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Executive Summary l. Carbon dioxide (CO2) is one of the gases of the atmosphere important in determining the Earth's climate. In the last generation the CO2 concentration in the atmosphere has increased from 3l5 parts per million (ppm) by volume to over 340 ppmv. (Chapters 3, 4) 2. The current increase is primarily attributable to burning of coal, oil, and gas; future increases will similarly be determined primarily by fossil fuel combustion. Deforestation and land use changes have probably been important factors in atmospheric CO2 increase over the past l00 years. (Chapters 2, 3) 3. Projections of future fossil fuel use and atmospheric concen- trations of CO2 embody large uncertainties that are to a considerable extent irreducible. The dominant sources of uncertainty stem from our inability to predict future economic and technological developments that will determine the global demand for energy and the attractiveness of fossil fuels. We think it most likely that atmospheric CO2 con- centration will pass 600 ppm (the nominal doubling of the recent level) in the third quarter of the next century. We also estimate that there is about a l-in-20 chance that doubling will occur before 2035. (Chapters 2, 3) 4. If deforestation has been a large net source of CO2 in recent decades, then the models that we are using to project future atmospheric concentrations are seriously flawed; the fraction of man-made CCÂ«2 remaining airborne must then be lower, and CO2 increase will probably occur more slowly than it otherwise would. (Chapter 3) 5. Estimates of effects of increasing CO2 on climate also embody significant uncertainties, stemming from fundamental gaps in our under- standing of physical processes, notably the processes that determine cloudiness and the long-term interactions between atmosphere and ocean. (Chapter 4) 6. Several other gases besides CO2 that can affect the climate appear to be increasing as a result of human activities; if we project
increases in all these gases, climate changes can be expected sig- nificantly earlier than if we consider CO2 alone. (Chapter 4) 7. From climate model simulations of increased C02 we conclude with considerable confidence that there would be global mean temperature increase. With much less confidence we infer other more specific regional climate changes, including relatively greater polar temperature increase and summer dryness in middle latitudes (e.g., the latitudes of the United States). (Chapter 4) 8. Results of most numerical model experiments suggest that a doubling of CO2, if maintained indefinitely, would cause a global surface air warming of between l.5Â°C and 4.5Â°C. The climate record of the past hundred years and our estimates of C02 changes over that period suggest that values in the lower half of this range are more probable. (Chapters 4, 5) 9. By itself, CO2 increase should have beneficial effects on photosynthesis and water-use efficiency of agricultural plants, especially when other factors are not already limiting growth. (Chapters 3, 6) l0. Analysis of the effects of a warmer and drier climate on rain- fed agriculture in the United States suggests that over the next couple of decades negative effects of climate change and positive effects from C02 fertilization both will be modest and will approximately balance. The outlook is more troubling for agriculture in lands dependent on irrigation. Longer-term impacts are highly uncertain and will depend strongly on the outcome of future agricultural research, development, and technology. (Chapter 6) ll. Changes in temperature and rainfall may be amplified as changes in the annual discharge of rivers. For example, a 2Â°C warming could severely reduce the quantity and quality of water resources in the western United States. (Chapter 7) l2. (a) If a global warming of about 3 or 4Â°C were to occur over the next hundred years, it is likely that there would be a global sea-level rise of about 70 cm, in comparison with the rise of about l5 cm over the last century. More rapid rates could occur subsequently, if the West Antarctic Ice Sheet should begin to disintegrate. (Chapter 8) (b) Such a warming might also bring about changes in Arctic ice cover, with perhaps a disappearance of the summer ice pack and asso- ciated changes in high-latitude weather and climate. (Annex l) l3. Because of their large uncertainties and significant implica- tions, it is important to confirm the various predictions of climate changes at the earliest possible time and to achieve greater precision. This can best be done through carefully designed monitoring programs of long duration emphasizing the ensemble of variables believed to influence climate or to reflect strongly the effect of C02. (Chapter 5)
l4. The social and economic implications of even the most carefully constructed and detailed scenarios of CO2 increase and climatic con- sequences are largely unpredictable. However, a number of inferences seem clear: (a) Rapid climate change will take its place among the numerous other changes that will influence the course of society, and these other changes may largely determine whether the climatic impacts of greenhouse gases are a serious problem. (b) As a human experience, climate change is far from novel; large numbers of people now live in almost all climatic zones and move easily between them. (c) Nevertheless, we are deeply concerned about environmental changes of this magnitude; man-made emissions of greenhouse gases promise to impose a warming of unusual dimensions on a global climate that is already unusually warm. We may get into trouble in ways that we have barely imagined, like release of methane from marine sediments, or not yet discovered. (d) Climate changes, their benefits and damages, and the benefits and damages of the actions that bring them about will fall unequally on the world's people and nations. Because of real or perceived inequities, climate change could well be a divisive rather than a unifying factor in world affairs. (Chapter 9) l5. Viewed in terms of energy, global pollution, and worldwide environmental damage, the "CO2 problem" appears intractable. Viewed as a problem of changes in local environmental factorsârainfall, river flow, sea levelâthe myriad of individual incremental problems take their place among the other stresses to which nations and individuals adapt. It is important to be flexible both in definition of the issue, which is really more climate change than CO2, and in maintaining a variety of alternative options for response. (Chapter 9) l6. Given the extent and character of the uncertainty in each segment of the argumentâemissions, concentrations, climatic effects, environ- mental and societal impactsâa balanced program of research, both basic and applied, is called for, with appropriate attention to more signifi- cant uncertainties and potentially more serious problems. (Chapter l) l7. Even very forceful policies adopted soon with regard to energy and land use are unlikely to prevent some modification of climate as a result of human activities. Thus, it is prudent to undertake applied research and developmentâand to consider some adjustmentsâin regard to activities, like irrigated agriculture, that are vulnerable to climate change. (Chapters l, 9) l8. Assessment of the CO2 issue should be regarded as an iterative process that emphasizes carry over of learning from one effort to the next. (Chapter l) l9. Successful response to widespread environmental change will be facilitated by the existence of an international network of scientists
conversant with the issues and of broad international consensus on facts and their reliability. Sound international research and assessment efforts can turn up new solutions and lubricate the processes of change and adaptation. (Chapter l) 20. With respect to specific recommendations on research, develop- ment, or use of different energy systems, the Committee offers three levels of recommendations. These are based on the general view that, if other things are equal, policy should lean away from the injection of greenhouse gases into the atmosphere. (a) Research and development should give some priority to the enhancement of long-term energy options that are not based on com- bustion of fossil fuels. (Chapters l, 2, 9) (b) We do not believe, however, that the evidence at hand about CO2-induced climate change would support steps to change current fuel-use patterns away from fossil fuels. Such steps may be necessary or desirable at some time in the future, and we should certainly think carefully about costs and benefits of such steps; but the very near future would be better spent improving our knowledge (including knowl- edge of energy and other processes leading to creation of greenhouse gases) than in changing fuel mix or use. (Chapters l, 2, 9) (c) It is possible that steps to control costly climate change should start with non-CO2 greenhouse gases. While our studies focused chiefly on C(^, fragmentary evidence suggests that non-CO2 greenhouse gases may be as important a set of determinants as CO2 itself. While the costs of climate change from non-CO2 gases would be the same as those from CO2, the control of emissions of some non-CO2 gases may be more easily achieved. (Chapters l, 2, 4, 9) 2l. Finally, we wish to emphasize that the CO2 issue interacts with many other issues, and it can be seen as a healthy stimulus for acquiring knowledge and skills useful in the treatment of numerous other important problems. (Chapter l)