Warning of a forthcoming collapse of the Atlantic meridional overturning circulation
About
Tipping to an undesired state in the climate when a control parameter slowly approaches a critical value is a growing concern with increasing greenhouse gas concentrations. Predictions rely on detecting early warning signals (EWSs) in observations of the system. The primary EWSs are increase in variance, (loss of resilience), and increased autocorrelation (critical slow down). These measures are statistical in nature, which implies that the reliability and statistical significance of the detection depends on the sample size in observations and the magnitude of the change away from the base value prior to the approach to the tipping point. Thus, the possibility of providing useful early warning depends on the relative magnitude of several interdependent time scales in the system. These are (a) the time before the critical value is reached, (b) the (inverse) rate of approach to the tipping point, (c) the size of the time window required to detect a significant change in the EWS and finally, (d) the escape time for noise-induced transition (prior to the tipping). Conditions for early warning of tipping of the Atlantic meridional overturning circulation (AMOC) are marginally fulfilled for the existing past $\sim$150 years of proxy observations where indicators of tipping have recently been reported. Here we provide statistical significance and data driven estimators for the time of tipping. We estimate a collapse of the AMOC to occur around the year 2057 under the assumption of a "business as usual" scenario of future emissions.