The impact of geoengineering aerosols on stratospheric temperature and ozone

Abstract

Anthropogenic greenhouse gas emissions are warming the global climate at anunprecedented rate. Significant emission reductions will be required soon to avoid a rapidtemperature rise. As a potential interim measure to avoid extreme temperature increase, ithas been suggested that Earth’s albedo be increased by artificially enhancingstratospheric sulfate aerosols. We use a 3D chemistry climate model, fed by aerosol sizedistributions from a zonal mean aerosol model, to simulate continuous injection of 1–10 Mt/a into the lower tropical stratosphere. In contrast to the case for all previous work, theparticles are predicted to grow to larger sizes than are observed after volcanic eruptions.The reason is the continuous supply of sulfuric acid and hence freshly formed small aerosolparticles, which enhance the formation of large aerosol particles by coagulation and, to alesser extent, by condensation. Owing to their large size, these particles have areduced albedo. Furthermore, their sedimentation results in a non-linear relationshipbetween stratospheric aerosol burden and annual injection, leading to a reductionof the targeted cooling. More importantly, the sedimenting particles heat thetropical cold point tropopause and, hence, the stratospheric entry mixing ratio ofH2O increases. Therefore, geoengineering by means of sulfate aerosols is predicted to acceleratethe hydroxyl catalyzed ozone destruction cycles and cause a significant depletion of theozone layer even though future halogen concentrations will be significantly reduced.