The Whitehouse Effect-shortwave Radiative Forcing of Climate by Anthropogenic Aerosols: an Overview

Loadings of tropospheric aerosols have increased substantially over the past 150 yr as a consequence of industrial activities. These aerosols enhance reflection of solar radiation by the Earth-atmosphere system both directly, by scattering light in clear air and, indirectly, by increasing the reflectivity of clouds. The magnitude of the resultant decrease in absorption of solar radiation is estimated to be comparable on global average to the enhancement in infrared forcing at the tropopause due to increases in concentrations of CO* and other greenhouse gases over the same time period. Estimates of the aerosol shortwave forcing are quite uncertain, by more than a factor of two about the current best estimates. This article reviews the atmospheric chemistry and microphysical processes that govern the loading and light scattering properties of the aerosol particles responsible for the direct effect and 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 short residence time of these aerosols (ca. 1 week) and the episodic removal in precipitation, and in the dependence of light scattering on particle 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 greenhouse 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 inferences of climate sensitivity to radiative forcing. Copyright c 1996 Elsevier Science Ltd I N T R O D U C T I O N The driving force of the Earth’s climate is absorption of solar radiation at the surface and, to lesser extent, by the atmosphere. Absorption of solar radiation of course results in heating of the system. As the temperature of the system increases, it emits increasing amounts of thermal infrared radiation. To excellent approximation, the absorption of solar (shortwave) radiation by the Earth-atmosphere system is balanced by emission of thermal infrared (longwave) radiation, so that the earth may be considered to be in radiative equilibrium-more accurately, steady state-at least in global and annual average. A schematic illustration of the Earth’s radiation balance is given in Fig. 1. Here all the fluxes represent annual and global average values, in units of W mp2. The average flux of shortwave solar radiation incident upon the Earth-atmosphere system is 1/4F-r z 343 W rnp2; F-r is the solar flux outside the atmosphere (the solar “constant”), about 1370 W mp2. The factor of 4 results from the ratio of the area of the Earth to that of the subtended disk. Of this incoming solar radiation, a fraction I? zz 0.3 is reflected, where l? is the global-mean planetary albedo (Ramanathan, 1987; Ramanathan er uL, 1989); this flux corresponds to about 106 W mp2. The principal contribution to the reflected shortwave radiation is from clouds; other contributions come from Rayleigh scattering by air, scattering by atmospheric aerosol particles, and surface reflectivity. The complement of the albedo 1 R corresponds to absorbed shortwave radiation and represents an average absorbed flux of 1/4&‘r(l I?) z 237 Wme2, that is matched, on an annual and global basis, by longwave emission. Clouds, water vapor, and other infrared absorbing gases in the atmosphere, both absorb and emit longwave radiation. The presence of these substances in the atmosphere leads to