The role of anthropogenic aerosols in the anomalous cooling from 1960 to 1990 in the CMIP6 Earth System Models

Abstract. The Earth System Models (ESMs) that participated in the 6th Coupled Model Intercomparison Project (CMIP6) tend to simulate excessive cooling in surface air temperature (TAS) between 1960 and 1990. The anomalous cooling is pronounced over the Northern Hemisphere (NH) midlatitudes, coinciding with the rapid growth of anthropogenic sulfur dioxide (SO2) emissions, the primary precursor of atmospheric sulphate aerosols. Historical simulations with and without anthropogenic aerosol emissions indicate that the anomalous cooling within the ESMs is potentially due to in part from overestimated anthropogenic aerosols and the enhanced aerosol-forcing-sensitivity. Structural uncertainties between ESMs that contribute to these two factors have a larger impact on the anomalous cooling than internal variability. CMIP6 simulations can also help us to quantify the relative contributions of aerosol-forcing-sensitivity by aerosol-radiation interactions (ARI) and by aerosol-cloud interactions (ACI). However, even when the aerosol-forcing-sensitivity is similar between ESMs, the relative contributions of ARI and ACI may be substantially different. The ACI accounts for 64 to 87 % of the aerosol-forcing-sensitivity and is the main source of differences between the ESMs. The ACI can be further decomposed into a cloud-amount term (which depends linearly on cloud fraction) and a cloud-albedo term (which is independent of cloud fraction, to the first order). The large uncertainties of cloud-amount term are responsible for the aerosol-forcing-sensitivity differences and further the anomalous cooling differences among ESMs. The metrics used here therefore provide a simple way of assessing the physical mechanisms contributing to anomalous twentieth century cooling in any given ESM, which may benefit future model developments.