Clear-Sky Direct Aerosol Radiative Forcing Uncertainty Associated with Aerosol Optical Properties Based on CMIP6 models

The direct perturbation of anthropogenic aerosols on Earth’s energy balance [i.e., direct aerosol radiative forcing (DARF)] remains uncertain in climate models. These uncertainties critically depend on aerosol optical properties, primarily aerosol optical depth (AOD), single scattering albedo (SSA), and the asymmetry factor g. In this study, we investigate the intermodel spread of DARF across 14 global models within phase 6 of the Coupled Model Intercomparison Project (CMIP6), using unified radiative transfer calculation and aerosol optical parameter assumptions. The global mean DARF for clear sky in 2014 with respect to 1850 is estimated as −0.77 ± 0.52 W m−2 assuming an externally mixed state and −0.68 ± 0.53 W m−2 for an internally mixed state. We further conduct a quantitative analysis and find that globally, for the external mixing assumption, AOD is the dominant factor, whose intermodel spread results in 36% of the total DARF uncertainty. For the internal mixing assumption, SSA becomes the major factor, which also leads to 36% DARF uncertainty. The g parameter and aerosol vertical distribution combined contribute to ∼30% of the DARF uncertainty. Regionally, DARF uncertainty is typically more sensitive to SSA where the absorbing aerosol fraction is high, such as South Asia and central Africa. Substantial differences between model-averaged and observed aerosol optical parameters are still noticed, with external mixing in general yielding closer agreement with observations. Our results highlight the importance of aerosol scattering and absorption properties in DARF estimation.