Lily Caroline Hahn , Nicholas Lutsko, Ian Eisenman, Matthew T. Luongo, Shang-Ping Xie

Climate models vary widely in projections of 21st century global warming and projections of the Atlantic Meridional Overturning Circulation (AMOC). However, the extent to which this uncertainty in AMOC contributes to uncertainty in warming has not yet been quantified. To investigate this, we perform climate model experiments that increase CO2 concentrations while imposing the range of AMOC declines found in the Coupled Model Intercomparison Project Phase 6 (CMIP6). We find that intermodel spread in the AMOC decline, imposed within a single model, reproduces 20% of the total intermodel spread in global warming. An idealized energy-balance model indicates that changes in ocean heat uptake and climate feedbacks contribute approximately equally to enhanced warming in our experiment with a smaller AMOC decline. A smaller AMOC decline produces greater ocean-to-atmosphere heating in the North Atlantic, which increases near-surface warming and reduces the lower-tropospheric stability to produce less-negative lapse-rate and shortwave cloud feedbacks. In the northern tropics, surface warming is enhanced by the wind-evaporation-SST feedback and by more-positive longwave cloud and water vapor feedbacks due to a northward shift of the Intertropical Convergence Zone. A Green’s function analysis confirms that a less-negative global feedback with a smaller AMOC decline is predominantly driven by sea-surface warming in the extratropical North Atlantic. Spread in the AMOC decline is correlated with similar differences in ocean heat uptake and climate feedbacks across CMIP6 models as in our experiments. These results suggest that model uncertainty in global warming may be substantially reduced by constraining projections of AMOC.