THE WHITEHOUSE EFFECT - SHORTWAVE RADIATIVE FORCING OF CLIMATE BY ANTHROPOGENIC AEROSOLS - AN OVERVIEW

Loadings of tropospheric aerosols have increased substantially over th e past 150 yr as a consequence of industrial activities. These aerosol s enhance reflection of solar radiation by the Earth-atmosphere system both directly, by scattering light in clear air and, indirectly, by i ncreasing the reflectivity of clouds. The magnitude of the resultant d ecrease in absorption of solar radiation is estimated to be comparable on global average to the enhancement in infrared forcing at the tropo pause due to increases in concentrations of CO2 and other greenhouse g ases over the same time period. Estimates of the aerosol shortwave for cing are quite uncertain, by more than a factor of two about the curre nt best estimates. This article reviews the atmospheric chemistry and microphysical processes that govern the loading and light scattering p roperties of the aerosol particles responsible for the direct effect a nd delineates the basis for the present estimates of the magnitude and uncertainty in the resultant radiative forcing. The principal sources of uncertainty are in the loading of anthropogenic aerosols, which is highly variable spatially and temporally because of the relatively sh ort residence time of these aerosols (ca. 1 week) and the episodic rem oval in precipitation, and in the dependence of light scattering on pa rticle size, and in turn on relative humidity. Uncertainty in aerosol forcing is the greatest source of uncertainty in radiative forcing of climate over the industrial period. At the high end of the uncertainty range, the aerosol forcing is comparable to the anthropogenic greenho use forcing, and substantially greater in industrialized regions. Even at the low end of the range, the aerosol forcing cannot be neglected in considerations of influences on climate over the industrial period. This uncertainty greatly limits the ability to draw empirical inferen ces of climate sensitivity to radiative forcing. Copyright (C) 1996 El sevier Science Ltd