Estimates of the direct and indirect radiative forcing due to troposphericaerosols: A review

This paper reviews the many developments in estimates of the direct and ind irect global annual mean radiative forcing due to present-day concentration s of anthropogenic tropospheric aerosols since Inter governmental Panel on Climate Change [1996]. The range of estimates of the global mean direct rad iative forcing due to six distinct aerosol types is presented. Additionally , the indirect effect is split into two components corresponding to the rad iative forcing due to modification of the radiative properties of clouds (c loud albedo effect) and the effects of anthropogenic aerosols upon the life time of clouds (cloud lifetime effect). The radiative forcing for anthropog enic sulphate aerosol ranges from -0.26 to -0.82 W m(-2). For fossil fuel b lack carbon the radiative forcing ranges from +0.16 W m(-2) for an external mixture to +0.42 W m(-2) for where the black carbon is modeled as internal ly mixed with sulphate aerosol. For fossil fuel organic carbon the two esti mates of the likely weakest limit of the direct radiative forcing are -0.02 and -0.04 W m(-2). For biomass-burning sources of black carbon and organic carbon the combined radiative forcing ranges from -0.14 to -0.74 W m(-2). Estimates of the radiative forcing due to mineral dust vary widely from +0. 09 to -0.36 W m(-2): even the sign of the radiative forcing is not well est ablished due to the competing effects of solar and terrestrial radiative fo rcings. A single study provides a very tentative estimate of the radiative forcing of nitrates to be -0.03 W m(-2). Estimates of the cloud albedo indi rect radiative forcing range from -0.3 to approximately -1.8 W m(-2). Altho ugh the cloud lifetime effect is identified as a potentially important clim ate forcing mechanism, it is difficult to quantify in the context of the pr esent definition of radiative forcing of climate change and current model s imulations. This is because its estimation by general circulation models ne cessarily includes some level of cloud and water vapor feedbacks, which aff ect the hydrological cycle and the dynamics of the atmosphere. Available mo dels predict that the radiative flux perturbation associated with the cloud lifetime effect is of a magnitude similar to that of the cloud albedo effe ct.