Climate and Carbon Cycle Responses to a 21st century AMOC Collapse under a 2 °C Stabilization Pathway

Abstract. The Atlantic Meridional Overturning Circulation (AMOC) is a key component of the climate system, yet the climate and carbon cycle responses to a collapse under emission pathways consistent with the Paris Agreement remain poorly understood. Using the comprehensive GFDL ESM2M Earth System Model with the Adaptive Emissions Reduction Approach, we impose a freshwater-induced strong AMOC weakening to 20 % of its preindustrial strength, initiated in year 2026 and achieved within 60 years. The counterfactual simulations without freshwater hosing otherwise follow a pathway in which global warming stabilizes at 2 °C and the AMOC weakens only modestly and partially recovers. Relative to the 2 °C scenario without AMOC collapse, a strong AMOC weakening cools global mean surface air temperature by −0.8 °C (5-member ensemble range: −0.7 to −0.9) by 2171–2200, offsetting 40 % of global warming. Pronounced cooling is simulated in the North Atlantic region, reaching up to -5.4 °C (-8.0 to -3.3) in winter over Iceland relative to 1861–1900 conditions. The global cooling is primarily driven by larger negative feedback from clouds, driven by an increase in low-level clouds in the North Atlantic region, with smaller contributions from enhanced global ocean heat storage and reduced atmospheric CO₂. The total ocean heat content increases by an additional 488 ZJ (442–531), primarily south of 20° N, associated with reduced northward heat transport and enhanced heat uptake in the North Atlantic. The additional heat increases global thermosteric sea level rise by an additional 10 % (8–12), with enhanced rise in the western and tropical North Atlantic and northern Indian Ocean, but pronounced reductions in the eastern North Atlantic. Atmospheric CO₂ declines by 13 ppm due to anomalous land carbon uptake of 44 GtC (33–53), dominated by enhanced carbon storage in the Amazon region under cooler and wetter conditions. In contrast, global ocean carbon storage decreases by 14 GtC, mainly north of 20° N. The AMOC-induced cooling temporarily breaks the near-linear relationship between cumulative CO₂ emissions and warming, increasing the remaining emission budget for limiting warming to 2 °C by 63 % (54–72). Compared to identical freshwater forcing under preindustrial conditions, the surface temperature, ocean heat content, and sea-level responses to an AMOC collapse are substantially damped in a 2 °C world, indicating reduced climate sensitivity to AMOC collapse in a warmer world. These results demonstrate that a strong AMOC weakening would profoundly alter the climate–carbon cycle system and underscore the importance of explicitly accounting for AMOC risks in long-term climate assessments.