Effect of changes in radiatively active species upon the lower stratospheric temperatures

Abstract

A one‐dimensional radiative‐convective model is employed to investigate the thermal effects in the lower stratosphere due to changes in the concentrations of radiatively active species. In particular, we consider the comparative influences due to species that exert surface‐troposphere radiative forcings of opposite signs. Two examples of such competing surface‐troposphere forcings are (1) increases in the well‐mixed greenhouse gases versus increases in tropospheric aerosols and (2) stratospheric ozone loss versus increase in tropospheric ozone. The radiative equilibrium of the lower stratosphere is perturbed both by the local changes in the concentrations of radiatively active species and by the changes in species' concentrations occurring in the troposphere and the middle/upper stratosphere. Perturbations in the concentrations of each of the species, as considered above, leads to a temperature decrease in the lower stratosphere. Relative to the well‐mixed greenhouse gases only case, simultaneous increases in these gases and tropospheric aerosols cause a reduction of the net surface‐troposphere radiative forcing, thereby diminishing the surface warming. However, since tropospheric aerosols contribute to a cooling of the lower stratosphere, the temperature decrease there is enhanced above that due to trace gases alone, with the aerosol‐induced effects scaling approximately linearly with their optical depth. A complete offset of the greenhouse gas surface‐troposphere forcing by tropospheric aerosols, while resulting in a null change in the surface temperature, would double the cooling of the lower stratosphere. Increases in tropospheric ozone would enhance the lower stratospheric cooling over and above that caused by the stratospheric ozone depletion. This is in contrast to the cooling and warming effects exerted upon the surface‐troposphere system by the stratospheric and the tropospheric ozone changes, respectively. Tropospheric ozone increases of 20% or more can yield a lower stratospheric cooling that is a significant fraction of the effects due to the observed stratospheric ozone loss. Both the surface effects and the enhancement of the lower stratospheric cooling scale approximately linearly with tropospheric ozone increases.