Aerosol and greenhouse gases forcing: Cloud feedbacks associated to the climate response

Abstract

If the climate impact of the increased atmospheric concentration in greenhouse gases has received a very large attention, the increased atmospheric loading in anthropogenic aerosols has been ignored for a long time. Recent studies, however, all converge to show its large importance for the climate evolution throughout the twentieth century. During this period the negative aerosol forcing might have had the same amplitude than the positive greenhouse forcing. But aerosols act in several manners, and their impact is still hard to quantify with precision. The aerosol direct effect consists in aerosol backscattering of visible radiation, thereby increasing the planetary albedo. It is active notably, but not exclusively, in clear-sky situations. To predict the direct aerosol forcing, for example between present-day and pre-industrial conditions, requires the knowledge of (1) pre-industrial and present-day distributions of the different aerosol types, (2) prediction of their optical properties, and (3) adequate treatment of the radiative transfer equation for a small perturbation as caused by a thin aerosol layer (Charlson et al., 1992). This direct effect have been represented under the simple form of a surface albedo perturbation by Mitchell et al. (1995) and Roeckner et al. (1995) to estimate its impact in transient climate scenarios using coupled ocean/atmosphere models. The same direct effect has been also considered more comprehensively in the experiments of Taylor and Penner (1994) where the equilibrium climate changes in response to perturbations of the greenhouse gases and sulfate aerosols, from the pre-industrial epoch to the the present one, were considered. All experiments show the significance of the aerosol forcing. Charlson et al. (1987) have shown that an indirect aerosol effect can add up to the direct effect.