Impact of future climate and emission changes on stratospheric aerosols and ozone

Global climatological distributions of key aerosol quantities (extinction, optical depth, mass, and surface area density) are shown in comparison with results from a three-dimensional global model including stratospheric and tropospheric aerosol components. It is shown that future trends in global and regional anthropogenic emissions of sulfur dioxide may induce substantial changes in the lower stratospheric budget of sulfate aerosols: with the Intergovernmental Panel on Climate Change (IPCC) Special Report on Emission Scenarios’ (SRES) upper limit, ‘‘A2’’ scenario, the integrated stratospheric sulfate mass is predicted to increase from 0.15 Tg-S to 0.20 Tg-S in the year 2030, and the 1.02-µm average optical depth from 1.5X10-3 to 2.2X10-3 with a 50% increase in shortwave radiative forcing. The latter, in turn, is found to be about 23% of the total forcing by sulfate aerosols (tropospheric 1 stratospheric). Convective upward transport of sulfur dioxide to the tropical tropopause is found to be a key point for understanding the global distribution of sulfate in the lower stratosphere. Large increases of anthropogenic sulfur production at tropical latitudes by developing countries may explain these rather large predicted changes of stratospheric sulfate. Effects of future climate changes on stratospheric aerosols are also discussed: it is shown that the largest perturbation is on the probability of polar stratospheric cloud (PSC) formation, and that is driven primarily by greenhouse gas–induced temperature changes. In particular, the model-calculated wintertime Arctic increase of total aerosol optical depth is close to a factor of 2, with PSC optical depth and surface area density increasing by a factor of 5. This is mainly due to the predicted decrease of sudden stratospheric warming frequency in the Northern Hemisphere and the associated higher stability of the polar vortex. Enhanced ozone losses result from faster heterogeneous chemical reactions on both sulfate and PSC aerosol surfaces. The chemically driven total ozone recovery in 2030 relative to 2000 is predicted to decrease from 14.5% to 13.7% when taking into account both climate and surface emission changes: effects related to climate changes (perturbed stratospheric circulation, water vapor distribution, PSC frequency, etc.) account for about 2/3 of the calculated slow down of the O3 recovery rate.