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Interception of sunlight by the high-altitude worldwide dust cloud generated by impact of a large asteroid or comet would lead to substantial land surface cooling, according to our three-dimensional atmospheric general circulation model (GCM). This result is qualitatively similar to conclusions drawn from an earlier study that employed a one-dimensional atmospheric model, but in the GCM simulation the heat capacity of the oceans substantially mitigates land surface cooling, an effect that one-dimensional models cannot quantify. On the other hand, the low heat capacity of the GCM land surface allows temperatures to drop more rapidly in the initial stages of cooling than in the one-dimensional model study. These two differences between three-dimensional and one-dimensional model simulations were noted previously in studies of “nuclear winter”; GCM-simulated climatic changes in the Alvarez-inspired scenario of “asteroid/comet winter,” however, are more severe than in “nuclear winter” because the assumed aerosol amount is large enough to intercept all sunlight falling on Earth. Impacts of smaller objects—which would occur much more frequently than the Cretaceous/Tertiary event deduced by Alvarez and coworkers—could also lead to dramatic, though of course less severe, climatic changes, according to our GCM. Our conclusion is that it is difficult to imagine an asteroid or comet impact leading to anything approaching complete global freezing, but quite reasonable to assume that impacts at the Alvarez level, or even smaller, dramatically alter the climate in at least a “patchy” sense.

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