Sensitivity of Historical Climate Simulations to Uncertain Aerosol Forcing.

The relative importance of anthropogenic aerosol in decadal variations of historical climate is uncertain, largely due to uncertainty in aerosol radiative forcing. We analyze a novel large ensemble of simulations with HadGEM3‐GC3.1 for 1850–2014, where anthropogenic aerosol and precursor emissions are scaled to sample a wide range of historical aerosol radiative forcing with present‐day values ranging from –0.38 to –1.50 Wm–2. Five ensemble members are run for each of five aerosol scaling factors. Decadal variations in surface temperatures are strongly sensitive to aerosol forcing, particularly between 1950 and 1980. Post‐1980, trends are dominated by greenhouse gas forcing, with much lower sensitivity to aerosol emission differences. Most realizations with aerosol forcing more negative than about –1 Wm–2 simulate stronger cooling trends in the mid‐20th century compared with observations, while the simulated warming post‐1980 always exceeds observed warming, likelydue to a warm bias in the transient climate response in HadGEM3‐GC3.1. Plain Language Summary: Anthropogenic aerosols have an overall cooling effect on climate due to their interaction with incoming solar radiation and influence on cloud properties. Their emissions have offset some of the historical warming induced by increasing greenhouse gases. However, the magnitude of the cooling induced by anthropogenic aerosol remains poorly constrained. In this study, we use a state‐of‐the‐art climate model, HadGEM3‐GC3.1, driven by different levels of aerosol emissions. This experimental setup tests the sensitivity of simulated historical temperatures to the strength of aerosol forcing in a climate model, all other factors remaining equal. Our results show that the period from 1951 to 1980 is particularly sensitive to aerosol forcing, coinciding with a period of rapid increases in global aerosol emissions and observed cooling over many regions, while temperature trends from 1980 onwards are primarily driven by increases in greenhouse gas concentrations. The observed temperatures over 1951–1980 are best reproduced by simulations with lower aerosol emissions than the standard configuration, implying that this model responds too strongly to aerosol forcing. Concurrently, the simulated temperatures warm faster than observed temperatures from 1980 onwards, suggesting that this climate model also responds more strongly to greenhouse gas forcing than observations suggest. Key Points: Simulations sampling aerosol forcing uncertainty suggest that aerosol forcing in the HadGEM3‐GC3.1 CMIP6 historical simulations is too largeComparing two key periods (1950–1980 and 1981–2010) with observations suggests a positive bias in the transient climate response in HadGEM3Most realizations with aerosol forcing more negative than about –1 Wm–2 cool too much in the mid‐20th century [ABSTRACT FROM AUTHOR]