Estimating the critical level of CO2 causing a collapse of the Atlantic Meridional Overturning Circulation

A collapse of the Atlantic Meridional Overturning Circulation (AMOC) will have serious impacts on global climate. The AMOC is responsible for the relatively mild climate in Northern europe and is a central tipping element in the climate system, interacting in complicated ways with other climatic tipping elements. In recent years, a weakening in circulation has been reported, but assessments by the Intergovernmental Panel on Climate Change (IPCC), based on the Coupled Model Intercomparison Project (CMIP) model simulations, suggest that a full collapse is unlikely within the 21st century. Tipping to an undesired state in the climate is, however, a growing concern with increasing greenhouse gas concentrations. Predictions based on detected early-warning signals in observational fingerprints, primarily an increase in variance (loss of resilience) and increased autocorrelation (critical slowing down), have recently been reported for the AMOC. Here, we construct a fingerprint-based proxy for the AMOC from 1870 to the present, constructed from seven previously proposed fingerprints. Assuming that the change in AMOC strength is governed by greenhouse forcing, represented by the atmospheric log[CO2], we estimate the critical level of CO$_2$ to be around 715 ppm. From this, we assess the risk of tipping in the five standard SSP scenarios.