At regional scales, multimodel ensembles (MMEs) of current Earth System Models (ESMs) produce very heterogeneous projections of the human influence on precipitation. The uncertainty is particularly acute for predictions of the hydrological effects of anticipated future reductions in anthropogenic aerosols. We focus on the current generation of ESMs assembled for the Coupled Model Intercomparison Project version 6 (CMIP6). As a case study, in this talk we consider the significant uncertainties in historical simulations of the changes in precipitation over the United States due to the steep reductions in sulfate aerosols following the imposition of emission controls in the 1960s.

From the observational record, we show that anthropogenic aerosol and greenhouse gas (GHG) emissions are the primary drivers of precipitation changes over the United States. GHG emissions increase mean and extreme precipitation from rain gauge measurements across all seasons, while the decadal-scale effect of global aerosol emissions decreases precipitation. Local aerosol emissions further offset GHG increases in the winter and spring but enhance rainfall during the summer and fall. At the scale of the United States, individual climate models reproduce observed changes but cannot confidently determine whether a given anthropogenic agent has increased or decreased rainfall. We discuss the implications for the uncertain projections of the hydrological responses to putative future reductions in anthropogenic aerosols.