• The impact of HLLV emissions on the stratosphere and upper atmosphere needs more investigation, as does the impact of changes in upper atmosphere constituents on the troposphere. 4.2 CONCLUSIONS There is no question that man is capable of, and indeed has been, modifying the local and regional climate. Our ability to perturb the global chemical content of the atmosphere has been documented for CO2 and other substances. Global climate models indicate strongly that these perturbations may change the climate of the world, but the exact nature and extent of the change cannot at present be estimated with any certainty, largely because of the great natural variability of climate. The specific conclusions of this study can be summarized: • A consensus exists that warming of the lower atmosphere may occur due to increasing concentrations of greenhouse gases, particularly CO2. A variety of climate models predict a 1° to 3°C warming for a doubling of the CO2 level, but the magnitude of the consequences are uncertain. • Noticeable warming should not occur before the end of this century. The current trend of global temperature has been towards cooling, probably due to natural causes. • The long residence time of CO2 in the atmosphere could mean that adverse impacts from global warming will persist for several centuries. • There are substantial uncertainties in the attempts to simulate the response of the atmosphere to climate-forcing perturbations. • The problem of increased particle loading is currently considered to be of lesser importance than the greenhouse effect. The direction of the particle effect is currently unknown. • Global waste heat impacts appear to be of negligible importance for the next century or so. If per capita energy use and population continue to increase rapidly, waste heat may become an important global issue after 2100. • Regional and local waste heat impacts may be significant but will be highly site specific and will depend on cooling technology. • The rate of climatic change is probably at least as important as the magnitude of climatic change.
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