The collapse of the AMOC "imminent"

To those of us following this this is not news 

A Major Ocean Current May Be Hurtling Towards Collapse

Gizmodo,

23 February, 2021

The ocean may have less time than we thought before massive, irreversible shifts take place. A new study finds that a crucial ocean system may reach its “tipping point” sooner than predicted if the rate of climate change continues at a breakneck pace.

When we talk about climate change, the concept of tipping points is basically adding fuel to the already nightmare fire. Scientists now agree that climate changes could drive crucial systems on our planet to change so much that they reach a tipping point where recovery is impossible and change is inevitable.

One of the most pressing worries scientists have when it comes to tipping points lies underwater. Specifically, the Atlantic meridional overturning circulation, known as the AMOC, which helps shepherd warmer water to the North Atlantic. Among other things, it helps ensure Europe has relatively mild winters given its high latitude. Messing with it could be one of the fastest ways to make not just the region’s but the world’s weather run amok.

In the new study published Monday in the Proceedings of the Natural Academy of Sciences, scientists considered not just the amount of change to the oceans that could precede a tipping point, but also the rate of change. Think of it as the difference between pouring a cup of hot water very slowly into a bucket of cold water versus dumping it in all at once. While the same amount of water is being added both times, the rate at which water is being added is quite different

To measure the impact the rate of change may have on the AMOC, the new study ran several experiments on a global ocean model. The current has been under intense scrutiny in recent years because cold, fresh water from melting Greenland glaciers has essentially been causing the current to slow down, though not stop.

“The AMOC is at risk of collapsing when a certain level of freshwater flow into the North Atlantic from increasing ice melt in Greenland is reached,” Johannes Lohmann, one of the authors of the study, said in an email. “These tipping points have been shown previously in climate models, where meltwater is very slowly introduced into the ocean. In reality, increases in meltwater from Greenland are accelerating and cannot be considered slow.”

The study modeled the increase in freshwater flowing. Lohmann said using “a large ensemble of simulations, we systematically varied the rate of change and the ocean’s initial conditions, and investigated how the collapse of the AMOC depended on these factors.”

The models ended up showing that in some cases with a more rapid rate of change, the AMOC actually collapsed before previous predictions indicated it would. If we stick to the cup of water analogy, previous studies essentially found a full cup of hot water needed to be added to the bucket for collapse, but the new findings show dumping in the water faster means you need less than a cup to trigger the collapse. The study shows that “the safe levels of global warming before such a collapse occurs may be smaller than previously thought, and may also be difficult to predict with certainty,” Lohmann said.

This study isn’t the final word on how fast the AMOC may change. Some of the modeling Lohmann and his coauthors use may merit a closer and more critical look, Dave Sutherland, an associate professor in the department of Earth sciences at the University of Oregon, who was not involved in the study, said over email. Sutherland pointed out that the study does not account for some of the specifics of the location of freshwater in Greenland, even as the findings are “important and timely” to help determine the fate of the AMOC.

“Bottom line is I think this study is important and points out the complex dynamics inherent in our climate system,” Sutherland said. “I’m worried by the details (though I’m sure some reviewers were, too), and think there could be other climate feedbacks or unresolved processes that might change their results, if not their ultimate conclusions.”

Lohmann said that the study’s findings do need to be tested further, but pointed out that the possibility of a rapid AMOC collapse should sound an alarm bell.

“Due to the potentially increased risk of abrupt climate change in parts of the Earth system that we show in our research, it is important that policymakers keep pushing for ambitious short- and mid-term climate targets to slow down the pace of climate change, especially in vulnerable places like the Arctic,” Lohmann said.