Arctic amplification is the main cause of the Atlantic meridional overturning circulation weakening under large CO₂ increases

Models project weakened Atlantic meridional overturning circulation (AMOC) under increasing CO₂, often attributed to increased ocean stratification caused by surface warming and freshening in the subpolar North Atlantic and by sea-ice melting. Here I analyze novel coupled model simulations with and without large Arctic amplification (AA)—the enhanced Arctic warming relative to the rest of the world under increasing CO₂—to show that AA largely explains AMOC’s weakening as CO₂ increases to above 570 ppm, due to increased runoff from Greenland and large evaporation decreases caused by reduced ocean-air temperature gradients from the Labrador Sea to the Nordic Seas. Together with a small role from ocean warming, the freshening reduces surface salinity, ocean density and mixed-layer depth, weakening deep water formation and AMOC. AMOC weakens only slightly under increasing CO₂ when AA is small due to less runoff from Greenland and smaller evaporation decreases. The results suggest a dominant role of Arctic and subpolar surface forcing over ocean dynamics in controlling AMOC’s strength and that enhanced Arctic warming, rather than changes at lower latitudes, will largely determine future AMOC’s fate under large CO₂ increases.