State dependence of aerosol forcing and its implications for climate response

Abstract. Aerosols offset greenhouse-gas warming, but their radiative forcing is often treated as a property of the aerosol perturbation, without accounting for the climate state in which it occurs. Here we show that this assumption is incomplete using atmosphere-only simulations with two GFDL atmospheric model configurations, GFDL-AM4.0 and GFDL-AM4-MG2, across climates from 8 K colder to 8 K warmer than present day. In both models, the same present-day-minus-preindustrial aerosol-emission perturbation produces weaker cooling as climate warms. This weakening arises from changes in both aerosol–radiation and aerosol–cloud interactions. Direct aerosol forcing weakens as aerosol optical depth decreases, reflecting changes in aerosol burden and the optical effectiveness of aerosol mass. Aerosol–cloud forcing weakens through model-dependent mechanisms: shortwave liquid-cloud adjustments dominate in GFDL-AM4.0, consistent with reduced cloud susceptibility in warmer climates, whereas GFDL-AM4-MG2 shows an additional longwave pathway involving aerosol–cloud–ice interactions and high-cloud responses. Aerosol forcing also weakens as climate feedbacks become less negative, revealing consistent within-model co-variation of forcing and feedback. These results highlight the importance of accounting for background climate state when interpreting the role of aerosol forcing in historical attribution, climate model tuning, and future climate projections.