Tropospheric sulfate aerosols produced by atmospheric oxidation of SO2 emitted from fossil fuel combustion scatter solar radiation and enhance the reflectivity of clouds. Both effects decrease the absorption of solar radiation by the earth-atmosphere system. This cooling influence tends to offset the warming influence resulting from increased absorption of terrestrial infrared radiation by increased atmospheric concentrations of CO2. The sulfate forcing is estimated to be offsetting 70% of the forcing by CO2 derived from fossil fuel combustion, although the uncertainty of this estimate is quite large--range 28 to 140%, the latter figure indicating that the present combined forcing is net cooling. Because of the vastly different atmospheric residence times of sulfate aerosol (about a week) and CO2 (about 100 years), the cooling influence of sulfate aerosol is exerted immediately, whereas most of the warming influence of CO2 is exerted over more than 100 years. Consequently the total forcing integrated over the entire time the materials reside in the atmosphere is net warming, with the total CO2 forcing estimated to exceed the sulfate forcing by a factor of 4 (uncertainty range 2 to more than 10). The present situation in which the forcing by sulfate is comparable to that by CO2 is shown to be a consequence of the steeply increasing rates of emissions over the industrial era.
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