Warning of a forthcoming collapse of the Atlantic meridional overturning circulation

Abstract Tipping to an undesired state in the climate, viewed as a complex system, when a control parameter slowly approaches a critical value (λ( t ) → λ c ) is a growing concern with increasing greenhouse gas concentrations. Predictions can rely on detecting early warning signals (EWS) in observations of the system. The primary EWS are increase in variance, due to loss of resilience, and increased autocorrelation or 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 λ c is reached, (b) the (inverse) rate of approach to the bifurcation 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 bifurcation). Conditions for early warning of tipping of the AMOC are marginally fulfilled for the existing past ~150 years of proxy observations where indicators of tipping have recently been reported. Here we provide statistical significance and estimate a collapse of the AMOC to occur around year 2050.