Albedo modification, often conceived as reducing solar insolation or as augmenting stratospheric SO2, increases the amount of reflected solar radiation and is a potential geoengineering strategy to counterbalance global warming caused by greenhouse gas emissions. Recent societal and scientific attention has highlighted knowledge gaps about how albedo modification will influence atmospheric processes and impact ecosystems. Existing studies suggest that albedo modification could lower temperatures, but also may decrease precipitation, with potentially large and unintended consequences for ecosystems, particularly in drylands, where water is the most limiting resource. Here, we investigated the ecohydrological consequences of albedo modification on global drylands using a daily time step, multiple soil layer simulation model of ecosystem water balance. We simulated how changes in precipitation and temperature after global albedo modification, obtained from the Geoengineering Model Intercomparison Project (GeoMIP), influenced temporal and spatial patterns of ecological drought and soil moisture extraction from different soil depths by plants. We evaluated responses under two GeoMIP scenarios: G1, which balances radiative forcing to preindustrial levels under 4-fold increased CO2 concentrations by decreasing insolation, and G4, which balances radiative forcing to 2020 levels under RCP4.5 by increasing stratospheric sulfate aerosols at a constant annual rate.
G1 decreased the warming of global mean temperature towards control conditions (0.05 ± 0.25 C), whereas G4 was not as effective (0.28 ± 0.31 C). G1 decreased global mean annual precipitation (-47 ± 16 mm) and more so at tropical and subtropical latitudes. G4 was more effective (0.4 ± 9 mm) albeit with large increases for East Asia and northern Africa and decreases for areas in Africa, southwestern Europe, and most of the Americas. G1 and G4 suggest that transpiration may not change much or even increase compared to climate-change only scenarios. Transpiration increases if CO2 fertilization outweighs reductions in hot-temperature mortality and nitrogen cycle limitation. Thus, soil moisture may increase regionally despite higher biomass density and may be enhanced by a reduction in evaporation due to lower temperatures. Increased biomass density, however, may increase interception of precipitation, resulting in reduced recharge of soil moisture particularly at depth. Therefore, the projected changes in the duration and depth distribution of ecological droughts depended on the combination of scenario, region, and model. Our simulations highlight the changes in soil moisture with depth and the extent of ecological droughts under global albedo modification, with important implications for the distribution of plant functional groups.