Enhanced shortwave absorption by water vapor increases effective climate sensitivity via accelerated AMOC recovery

Abstract

Climate models exhibit substantial inter-model spread in climate sensitivity, typically attributed to uncertainty in cloud feedbacks. In contrast, the influence of clear-sky shortwave absorption (SWA) remains underexplored, despite its substantial uncertainty. Using a single-model framework, we systematically perturb SWA and impose CO₂ quadrupling on distinct mean states that differ in SWA, allowing assessment of its impact on both the mean climate and the CO₂-driven response. Enhanced SWA reduces surface shortwave radiation, leading to Arctic cooling. Under higher SWA, CO₂ forcing drives increased advection of colder Arctic air into the subpolar North Atlantic, enhancing turbulent heat loss and facilitating AMOC recovery. This accelerated recovery amplifies warming in the subpolar North Atlantic, strengthens lapse rate and shortwave cloud feedbacks, and ultimately increases climate sensitivity over time. These findings reveal a previously overlooked pathway by which clear-sky SWA modulates long-term climate feedback, underscoring the need to better constrain SWA in climate models.