AMOC collapse is unlikely in the near future

A new study published in Science (Zhou and McManus, 2024) reconstructed the land ice melting (or discharge) rate during the intermittent events of ice melting in the last glacial period (i.e., Heinrich events). The study showed that the present-day Greenland Ice Sheet melting rate (since the 1980s) is comparable to that during weak-to-moderate strength Heinrich events. This is concerning because paleo reconstructions and modeling studies have suggested that the Atlantic Meridional Overturning Circulation (AMOC) was severely disrupted during those events. However, the rate of ice melting flux is not the only factor. Another critical factor is the duration – how long the enhanced ice melting flux continues to increase the vertical stability of the high-latitude North Atlantic Ocean to the point where the AMOC shuts down. A proxy for estimating the duration of enhanced ice melting flux is the mean strength of the AMOC. During the Heinrich events, the AMOC was already very weak. However, the AMOC in modern times is much stronger, suggesting that the enhanced ice melting flux will have to continue for hundreds or even a thousand years before the AMOC shuts down. However, because the melting is causing the Greenland Ice Sheet to recede from Greenland’s coasts, where ice melting occurs, its ice discharge should not persist long enough to cause a major disruption of the AMOC.

Note that this conclusion is consistent with the model experiments carried out by van Westen et al. (2024). In their model experiments, freshwater flux forcing was added to the subpolar North Atlantic at a rate of 0.1 ~ 0.66 Sv (10⁶ m³ sec^-1) for 2,200 years. Only after the mean AMOC was weakened to ~ 10 Sv (around the model year of 1,700~ 1,750), the simulated AMOC rapidly transitioned to a complete shutdown within 100 years.

Figure 1 from Zhou and Mcmanus (2024): Estimates of freshwater flux during Heinrich events (HEs) from the literature. Dashed-line bars are model-based fluxes. Solid-line bars are observation-based fluxes. The annotations at the bases of bars mark event-specific estimates. The absence of such annotation means that the estimate is for a generic freshwater-discharge event. The numbers above bars are the freshwater-flux values in Sv. Note the log scale of the y axis. Also shown are the historical GrIS freshwater flux and the projected GrIS freshwater flux in 2100, 2200, and 2300. The markers with the same color come from the same study. Studies on the HE freshwater flux are cited below the x-axis. Triangles pointing up, squares, and triangles pointing down denote RCP 8.5, 4.5, and 2.6 projections, respectively. GrIS, Greenland Ice Sheet; H, Heinrich event; YD, Younger Dryas. 1 Sv = 10⁶ m³ s^-1.

Zhou, Y. & McManus, J. F. Heinrich event ice discharge and the fate of the Atlantic Meridional Overturning Circulation. Science, 384, 983-986 (2024). https://doi.org/10.1126/science.adh8369.

van Westen, R. M., Kliphuis, M. & Dijkstra, H. A. Physics-based early warning signal shows that AMOC is on tipping course. Sci. Adv. 10, eadk1189 (2024). https://doi.org/10.1126/sciadv.adk1189.