Anthropogenic aerosols in the troposphere can influence the
microphysical properties of clouds and in turn their reflectivity
(albedo), thereby exerting a radiative forcing of climate. The
magnitude of this forcing is quite uncertain, but estimates based on
model calculations and measurements of anthropogenic sulfate loadings
suggest that this forcing is approximately -1 watt per meter squared,
global average, where the negative sign indicates a cooling influence.
Such a forcing is comparable to the shortwave forcing due to direct
light scattering by anthropogenic aerosols, and, likewise, (but of
opposite sign) to forcing by anthropogenic greenhouse gases. This
article presents the theoretical basis of this so-called indirect
forcing and reviews pertinent observational studies of the influence of
anthropogenic aerosols on cloud microphysical and radiative properties.
Approaches for representing this phenomenon in climate models are
described and results of initial studies with climate models are
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