Rate-dependent Tipping of the AMOC under CO2 increase in an Intermediate Complexity Model

The stability of the Atlantic Meridional Overturning Circulation (AMOC) under future climate change remains uncertain. While most climate models across the model hierarchy project a weakening or collapse under freshwater forcing, transient simulations under increasing CO₂ levels also commonly show a weakening or even a collapse of the AMOC. However, longer equilibrium experiments---primarily conducted with lower-complexity models due to computational costs---show more varied responses to CO₂ forcing. While most models show an initial weakening of the AMOC, some models equilibrate to a weak AMOC state only at very high CO₂ levels, while others equilibrate to a stronger-than-present AMOC. One such model is the intermediate complexity model CLIMBER-X, which (in equilibrium) shows that the AMOC strengthens until at least 16 times preindustrial CO₂ levels are reached. However, during the transient phase of increasing CO₂, the AMOC weakens. This suggests that the AMOC's transient response may differ from its equilibrium behavior. This raises the question: can the AMOC collapse under rapid and high CO₂ increase, even if a stable equilibrium state exists? 

We show that the AMOC exhibits rate-dependent tipping; when CO₂ increases fast enough and reaches sufficiently high levels, the AMOC can fully collapse. This occurs under very high forcing, starting from 7 times preindustrial CO₂ levels and a rate of 2.0% ppm/yr CO₂ increase. This collapse occurs despite the existence of a stable AMOC at equilibrium. By examining the physical processes through which the collapse occurs, we contribute to the understanding of the AMOC response in a warming climate. By also incorporating freshwater forcing, we assess the risks of rapid warming on the AMOC stability. Our results show that even models with a stable equilibrium AMOC under high CO₂ levels can experience weakening during the transient phase or even collapse. This highlights the need to assess both the rate and magnitude of CO₂ forcing when assessing the stability of the AMOC. While this effect occurs at very high CO₂ levels in CLIMBER-X, the role of the rate of CO₂ increase may become relevant at lower CO₂ levels when combined with freshwater forcing. Our findings demonstrate that the AMOC can undergo rate-dependent tipping under rapid and high CO₂increase, even if a stable AMOC exists at very high CO₂ levels.