Tropospheric sulfate aerosols produced by atmospheric oxidation of SO2
emitted from fossil fuel combustion scatter solar radiation and enhan
ce the reflectivity of clouds. Both effects decrease the absorption of
solar radiation by the earth-atmosphere system. This cooling influenc
e tends to offset the warming influence resulting from increased absor
ption of terrestrial infrared radiation by increased atmospheric conce
ntrations 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 cool
ing. Because of the vastly different atmospheric residence times of su
lfate aerosol (about a week) and CO2 (about 100 years), the cooling in
fluence of sulfate aerosol is exerted immediately, whereas most of the
warming influence of CO2 is exerted over more than 100 years. Consequ
ently the total forcing integrated over the entire time the materials
reside in the atmosphere is net warming, with the total CO2 forcing es
timated to exceed the sulfate forcing by a factor of 4 (uncertainty ra
nge 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.