most abundant ones, all have long residence times in the air (tens of years) so that they can accumulate. Figure 4.l illustrates typical time histories of CFC-ll and CFC-l2 at ground level. Note that, like CO2, with its fairly long atmospheric residence time, the concentra- tion in air will usually increase even if the rate of emissions decreases; this is the case for CFC-ll and CFC-l2 during the past few years. Both the sources and sinks of the chlorofluorocarbons are believed to be known. The emissions from industrial production and produce uses (such as aerosol propellants), for which good estimates are published by the Chemical Manufacturers Association, represent the only source of any consequence. Photochemical destruction, mainly in the stratosphere, and very slow uptake by the oceans are the only known significant sinks. Theoretically, chlorofluorocarbons are implicated as potential destroyers of stratospheric ozone, which in turn could result in human health and ecological damage from increased ultraviolet radiation. Since some consideration has been given to restricting their emissions, an extrapolation of current or past growth rates to predict future atmospheric concentrations may be unwise at this time. Nitrous oxide, it is likely that most nitrous oxide in the air has come from denitrification in the natural or cultivated biosphere. One would therefore expect to find the largest part of atmospheric nitrous oxide to be derived from nature, unrelated to human activity. Recent, careful measurements by a few investigators (Weiss, l98l; U.S. Govern- ment, l982) have suggested a small growth rate of the concentration of nitrous oxide in ground level air at remote locations as illustrated in
286 380 340 300 260 220 CFC-11 CFC-12 11.3 PPT/YR 15.7 PPT/YR I 1 1.0 PPB/YR 220 200 180 160 HO 310 30C 1977 1978 1979 1980 1981 1982 . FIGURE 4.l Upper panels: Measurements of chlorofluorocarbon (CFC-ll, CFC-l2) concentration in the atmosphere, Mauna Loa Observatory. Bottom panel: Measurements of atmospheric concentration of nitrous oxide in ground-level air at remote locations. [Source: Geophysical Monitoring for Climate Change (GMCC), Air Resources Laboratory, Rockville, Md.] Figure 4.l. The source of the small increase is unknown, but a prime candidate is the continued expanded use of nitrogen fertilizers around the world to improve agricultural productivity. If so, the current slow increase is likely to continue into the foreseeable future since food demands will grow with population size. It has been suggested that since nitrous oxide is stable in the troposphere and is implicated in potential ozone destruction, there might be a move to try to restrict fertilizer usage. Figure 4.l suggests, however, that the rate of increase of nitrous oxide in ground level air is so small, perhaps 0.25% per year, that many decades would pass before an increase of nitrous oxide would raise concern for ozone depletion. Methane. The most abundant hydrocarbon, often called natural gas, is increasing in the atmosphere. It is thought to be a natural con- stituent of the air arising as it does from many biological processes and perhaps seeping out of the Earth. Measurements in the l950s and l960s had large error bars, and there were spatial differences so that the observed temporal variability was not viewed as an upward trend. However, in the late l970s several investigators using gas chromatog-
287 1.7 1.6 i Z 1.5 O < c O 8 1.4 1.3 1.2 METHANE 1965 1970 YEAR 1975 1980 FIGURE 4.2 Northern hemisphere methane measurements, compiled from various sources, from l965 to l980 (log scale). The straight-line exponential best fit corresponds to an annual increase of about l.7% per year (Rasmussen et al., l98l). raphy have unequivocably demonstrated an upward trend. Rasmussen and Khalil (l98l) have combined their recent measurements with earlier ones to suggest that the trend existed since at least l965, as shown in Figure 4.2. Craig and Chow (l982) have found, from measurements of methane in ice cores, that concentrations prior to the sixteenth century were 0.7 ppmv. Rasmussen and Khalil suggest that the expansion in the number of farm animals and rice production might well explain, at least quali- tatively, the atmospheric methane growth. Other biological activities such as termite destruction of wood (Zimmerman et al., l982) and possible leakage from man's mining and use of fossil methane might also contribute to methane in the air; their contribution to a growth of methane in air is less clear. The higher concentrations far north of the equatorial region suggest the termite source to be minor. The relatively rapid recent increase with time, about as fast as for CO2, combined with the uncertainty as to its origin are both intriguing features of the methane growth in air. It is possible that the trend in the stable isotopic content of the carbon in methane might shed light on the source of the growth. Thus, atmospheric methane is now about -40%, while that derived from biological activity is about -60%.
288 If the growth in atmospheric methane is due to increased biological sources, the carbon in methane should become more negative with time. The interpretation of such isotopic trends will require an understanding of appropriate fractionation factors as the methane moves from one reservoir to another. There is no reason to expect the upward trend in atmospheric methane concentration to stop soon since the most likely sources of methane are related to population size. In the long run, those sources that are dependent on the size of a biospheric feature (e.g., cows or rice paddies) will ultimately be limited by space. Thus, the growth rate in atmospheric concentration illustrated in Figure 4.2 might continue for many decades but probably not for centuries. A better understanding of the source of the upward trend would improve this prediction. Tropospheric ozone. Tropospheric ozone was originally believed to be primarily a consequence of transport from stratospheric ozone by air motions. It can also be created within the troposphere by man and nature. Locally, as in the Los Angeles Basin, large amounts of ozone are derived from oxides of nitrogen, hydrocarbons, and sunlight. Few scientists believe that these local sources of pollution can increase the upper-tropospheric concentrations of ozone since ozone is so reactive that its lifetime in the lower atmosphere is relatively short, no more than a few days. Nevertheless, an analysis of a limited number of measurements of ozone in the 2- to 8-km layer in the northern hemisphere suggest an upward trend as evidenced in Figure 4.3 from Angell (l983) . Note that no trend can be found in southern Australia. It has been suggested by Liu et al. (l980) that this increase of mid- and upper-troposphere ozone concentration of the northern hemisphere results from photochemical reactions of the oxides of nitrogen and hydrocarbons emitted by high-flying jet aircraft. Since the lifetime of an ozone molecule in the upper troposphere is also relatively short, little accumulation takes place. An increase in concentration must therefore reflect a continual increase in aircraft emissions, if that is the source. During most of the period illustrated in Figure 4.3, the number of jet aircraft as increasing in the northern hemisphere. Some other gases. The ALE program of the Chemical Manufacturers Association and CSIRO have measured several other gases at the Australian Baseline station in Tasmania. At least two of three gases (in addition to some already noted above) show upward trends and may have absorption lines in the infrared window of -the electromagnetic spectrum, making them potential greenhouse gases: carbon tetrachloride (CCl4) and methyl chloroform (CI^CC^) . The growth of carbon tetra- chloride reported from Tasmania is about l% per year since l976, but the methyl chloroform is closer to l0% per year since l979. Very likely both of these gases possess both natural and man-made sources. On the other hand, measurements at the Mauna Loa Observatory exhibit no or insignificant increases in carbon monoxide (CO). It is likely that the list of atmospheric gases studied for their trends and potential green- house effects will grow in years to come: the study of greenhouse gases other than CO2 is still in its infancy. The atmospheric concentrations of these trace gases are not all inde- pendent of one another. Complicated chemical reactions among these
289 1 20 10 0 10 0 O if -10 10 0 -10 -20 LU Q Ozone, 2-8 km 1965 1970 1975 1980 YEAR FIGURE 4.3 Ozone measurements in the 2- to 8-km layer at various latitudes. (Source: Angell, l983.) gases, as well as with other gases not particularly radiatively active, can affect their concentrations. For instance, it has been argued (WMO, l983) that increases in carbon monoxide (CO) in the presence of NO can, by OH oxidation, produce an increase in O3 and methane (CH4). In addition to chemical reactions with today's atmospheric composi- tion, there would likely be new climate-chemistry interactions in the future. As the composition changes, the expected higher atmospheric water-vapor content will affect the atmospheric chemistry. An increase in OH accompanying an increase in H2O could reduce the O3 and CH4 otherwise present. Thus, unlike CO2, which generally does not undergo chemical changes in the air, these trace gases frequently do. Not only can the mean concentration be affected by other chemicals and sunlight, but distribu- tion particularly in the vertical can be influenced (ozone is a prime example). To estimate future concentrations will require more than estimates of natural and man-made emission rates, fundamental though those rates will be. Increased global coverage of the measurements of these gases will also help in separating natural from man-made sources of some of these gases. Climate effect. Most of the estimates of the climate effects of the trace species have been based on l-D radiative-convective models. Typically the calculation involves doubling a reference concentration
290 of the gas (for the chlorofluorocarbons, increases from 0 to l or 2 ppb are used) while other constituents are held constant. Table 4.l gives some estimates of the change in surface temperature due to either a doubling of their concentration or an increase from 0 to l ppb for the halocarbons. The table was adapted from Table 2a in HMO (l983). There are other published values, but they generally do not disagree by more than about ±30% with the figures given here. The models used to obtain these results generally gave a sensitivity to doubled CO2 between 2 and 3°C. Thus, none of the changes of individual trace gases approaches CO2 by itself, but it is clear that the summation of all of these potential changes could be of the same magnitude as CO2. It is worth noting that because the concentration of each of these gases is small enough to be treated as optically thin, the temperature effect is linearly proportional to their concentration, whereas the CO2 effect depends logarithmically on the concentration. The spectroscopic parameters of several of these gases is not well known, and even the band strengths of some have not been measured. The spectral transmittance and total band absorptions also need to be better determined. These improvements will be needed to develop more accurate radiative transfer models. It will also help in answering questions about band overlap between constituents and with water vapor. Such information is needed for better parameterization in climate models. TABLE 4.l Some Estimates of Surface Temperature Change due to Changes in Atmospheric Constituents Other Than CO2 Constituent Mixing Change From Ratio Surface (ppb) Temperature To Change (°C) Sourcel Nitrous oxide (N2O) 300 600 0.3-0.4 l,3 Methane (CH4) l500 3000 0.3 3,4 CFC-ll (CFCl3) 0 l 0.l5 l,5 CFC-l2 (CF2Cl2) 0 1 0.l3 l,5 CFC-22 (CF2HCl) 0 l 0.04 7 Carbon tetrachloride (CCl4) 0 l 0.l4 l,5 Carbon tetrafluoride (CF4) 0 l 0.07 2 Methyl chloride (CH3Cl3) 0 l 0.0l3 l,5 Methylene chloride (CH2Cl2) 0 l 0.05 l,5 Chloroform (CHCl3) 0 l 0.l l,5 Methyl chloroform (CH3CCl3) 0 l 0.02 7 Ethylene (C2H4) 0.2 0.4 0.0l l Sulfur dioxide (SO2) 2 4 0.02 l Ammonia (NH3) 6 l2 0.09 l Tropospheric ozone (O3) F(Lat,ht) 2 F(Lat,ht) 0.9 4,6 Stratospheric water vapor (H2O) 3000 6000 0.6 l ^Sources: l, Wang et al. (l976); 2, Wang et al. (l980); 3, Conner and Ramanathan (l980); 4, Hameed et al. (l980); 5, Ramanathan (l975) ; 6, Fishman et al. (l979); 7, Hummel and Reck (l98l).
29l References Angell, J. K. (l983). Global variation in total ozone and layer-mean ozone: an update through l98l. Manuscript, Air Resources Laboratory, Silver Spring, Md. Craig, H., and C. C. Chou (l982). Methane: the record in polar ice cores. Geophys. Res. Lett. 9:l22l-l224. Donner, L., and V. Ramanathan (l980). Methane and nitrous oxide: their effect on the terrestrial climate. J. Atmos. Sci. 37:ll9-l24. Fishman, J., V. Ramanathan, P. Crutzen, and S. Liu (l979). Tropospheric ozone and climate. Nature 282:8l8-820. Hameed, S., R. Cess, and J. Hogan (l980). Response of the global climate to changes in atmospheric composition due to fossil fuel burning. J. Geophys. Res. 85:7537-7545. Hummel, J. R., and R. A. Reck (l98l). The direct thermal effect of CHClF2, CH3CCl3 and CH2Cl2 on atmospheric surface temperatures. Atmos. Environ. l5:379-382. Liu, S. C., D. Kley, M. McFarland, J. Mahlman, and H. Levy, II (l980). On the original of tropospheric ozone. J. Geophys. Res. J3I5:7546-7552. Ramanathan, V. (l975). Greenhouse effect due to chlorofluorocarbons: climatic implications. Science l90:50-52. Rasmussen, R. A., and M. A. K. Khalil (l98l). Atmospheric methane (CH4): trends and seasonal cycles. J. Geophys. Res. 86:9826-9832. U.S. Government (l982). Summary report for l98l. Geophysical Monitoring for Climatic Change No. l0. Wang, W. C., Y. Yung, A. Lacis, T. Mo, and J. Hansen (l976). Greenhouse effects due to man-made perturbation of trace gases. Science l94:685-690. Wang, W. C., J. P. Pinto, and Y. Yung (l980). Climatic effects due to halogenated compounds in the Earth's atmosphere. J. Atmos. Sci. 17:333-338. Weiss, R. F. (l98l). The temporal and spatial distribution of tropospheric nitrous oxide. J. Geophys. Res. 86:7l85-7l96. WMO (l983). Report of the Meeting of Experts on Potential Climatic Effects of Ozone and Other Minor Trace Gases. Report No. l4. WMO Global Ozone Research and Monitoring Project, Geneva, 38 pp. Zimmerman, P. R., J. P. Greenberg, S. O. Wandiga, and P. J. Crutzen (l982). Termites: a potentially large source of atmospheric methane, carbon dioxide and molecular hydrogen. Science 2l8:563-565.
