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Abstract Anthropogenic aerosol effective radiative forcing (ERF) remains one of the largest uncertainties in quantifying the human influence on climate. Estimates of aerosol ERF depend on the background meteorological state and composition, including factors such as cloud regimes, circulation, and pre‐existing aerosol and precursor levels. Here, we investigate the sensitivity of anthropogenic aerosol ERF to these background conditions using the GFDL‐ESM4.1 model. Following and extending the CMIP6 AerChemMIP protocol, we conducted pairs of simulations under different base states, by comparing present‐day versus pre‐industrial sea surface temperatures, sea‐ice, and atmospheric composition, to diagnose aerosol ERF. We performed experiments isolating individual aerosol species (sulfate, black carbon, organic carbon, and ammonia) to examine their ERF dependence on such base states. We find that the diagnosed global‐mean aerosol ERF is relatively robust but exhibits species‐specific sensitivity across approaches, especially for black carbon and ammonia, while regional aerosol ERF differs strongly depending on the background climate and composition, demonstrating sensitivity to meteorological, and in particular, background composition conditions. Specifically, black carbon ERF is much more positive in a present‐day background due to enhanced absorption from internal mixing with co‐emitted aerosols. We also find that aerosol ERFs are not strictly additive across species, reflecting nonlinear interactions linked to cloud and aerosol microphysical processes under different background states. Our findings highlight the importance of accounting for background climate and composition conditions and nonlinear multi‐species interactions when assessing aerosol ERF in climate models.
Zhang et al. (Tue,) studied this question.