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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
2 CARBON DIOXIDE AND CLIMATE: 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 CO2. 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
Summary of Conclusions and Recommendations 3 to be too 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 CO2- 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 CCvinduced 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
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, it 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.
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
6 CARBON DIOXIDE AND CLIMATE: 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 CO2 to be ' 'near 3Â°C with a probable error of
Summary of Conclusions and Recommendations 7 Â±1.5Â°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 45Â° 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
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. DEVELOPMENT OF MONITORING AND EARLY DETECTION STRATEGIES A comprehensive set of variables should be monitored in order to discriminate CO2-induced changes from changes in climate caused by other factors. These variables should include CO2 concentration in the atmosphere, the solar irradiance, the spectral distribution of solar and terrestrial radiation (at the top and bottom of the atmosphere), and concentrations of aerosol and minor constituents in the atmosphere. A set of indices that have a large signal-to-noise ratio with respect to CO2-induced changes should be identified and monitored. Emphasis should be placed on the compilation and analysis of past climatic data to acquire more reliable reconstructions of past variations of climate on a variety of space scales. The operational monitoring of the ocean's response to climatic change may provide an early indication of climate change. Of particular value appear to be such indices as potential temperature and salinity changes on isopycnals in the wind-driven gyres.