The Effect of Atmospheric Aerosols on Climate with Special Reference to Temperature near the Earth's Surface

Abstract

A generalized model of the effect of an optically thin atmospheric aerosol on the terrestrial heat budget is proposed, and applied to the problem of estimating the impact of the aerosol on temperatures near the earth's surface. The distinction between warming and cooling near the surface attributable to the aerosol is found on the basis of this model to depend on whether the ratio of absorption a to backscatter b of incoming solar radiation by the aerosol is greater or less than the critical ratio ????????????(a/b)O?=?C(1?A)(1?Ak)/[D(1+A)?C(1?A)], where A is the surface albedo, C the fraction of sensible to total (sensible plus latent) solar heating of the surface, D the fraction of aerosol that is in convective contact with the surface, and k a multiple of b that measures the relative aerosol backscattering efficiency with respect to solar radiation reflected upward from the surface. A distinction is drawn between a stratospheric aerosol (D=0) which generally cools the atmosphere near the surface, and a tropospheric aerosol (D?1) which may either cool or warm the atmosphere near the surface depending on various properties of the aerosol and of the surface itself. Over moist surfaces, such as vegetated areas and oceans, the critical ratio (a/b)o is of order 0.1. Over drier surfaces, such as deserts and urban areas, (a/b)o is of order unity. If the actual ratio a/b of most tropospheric aerosols is of order unity, as inferred by previous authors, then the dominant effect of such aerosols is warming except over deserts and urban arms where the effect is somewhat marginal between warming and cooling. Further aerosol climatic effects are found likely to include a slight decrease of cloudiness and precipitation, and an increase of ?planetary? albedo above the oceans, although not necessarily above the continents. Suggestions by several previous authors to the effect that the apparent worldwide cooling of climate in recent decades is attributable to large-scale increases of particulate pollution of the atmosphere by human activities are not supported by this analysis.