A comparison of geoengineering methods: assessment of precipitation side effects

Abstract:

Intentional modification of Earth’s climate by geoengineering can restore global mean temperature in climate model simulations but is expected to cause regional inequalities in temperature change and shifts in precipitation which may depend on the geoengineering method employed. In simulations of twenty-first century climate using the UKMO HadGEM2 climate model, we have assessed the effectiveness of two regional scale geoengineering methods (crop and desert albedo modification) and four large scale geoengineering methods (ocean albedo modification, marine cloud brightening by sea salt, cirrus cloud thinning and stratospheric sulphur). We projected anthropogenic emissions based on RCP4.5, applied geoengineering from 2020 to 2069 and quantified the impact on temperature and precipitation for 2040-2059 compared to a no-geoengineering control simulation. We found forcing for crop albedo modification was largely insignificant (-0.3 ± 0.3 Wm^-2). Desert albedo modification had a catastrophic impact on tropical precipitation drying the Amazon, the Sahel, India and China. Of the large scale geoengineering simulations, only stratospheric sulphur and ocean albedo modification were potentially scalable to temporarily return global mean temperature to the late twentieth century climate. Cirrus cloud thinning was the only method that increased global mean precipitation (+0.7%) while in other respects the four methods were remarkable in the consistency of their precipitation response to geoengineering compared to the control simulation (Figure 1). Over land, precipitation reduced less (between -0.5% and +1.8%) than global precipitation (between -3.8% and +0.7%). A northward shift in tropical precipitation over the Atlantic and eastern Pacific was found for all four methods, likely driven by cloud rapid adjustments and changes in atmospheric circulation. After geoengineering, during 2080-2099, significant differences in maritime tropical precipitation persisted despite regional precipitation elsewhere being generally within a 95% confidence interval of the control. We conclude that delivering equitable and predictable changes in tropical precipitation is currently the greatest climate related challenge for geoengineering.