Human influence on joint changes in temperature, rainfall and continental aridity

Large-scale mechanisms causing regional drying are not well understood. Models and observational data reveal that human-caused changes in GHGs and aerosols led to detectable global and hemispheric signals in the joint behaviour of precipitation, temperature and aridity since the 1950s. Despite the pervasive impact of drought on human and natural systems, the large-scale mechanisms conducive to regional drying remain poorly understood. Here we use a multivariate approach(1,2)to identify two distinct externally forced fingerprints from multiple ensembles of Earth system model simulations. The leading fingerprint,F-M1(x), is characterized by global warming, intensified wet-dry patterns(3)and progressive large-scale continental aridification, largely driven by multidecadal increases in greenhouse gas (GHG) emissions. The second fingerprint,F-M2(x), captures a pronounced interhemispheric temperature contrast(4,5), associated meridional shifts in the intertropical convergence zone(6-9)and correlated anomalies in precipitation and aridity over California(10), the Sahel(11,12)and India.F-M2(x) exhibits nonlinear temporal behaviour: the intertropical convergence zone moves southwards before 1975 in response to increases in hemispherically asymmetric sulfate aerosol emissions, and it shifts northwards after 1975 due to reduced sulfur dioxide emissions and the GHG-induced warming of Northern Hemisphere landmasses. Both fingerprints are statistically identifiable in observations of joint changes in temperature, rainfall and aridity during 1950-2014. We show that the reliable simulation of these changes requires combined forcing by GHGs, direct and indirect effects of aerosols, and large volcanic eruptions. Our results suggest that GHG-induced aridification may be modulated regionally by future reductions in sulfate aerosol emissions.