Anthropogenic Forcing Decreased Concurrent Soil Drought and Atmospheric Aridity in the Historical Period 1850-2013

As awareness of the importance of anthropogenic climate change has increased, attention is being focused on "compound extremes," such as co-occurring soil drought (low amounts of soil moisture, SM) and atmospheric aridity (high vapor-pressure deficits, VPD), because of the disproportionate impacts of such extreme conditions on natural and societal systems. Few advances, however, have been made in isolating the net effect of anthropogenic forcing on the occurrence of such compound extremes. Nine Earth System models (ESMs) under natural-only and fully-forced simulations from Phase 6 of the Coupled Model Intercomparison Project (CMIP6) indicated that the presence of anthropogenic forcing in the historical period 1850-2013 weakened the coupling between SM and VPD and thus decreased the frequency of compound drought and aridity globally. The anthropogenically induced decrease in the strength of land-atmospheric feedbacks, which offset the drying trend effects of global warming by countering the expected natural correlation between SM and VPD, appears to have been responsible for the relative rarity of concurrent drought and aridity in the historical period. We also modeled independently the anthropogenic forcings of aerosols and greenhouse gases (GHGs) to further highlight that the widespread weakening of land-atmospheric feedbacks may have been associated primarily with the cooling effects induced by increases in anthropogenic emissions of aerosols, because the increase in intensity and frequency of compound drought and aridity that might have been expected from rising GHG concentrations was widely counter-balanced until very recently by an aerosol-driven cooling, particularly for the middle and high latitudes of the Northern Hemisphere. This finding indicates that the trade-off between these two main anthropogenic forcings may determine future patterns of concurrent drought and aridity in a changing climate. If global emissions of anthropogenic aerosols decrease in the future, as expected, our results imply a renewed strengthening of land-atmospheric feedbacks, and thus an intensification of concurrent drought and aridity.