Improved Monitoring of Early-Warning Signs of an AMOC Collapse

Abstract

This paper reviews a recent study predicting that the Atlantic Meridional Overturning Current circulation may collapse in the middle of this century and concludes that there are too many uncertainties to allow a reliable prediction. Even if the model used in that study is fairly adequate, its predictions depend heavily on the quality of the involved approximations, the choice of the circulation proxy, the method of trend estimation, the size of the rolling window used for the estimation of the second moments, the determination of the start time of ramping, etc. Forced to set lower targets, the focus is consequently shifted to the improved detection of early-warning signs like an increase in variance and autocorrelation. Two new estimation procedures are introduced. The first concerns the estimation of the trend and includes a new criterion for comparing the quality of different estimates. The second is a procedure for detecting changes in the first-order autocorrelation without any delay caused by an estimation window. The results obtained with these methods suggest that the autocorrelation increases erratically rather than steadily which would make forecasting based on extrapolation impossible.