Atmospheric Chemistry And Global Warming

The links of trace gas chemistry to climate change extend beyond the observed increase in C02 concentrations. Increases in CH4, tropospheric 03, and anthropo-genically produced concentrations of the chlorofluorocarbons (CFCs) also contribute to global warming (see Fig. 8). If only the direct radiative effects of the trace gas increases are considered, 62% of the increase would be due to C02, 20% to CH4,4% to N20, and 14% to the CFCs. If chemical feedbacks are considered, the global warming due to changes with respect to changes in ozone concentrations (in both the stratosphere and troposphere) result in changes in ozone being as important as the changes in methane. With the international cooperation now in place to phase out the production and use of CFCs, future scenarios indicate that tropospheric ozone increases will replace methane as the second most important trace gas that contributes to the greenhouse effect (see Houghton et al., 1990; Houghton et al., 1996).

Furthermore, because tropospheric ozone is a relatively short-lived gas, especially when compared to other gases that contribute to global warming, increases in its concentration may have regional and seasonal effects that must be accounted for properly when temperature perturbations are being computed. Another consideration for regional climate change is the presence of particulates that are produced by fossil fuel combustion and biomass burning. Particles screen some of the incoming solar radiation and therefore result in a regional cooling effect. Studies to date show that both tropospheric ozone and man-made aerosols must be included in any scenarios attempting to simulate global climate and that the regional effects caused by these two constituents are comparable to the global perturbation of the longer-lived trace gases.

Lastly, the future buildup of methane and hydrogenated CFCs, the replacement gases to the CFCs, which can be removed from the atmosphere by OH oxidation, is complicated by the potential change in the oxidizing capacity of the troposphere. As 03 increases in the troposphere, it is likely that the global abundance of OH will also increase, since the primary formation mechanism of OH in the troposphere is initiated by the photolysis of 03 and the subsequent reaction of the excited oxygen atom with water vapor. If more OH is present, the removal of CH4 and the replacements for the CFCs, which are removed in the troposphere by OH, becomes more efficient and thus their rate of increase is slowed.

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