Climate Forcing by Anthropogenic Aerosols

TLDR

Tropospheric aerosols, especially anthropogenic sulfate, significantly influence radiative forcing, but their spatial and seasonal variability prevents simple compensation of climate effects. The study seeks to integrate aerosol climate forcing research to guide policy on greenhouse gas and sulfur dioxide emissions by evaluating past, present, and future impacts. Sulfate aerosols cool the planet by directly scattering shortwave solar radiation and by enhancing cloud reflectivity, thereby raising planetary albedo. Anthropogenic sulfate aerosols currently impose a cooling forcing of about –1 to –2 W m⁻², roughly equal in magnitude to greenhouse gas warming but opposite in sign, thereby substantially offsetting global warming.

Abstract

Although long considered to be of marginal importance to global climate change, tropospheric aerosol contributes substantially to radiative forcing, and anthropogenic sulfate aerosol in particular has imposed a major perturbation to this forcing. Both the direct scattering of shortwavelength solar radiation and the modification of the shortwave reflective properties of clouds by sulfate aerosol particles increase planetary albedo, thereby exerting a cooling influence on the planet. Current climate forcing due to anthropogenic sulfate is estimated to be -1 to -2 watts per square meter, globally averaged. This perturbation is comparable in magnitude to current anthropogenic greenhouse gas forcing but opposite in sign. Thus, the aerosol forcing has likely offset global greenhouse warming to a substantial degree. However, differences in geographical and seasonal distributions of these forcings preclude any simple compensation. Aerosol effects must be taken into account in evaluating anthropogenic influences on past, current, and projected future climate and in formulating policy regarding controls on emission of greenhouse gases and sulfur dioxide. Resolution of such policy issues requires integrated research on the magnitude and geographical distribution of aerosol climate forcing and on the controlling chemical and physical processes.