Does Fossil Fuel Combustion Lead To Global Waunini?t

Tropospheric sulfate aerosols produced by atmospheric oxidation of S02 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 C02. The sulfate forcing is estimated to be offsetting 70% of the forcing by C02 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 C02 (about 100 years), the cooling influence of sulfate aerosol is exerted immediately, whereas most of the warming influence of C02 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 C02 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 C~ is shown to be a consequence of the steeply increasing rates of emissions over the industrial era.