That's an old story from July last year that was based on modelling with ridiculously implausible inputs such as freshwater flowing in to the Atlantic sufficiently fast enough to raise sea levels by 6cms per year
Prof Jonathan Bamber, Director of the Bristol Glaciology Centre, University of Bristol, said:
“The authors of this study look at the stability of a key component of the ocean circulation called the Atlantic Meridional Overturning Circulation that has received a great deal of attention over the last decades as paleoclimate data suggests it is bistable and has an “off” mode and an “on” mode. The most recent and sophisticated generation of climate models, called GCMS, do not tend to reproduce this bistable behaviour but with this model they have managed to achieve it. They did this by imposing a huge freshwater forcing to the North Atlantic that is entirely unrealistic for even the most extreme warming scenario over the next century. Their freshwater forcing applied to the North Atlantic is equivalent to 6 cm/year of sea level rise by the end of the experiment, which is more than seen during the collapse of the ice sheet that covered North America during the last glaciation. To what extent such an experiment can be used to infer robust behaviour in the climate system is difficult to know but it is interesting that they do see a collapse in the AMOC, even in this artificial case.
“The authors also investigate a parameter related to freshwater transport by the AMOC that has been linked to its stability. They find that this parameter is declining and when it reaches a minimum this is an indication of an impending AMOC collapse. While that may correct, it is unclear when and if the parameter will reach a minimum and whether it will continue to decline. In fact, some observational data indicate it is not decreasing so the title of the paper is a little misleading. It would be more accurate to say that “Physics-based early warning signal shows that the AMOC may be on a pathway to a tipping point at some unknown time in the future”. it would be interesting to know why this GCM shows bistability when others have not and whether a more realistic experiment with a more realistic present-day climate and AMOC strength also shows bistability. No doubt this work will spur on further study on this important topic.”