Abstract/Summary

The relative importance of anthropogenic aerosol in decadal variations of historical climate is uncertain, largely due to uncertainty in aerosol radiative forcing. We analyse 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 realisations with aerosol forcing more negative than about -1 Wm-2 simulate stronger cooling trends in the mid-twentieth century compared to observations, while the simulated warming post-1980 always exceeds observed warming, likely due to a warm bias in the Transient Climate Response in HadGEM3-GC3.1.