Ocean circulation is expected to change under greenhouse gas forcing in a number of ways, most notably including a weakening of the Atlantic Meridional Overturning Circulation (AMOC). But there is active debate about whether those circulation changes will enhance or reduce the rate of global surface warming. Here we use numerical simulations to study the climate response to greenhouse gas forcing with and without the impacts of ocean circulation changes. We first use an ocean-only model (MITgcm) to study how circulation changes impact global warming in the absence of any atmospheric response. We find that circulation changes enhance global warming by decreasing the efficiency of ocean heat uptake. Importantly, AMOC weakening causes relatively more of the anomalous heat to be stored near the surface ocean rather than in the deep ocean. We then use a coupled atmosphere–ocean climate model (CESM1) to study how circulation changes impact global warming when including the atmospheric response. Here, we find that circulation changes reduce global warming by increasing the efficiency of both ocean heat uptake and radiative response to surface warming (through the effect of sea-surface temperature patterns on radiative feedbacks). Importantly, the atmosphere responds to AMOC weakening by converging anomalous heat in the subpolar North Atlantic, which causes further AMOC weakening and subpolar cooling due to reduced northward ocean heat transport. As a result, the subpolar low cloud cover increases, reflecting more sunlight back to space and cooling the global surface ocean. Altogether, the atmospheric response to ocean circulation changes acts to reduce global surface warming.

The results suggest that arguments that AMOC weakening will enhance global warming do have merit, but only when neglecting the atmospheric response to sea-surface temperature changes. When the atmospheric response is included, AMOC weakening strongly reduces global warming by increasing both the depth of ocean heat storage and radiative damping to space.