Multi-Stability of the Present-Day Atlantic Meridional Overturning Circulation

The Atlantic Meridional Overturning Circulation (AMOC) plays a key role in the climate system, especially in the global meridional heat transport. Historical reconstructions indicate that the AMOC has weakened by about 15% since the mid-20th century. Paleoclimate records, ocean theory, as well as a hierarchy of climate models suggest that the AMOC is a tipping element, sensitive to changes in buoyancy fluxes at the air-sea interface, and could transition into a substantially weaker or fully collapsed state. Such a transition would have significant climate impacts on decadal to centennial timescales, potentially exceeding societal adaptability. Assessing the probability of such a transition, particularly before 2100, requires evaluating whether a collapsed AMOC state is possible under current forcing conditions. While conceptual and intermediate-complexity models have long identified collapsed states, comprehensive global climate models have only recently done so. Based on integrating model diagnostics with observations and current AMOC theory this review article critically evaluates the current arguments for and against the evidence of a multi-stable AMOC regime. We conclude that the evidence base in favor of such a regime has broadened over the last years and that the present-day AMOC is in such a regime.