Reconciled warning signals in observations and models imply approaching AMOC tipping point

Abstract

Paleoclimate proxy records and models suggest that the Atlantic Meridional Overturning Circulation (AMOC) can transition abruptly between a strong and a weak state. Empirical warning signals in observational fingerprints indeed suggest a stability decline and raise concerns that the system may be approaching a tipping point. However, state-of-the-art Earth System Models (ESMs) do not consistently show such a stability loss, hence inconclusive in their projections of AMOC collapse under global warming. It remains unclear whether warning signals of AMOC tipping are overlooked in ESMs or overinterpreted in observations, calling for further investigations of AMOC stability. Here, based on the concept of critical slowing down, AMOC stability decline that can be interpreted as a warning signal of AMOC tipping is identified in the eastern SPNA in both observations and ESMs. This warning signal is in accordance with a physical indicator of AMOC stability, AMOC-induced freshwater convergence into the Atlantic basin. The observed signal can be reconciled with the modeled one only under warming exceeding the Paris Agreement goal, suggesting that AMOC stability is overestimated in ESMs. Our results suggest that the observed AMOC may indeed be losing stability and could thus reach a tipping point much earlier than state-of-the-art ESMs suggest.