The all-sky Aerosol Direct Radiative Effect (ADRE) varies considerably
among global climate models (GCMs), which results from differences in
both aerosol and atmospheric states and radiative transfer calculations.
This study uses a novel regression method to analyze the inter-model
spread of ADRE among the GCMs of the Sixth Coupled Model Intercomparison
Projects (CMIP6). The key state variables examined include scattering
and absorbing aerosol optical depth, surface albedo, and shortwave cloud
radiative effect. We find that differences in state variables and
radiative transfer explain 67% and 17%, respectively, of the global
ADRE anomaly. The ADRE anomaly in different models is dominated by
different factors, which sometimes leads to compensating effects. For
example, aerosol optical depth differences explain most of the ADRE
anomaly in the CNRM-ESM2-1 and GFDL-ESM4 models, while radiative
transfer evaluation of the aerosol single scattering effect dominates
the ADRE anomaly in the HadGEM3-GC31-LL, MPI-ESM-1-2-HAM, and MRI-ESM2-0
models.