Stratospheric aerosols: Effect upon atmospheric temperature and global climate

Abstract

A significant fraction of the aerosol population of the stratosphere is believed to consist of sulfate, presumably in the form of sulfuric acid. An increase in stratospheric aerosols could modify global climate by augmenting reflection of sunlight as well as enhancing the atmospheric greenhouse, two competing mechanisms with regard to changing the global surface temperature. Assuming that the aerosols consist of supercooled 75% aqueous sulfuric acid, we present a first-order estimate as to the effect of aerosol concentration upon global surface temperature. The model calculations illustrate that the increase in reflected sunlight constitutes the dominant contribution by aerosols; the normal aerosol concentration reduces the global surface temperature by roughly 0.7 K, and a doubling of concentration would provide a further decrease by the same amount. Increased aerosol concentration further results in heating of the stratosphere through absorption of infrared radiation emitted by the earth–atmosphere system. The model indicates that stratospheric temperature at 20 km could be raised by as much as 9 K, consistent with Southern Hemisphere observations following the eruption of Mt. Agung.