Earlier today my son in Sweden brought to my attention this video about the AMOC: https://www.youtube.com/watch?app=desktop&v=GDy7Q8iAtFg. It’s a nice video with some valid points (about how things might change if the AMOC collapsed), but the assertion that it may well happen within decades is not plausible. That the cold blob is the hint what’s ahead has little basis in fact. They mention RAPID and OSNAP as big AMOC monitoring programs. We are learning a lot from these activities and while they find AMOC exhibit mostly wind-driven variability, they find no evidence of a slow-down. There are better ways to detect a slow-down of the AMOC. The video discusses at greater length numerous modeling studies that do suggest a slow-down if not a collapse of the AMOC sometime in the coming decades.
Without actually citing the relevant papers (they are shown briefly in the video), these papers motivate the alarmist tone of the video. Some studies use the alleged increasing variability of sea surface temperature in the subpolar north Atlantic as a precursor for a shut-down of the AMOC. Those statistical studies include no physics and what is worse they don’t consider the Nordic Seas, the area that matters most for the state of the AMOC. Other studies use comprehensive all-encompassing numerical climate models to show a slow-down. In one such study a huge amount of fresh water is added to the subtropics (not subpolar water as you might have thought!) over a lengthy time, and eventually a collapse takes place. How credible is this finding? Other studies, based on sediment records, allege that the AMOC has already slowed down several Sverdrups (1 Sv = 10^6 m^3/s) since the 1950s. This is presented as fact even though these findings are far from universally accepted. These studies and others with diverse approaches have in common the objective of showing that the AMOC is or soon will be collapsing.
The consequences of an AMOC collapse are enormous, no question about that. We see what the world looked like during the recent and earlier glacial periods. But what seems to be happening is that the sudden warming that took place at the Bølling-Allerød (14,690 BP) and Younger Dryas transitions (11,700 BP), presumably due to the turn-on of the AMOC, is being taken as evidence that the AMOC can collapse just as quickly. But do we know this? Consider the Dansgaard-Oeschger events: they exhibit a sharp increase in temperature, almost certainly reflecting a sudden turn-on of the AMOC. But the D-O events or cycles are not symmetric. The sudden warming is followed by a gradual cooling over many 100s of years with sometimes a final collapse into a cold state a millennium or so later before the process starts anew. The point is that there is little geophysical evidence that the AMOC can collapse as sharply as it can turn on.
Admittedly, we are in a different situation from the past with persistent if not accelerating global warming, but if we really think a collapse is imminent, we should be looking more closely at where this must happen, namely in the Nordic Seas or more specifically what happens to the overflow water leaving the Nordic Seas. If this water does not sink deep enough that it can flow south as a deep western boundary current but instead remains in the subpolar North Atlantic, then it won’t be possible to import warm water from the south into the Nordic Seas, and that would be a problem. As of now all the evidence I am aware of is that the overflow from the Nordic Seas is steady. We also know that the density of these waters is stable. Were they to lighten significantly such that they can’t sink into the deep North Atlantic, then we may have a problem.
But it isn’t easy to lighten the Nordic Seas water. First, note that the various salinity anomalies that have cycled through have had little if any impact. Second, not even the huge fresh water release from the collapse of Lake Agassiz could shut down the AMOC although there is evidence it led to a climate chilling for O(100) years. By comparison, the melt water from Greenland, what some people worry about, is far more gradual with a circuitous path around the Labrador and Irminger Seas without notably impacting the warm salty North Atlantic Current.
No doubt variations in sea surface temperature patterns in the subpolar North Atlantic can affect local and regional weather and climate, but that is not what the above discussion is about. The comments above concern potential transitions between modern and glacial climates.