INDIRECT INFLUENCE OF OZONE DEPLETION ON CLIMATE FORCING BY CLOUDS

CHEMICAL depletion of ozone in the lower stratosphere decreases the di rect radiative forcing from greenhouse gases in the atmosphere(1). Her e we show that ozone depletion may also exert an indirect effect on ra diative forcing via its effect on the oxidation state of the atmospher e. Hydroxyl (OH) radicals in the troposphere are produced by photodiss ociation of tropospheric ozone in the presence of water vapour, and th is process is enhanced if the absorption of ultraviolet radiation by t he overlying stratospheric ozone column decreases. As OH oxidizes SO2 to sulphuric acid, which then forms cloud condensation nuclei(2), vari ations in tropospheric OH concentration can influence cloud albedo. We use a global two-dimensional model forced by observed changes in stra tospheric ozone to calculate the consequent changes in production of s ulphuric acid over the past decade, and thus to estimate the effect on cloud albedo. We find that this indirect effect of ozone depletion ma y decrease radiative forcing (via increased cloud reflectivity) by at least as much as the direct effect.