Reducing the Uncertainty of Aerosol Forcing in Near-Term Climate Projections

Anthropogenic aerosol emissions are predicted to decline sharply throughout the 21st century, in line with climate and air quality policies. However, the persistent uncertainty in aerosol forcing limits our understanding of how the climate will respond to reductions in anthropogenic aerosol emissions over the coming decades. We use statistical emulation on a perturbed parameter ensemble to predict radiative forcing and sample its uncertainty for anthropogenic aerosol emission scenarios prescribed by the Shared Socioeconomic Pathways (SSPs). We are then able to compare the relative uncertainty of scenario uncertainty and model uncertainty. Reductions in anthropogenic SO₂ emissions alone give a global mean radiative forcing up to 0.68 Wm^-2 by 2040 relative to present day, however, regionally radiative forcing can be more positive. We find the credible interval in global mean radiative forcing is approximately +/- 20% of the mean prediction. The results suggest model parametric uncertainty represents a considerable source of uncertainty in near term climate projections of aerosol forcing. To reduce this uncertainty multiple observational constraints capturing the different regional sources of forcing uncertainty will need to be applied.