The impacts of Unilateral Stratospheric Geoengineering

Stratospheric geoengineering proposals have been suggested on the premise that the cooling impacts of volcanic eruptions could be deliberately mimicked to offset the impacts of increased greenhouse gas concentrations in the future by counterbalance global warming. Here, we examine both the impacts of hemispherically asymmetric volcanoes in the observational record and the impact of prolonged deliberate injection of stratospheric aerosol into either the northern or southern hemisphere stratosphere or into both hemispheres equally to assess the impacts on Sahelian rainfall and agriculture (Haywood et al., 2013). While the frequency of volcanic eruptions during the past 100 years is too sparse for definitive attribution, there is a suggestion that volcanic eruptions that preferentially load the northern hemisphere are the harbinger of Sahelian drought. Simulations are then performed with the HadGEM2 couple atmospheric-ocean model to assess the impacts of these volcanic eruptions and deliberate unilateral stratospheric geoengineering. Figure 1 shows the impacts of the geoengineering simulations which show that stratospheric injection into the northern hemisphere induces a severe and prolonged Sahelian drought with undoubted detrimental consequences for the local population. Conversely injection into the southern hemisphere causes a significant greening of the Sahel with vegetation productivity enhanced by over 100%. On the face of it, this suggests potential advocacy of injection into the southern hemisphere: we will investigate potential other side-effects from such a strategy...... Haywood, J.M., A. Jones, N. Bellouin, and D.B. Stephenson, Asymmetric forcing from stratospheric aerosols impacts Sahelian drought, Nature Climate Change, Vol 3, No 7, 660-665, doi: 10.1038/NCLIMATE1857, 2013.