Unveiling the dominant control of the systematic cooling bias in CMIP6 models: quantification and corrective strategies

Abstract. Including sophisticated aerosol schemes in the models of the sixth Coupled Model Inter-comparison Project (CMIP6) has not improved historical climate simulations. In particular, the models underestimate the surface air temperature anomaly (SATa) when anthropogenic sulfur emissions increased in 1960~1990, making the reliability of the CMIP6 projections questionable. Biases in cooling among the models are correlated with sulfate burden and the deposition of sulfur is the process responsible. We show that the lifetime of atmospheric sulfur, defined by a new global index for sulfur deposition (an "Effective Sulfur Retention Timescale" (ESRT)), determines the cooling biases. Reducing the biases to within the observational uncertainty is consistent with a physically plausible ESRT of around one day, whereas the CMIP6 models overestimate this timescale. Based on targeting a reduction of ESRT, post-CMIP6 improvements to two models are shown to greatly improve SATa reproduction.