Stratospheric Temperature Adjustment Robustly Damps the State Dependence of CO 2 Radiative Forcing

Abstract

Abstract The equilibrium climate sensitivity (ECS) depends on background climate state, a phenomenon known as state dependence. Recent studies highlighted state‐dependent CO 2 radiative forcing as a mechanism for ECS state dependence, largely attributing it to enhanced instantaneous forcing ( F INS ) at higher CO 2 . However, about 30% of CO 2 effective radiative forcing comes from atmospheric adjustments, whose role and underlying mechanisms in forcing state dependence remain unclear. We focus on forcing induced by stratospheric temperature adjustment ( F STA ), a dominant component of atmospheric adjustment. Using multi‐model assessment in clear‐sky condition, we show that F STA decreases as CO 2 increases, partially offsetting F INS enhancement, reducing overall forcing state dependence from 40% ( F INS ) to 17% ( F INS + F STA ) under pre‐industrial climatology. We further decompose F STA into multiplicative components and show that the reduction is primarily driven by changes in instantaneous heating rate perturbation, which emerges from reduced stratospheric Planck emission and decreased stratospheric transmissivity in higher CO 2 climate states.