Global stability of the AMOC under CO2 forcing: Boundary crisis, long transients and oscillatory edge states

There is growing concern that the Atlantic Meridional Overturning Circulation (AMOC), a vital Earth system component, could weaken or even collapse under climate change. Despite the severe potential impacts associated with such a transition, it remains extremely challenging to reliably estimate the proximity to a critical threshold and to predict the AMOC's fate under future anthropogenic forcing. We argue that a global viewpoint on the dynamics beyond the detection of early-warning signals is needed for a robust risk assessment. Here we explore the phase space of an intermediate-complexity earth system model, PlaSim-LSG, featuring a multistable AMOC. For two different atmospheric carbon dioxide (CO₂) levels, we explicitly compute the Melancholia (M) state that separates the strong and weak AMOC attractors found in the model. The M state is a chaotic saddle embedded in the basin boundary between the competing states (an edge state). We show that, while being unstable, the M state can govern the transient climate for centuries. The M state exhibits strong AMOC oscillations on centennial timescales driven by sea ice and oceanic convection in the North Atlantic. Combining these insights with simulations under future CO₂ forcing scenarios (SSPs), we demonstrate that in our model the AMOC undergoes a boundary crisis at CO₂ levels projected to be reached in the next decade. Near the crisis, the AMOC behavior becomes highly unpredictable. Founded in dynamical systems theory, our results offer an interpretation of the so-called stochastic bifurcation recently observed in a CMIP6 earth system model under the same time-dependent forcing scenario.