1.6.2.3 Ocean: Atlantic Meridional Overturning Circulation (AMOC)

The AMOC system has been identified as having the potential to collapse from its current strong state. One potential cause of this tipping point is the increased freshwater influx from a mix of increased precipitation and ice melt due to climate change (more detail on the mechanisms and likelihood given in 1.4.2.1).

EWS of AMOC collapse (Boulton et al., 2014) have been detected in a fully coupled climate model that was forced with a linearly increasing freshwater flux (Hawkins et al., 2011) – specifically increases in temporal AR(1) and variance in the strength of the overturning circulation. Furthermore, circulation strength at different latitudes are tested in this model, allowing the possibility to see where EWS may work best in the real world. A significant detection of the movement towards tipping could be seen up to 250 years in advance after 550 years of monitoring. 

Direct measurements such as sea surface temperature and salinity across the Atlantic ocean can provide a real-world fingerprint of current AMOC strength. A recent study identified potential EWS of AMOC collapse using these measurements. In eight such indices, increases in AR(1) and variance are found over the last century and suggest that the AMOC could be approaching a tipping point to its weaker circulation mode (Boers, 2021). When extrapolated, these EWS of AMOC collapse give an indication of a mid-21st Century AMOC tipping point (Ditlevsen and Ditlevsen, 2023), although considerable uncertainty remains around these timelines (Ben-Yami et al., 2023; 1.4.2.1).

Proxy records, such as bivalve shell increments, can provide an opportunity to measure early warning indicators prior to historical transitions. Recent work using three bivalve records has found that the North Atlantic Subpolar Gyre, a subsystem of the AMOC, destabilised prior to the transition into the Little Ice Age in the 14^th century, with measurable EWS of AR(1) and variance prior to this transition (Arellano-Nava et al., 2022).