Effective climate sensitivity increases with enhanced shortwave absorption by water vapor due to its impact on AMOC recovery

Uncertainty in climate sensitivity remains a critical challenge for effective mitigation and adaptation strategies. While cloud radiative feedbacks are often highlighted as a major source of this uncertainty, here we explore the impact of clear-sky shortwave radiation absorption by water vapor (SWA). Using abrupt 4xCO₂ simulations with altered SWA, we show that higher SWA conditions lead to a larger increase in climate sensitivity over time due to the faster and stronger recovery of the initially weakened Atlantic Meridional Overturning Circulation (AMOC). Enhanced SWA reduces surface shortwave radiation, leading to global cooling, particularly in the Arctic, where increased salinity creates conditions favorable for AMOC recovery. This accelerated recovery amplifies warming in the subpolar North Atlantic, intensifying positive lapse rate and cloud feedbacks, ultimately leading to a larger increase in net climate feedback and climate sensitivity. This underscores the need to constrain clear-sky SWA uncertainties to improve projections of climate sensitivity and associated feedback mechanisms.