The impact of geoengineering aerosols on stratospheric temperature and ozone

Abstract

Anthropogenic greenhouse gas emissions are warming the global climate at an unprecedented rate. Significant emission reductions will be required soon to avoid a rapid temperature rise. As a potential interim measure to avoid extreme temperature increase, it has been suggested that Earth's albedo be increased by artificially enhancing stratospheric sulfate aerosols. We use a 3D chemistry climate model, fed by aerosol size distributions 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, the particles 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 aerosol particles, which enhance the formation of large aerosol particles by coagulation and, to a lesser extent, by condensation. Owing to their large size, these particles have a reduced albedo. Furthermore, their sedimentation results in a non-linear relationship between stratospheric aerosol burden and annual injection, leading to a reduction of the targeted cooling. More importantly, the sedimenting particles heat the tropical cold point tropopause and, hence, the stratospheric entry mixing ratio of H 2 O increases. Therefore, geoengineering by means of sulfate aerosols is predicted to accelerate the hydroxyl catalyzed ozone destruction cycles and cause a significant depletion of the ozone layer even though future halogen concentrations will be significantly reduced.

A comparison of the climate impacts of geoengineering by stratospheric SO2 injection and by brightening of marine stratocumulus cloud

“…To continue to balance the GHG-induced warming after their respective B decades, the level of geoengineering would have to be increased. For SO 2 injection, this would mean having to increase injection rates, although possible nonlinear effects could reduce the cooling efficiency of further injection (Heckendorn et al, 2009). For the cloud brightening approach, this would presumably entail further cloud modification, in terms of increasing both the regions where clouds were modified and the amount by which they were modified.…”

Climate tipping points and solar geoengineering

“…Solar SGE may directly cause damages, for instance, by reducing the upper ozone layer (Heckendorn et al (2009)). We model these damages analogously to DICE's specification of damages from climate change.…”

Modelling the size distribution of geoengineered stratospheric aerosols

“…It is known from a variety of observations, in particular from the space-borne global monitoring of stratospheric aerosols, as well as from two-and threedimensional model studies that in the absence of injections of volcanic material the stratospheric aerosol Rasch et al (2008) and Heckendorn et al (2009). layer is well mixed (SPARC, 2006). Furthermore, observations as well as models show that the vertical distribution of stratospheric aerosols does not exhibit strong gradients in central regions of the layer, i.e.…”

“…The aerosol size distributions were instead prescribed based on observational findings after the Mt Pinatubo eruption, which in summer 1991 injected approximately 20 Tg SO 2 into the stratosphere (SPARC, 2006), for a few months generating a reduction of solar insolation of about the right magnitude (−2 W m −2 ) to counteract the current anthropogenic greenhouse effect. In a more recent modelling study, Heckendorn et al (2009), hereafter referred to as H09, calculated the aerosol size distribution by an interactive and size-resolved treatment of aerosol microphysical processes originating from different methods of SO 2 injection into the stratosphere. They showed that the stratospheric aerosol layer resulting from a continuous injection of 20 Tg SO 2 per year would not be able to compensate a warming resulting from 2 W m −2 due to the anthropogenic greenhouse effect.…”