Isolating the evolving contributions of anthropogenic aerosols and greenhouse gases: A new CESM1 large ensemble community resource

The evolving roles of anthropogenic aerosols (AER) and greenhouse gases (GHG) in driving large-scale patterns of precipitation and SST trends during 1920-2080 are studied using a new set of "all-but-one-forcing" initial-condition large ensembles (LEs) with the Community Earth System Model version 1 (CESM1), which complement the original "all-forcing" CESM1 LE (ALL). The large number of ensemble members (15-20) in each of the new LEs enables regional impacts of AER and GHG to be isolated from the noise of the model's internal variability.

Our analysis approach, based on running 50-yr trends, accommodates geographical and temporal changes in patterns of forcing and response. AER are shown to be the primary driver of large-scale patterns of externally forced trends in ALL before the late 1970s, and GHG to dominate thereafter. The AER and GHG forced trends are spatially distinct except during the 1970s transition phase when aerosol changes are mainly confined to lower latitudes.