Opposing transient and equilibrium effective radiative forcing from aerosol-cloud interactions

Aerosols influence clouds, and therefore Earth's radiation budget, through processes that operate across multiple and interacting time scales, making aerosol-cloud interactions (ACI) a persistent source of uncertainty in estimates of effective radiative forcing (ERF). Here we examine the time-dependent response of the local, convection-focused ERF_ACI using an ensemble of high-resolution simulations initialized from different atmospheric states and subjected to an instantaneous aerosol perturbation, together with simulations in which aerosol concentration changes with prescribed periods. We find that the transient ERF_ACI during the first  ~ 2 days is positive, driven by rapid microphysical invigoration, enhanced high-cloud fraction, and increased longwave trapping. In contrast, the equilibrium ERF_ACI becomes negative as upper-tropospheric warming increases static stability and reduces anvil cloud fraction. As a result, the time-mean forcing depends on the ratio between the environmental adjustment time scale (τ_adj) and the aerosol-perturbation time scale (τ_aer). For intermediate regimes, where τ_aer is only moderately longer than τ_adj, the system exhibits pronounced hysteresis: ERF_ACI depends not only on the instantaneous aerosol loading but also on its recent history. These results imply that snapshot-based observational constraints and near-instantaneous-equilibrium convective parameterizations may systematically misestimate ERF_ACI.