Uncertainty in the Evolution of Climate Feedback Traced to the Strength of the Atlantic Meridional Overturning Circulation

In most coupled climate models, effective climate sensitivity increases for a few decades following an abrupt CO2 increase. The change in the climate feedback parameter between the first 20 years and the subsequent 130 years is highly model dependent. In this study, we suggest that the intermodel spread of changes in climate feedback can be partially traced to the evolution of the Atlantic Meridional Overturning Circulation. Models with stronger Atlantic Meridional Overturning Circulation recovery tend to project more amplified warming in the Northern Hemisphere a few decades after a quadrupling of CO2. Tropospheric stability then decreases as the Northern Hemisphere gets warmer, which leads to an increase in both the lapse‐rate and shortwave cloud feedbacks. Our results suggest that constraining future ocean circulation changes will be necessary for accurate climate sensitivity projections. Plain Language Summary How much the Earth's climate will warm in response to increasing carbon dioxide concentration, a number known as climate sensitivity, is an essential metric of the impacts of anthropogenic climate change. Most current global climate models agree that the climate will become more sensitive as time passes, indicating an underestimation of future warming inferred from historical records. In this study, we report that the slow response of oceanic circulation has an influence on this time evolution of climate sensitivity. In the 15 state‐of‐the‐art global climate models we investigate, the models projecting restrengthening of Atlantic Meridional Overturning Circulation after a few decades of weakening tend to simulate a more significant increase in climate sensitivity. We propose a mechanism as follows: Atlantic Meridional Overturning Circulation strengthening causes more enhanced surface warming in the Northern Hemisphere, altering the vertical stability of the global atmosphere. The changes in atmospheric vertical stability then strengthen the radiative feedbacks that amplify greenhouse gas forcing, accounting for the larger increase in climate sensitivity in these models. Our findings emphasize the important contribution of ocean circulation to the intermodel spread in climate change projections. Key Points Uncertainty in the Atlantic Meridional Overturning Circulation is the main cause of the model spread in evolution of the warming pattern Warming in Northern Hemisphere extratropics tends to be surface trapped, leading to more positive lapse‐rate