most important changes in composition associated with HLLVs, namely, decreases in ozone and increases in H2O and NO^., would not have a significant effect on climate, at least as measured by changes in surface temperature. For example, Ramanathan et al. (50) computed a surface temperature increase of only 0.06 °K for a 10% increase in stratospheric water vapor. Since HLLVs are expected to produce at most a 0.5% increase in water vapor, their globally averaged effect through this mechanism can be ignored. Ozone decreases should be even less important, since they are concentrated strongly at levels well below the maximum in ozone heating at 50 km (31 mi). Even if ozone were to be decreased at all heights by the maximum value of 1%, surface temperature decreases would amount to no more than 0.01 °K (50). This decrease would be made even smaller by the expected increase in NOn which acts to warm the troposphere by absorbing solar radiation reflected from the surface. Thus, on a globally averaged basis, radiative mechanisms would be ineffective in causing significant climatic changes associated with HLLVs. It is conceivable that the radiative effects could be enhanced by the formation of “corridors” of H2O and NOj. enhancement (or ozone reduction). For example, if all the water vapor from the HLLVs were concentrated in a 5°-wide latitudinal band, water vapor increases of 10% within that band would be possible. However, this is not likely to result in a local 0.06 °K increase in surface temperature, for horizontal transport will smear out any synoptic scale heating on a much more rapid time scale than the several months it takes to build up significant water vapor increases. In any case, it was shown in Sec. 4 that the establishment of a water vapor “corridor” is highly unlikely below 60 km (36 mi). It should be noted that cloud cover is an extremely important parameter in the assessment of climate — and a critically uncertain one as well. However, it appears that changes in cloud cover associated with HLLVs will be insignificant. In the mesosphere, noctilucent cloud incidence is not likely to increase greatly, as shown in
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