A warm, freshwater mass rises and moves north across the Atlantic Ocean.
It’s headed toward the Arctic. There, frigid winds will scrape heat and liquid from its surface and it will sink, colder, saltier, and denser to the ocean bottom — and begin a slow journey back toward the lower latitudes, sliding under more warm water headed north.
All that heat scraped from its surface reshapes the local climate. It enters the atmosphere of the North Atlantic, slow-cooking the weather of places like Iceland, Britain, and northern Europe. That’s a big part of the reason those regions are, if not toasty, livable.
The cycle of fresher, warmer water moving north over southbound saltier, denser, colder water in the North Atlantic is called the AMOC (or Atlantic Meridional Overturning Circulation). It’s part of a complex, three-dimensional global system, driven by differences in salinity, density, and temperature, that circulates heat and matter through the world’s deep oceans.
It’s also changing. In a trend dating back to 2004, the AMOC has declined to about two-thirds of its former strength, which may have lead to some harsh winters in the UK and western Europe. A few scientists have even raised the spectre of a total AMOC collapse — a paradoxical warming scenario that would lead to far-harsher winters and expanding ice sheets in the North Atlantic. It would be a paradoxical, but not impossible, consequence of a warming climate.
In a paper published in Science Advances, Yale geophysicist Wei Liu and his coauthors demonstrate what they say is a significant bias in existing climate models toward AMOC stability, and show how increased heat and CO2 in the atmosphere could directly lead to AMOC collapse.
The result? Even as the rest of the world heats up, parts of Europe could see significant annual cooling over a period of several hundred years.
Liu cautions that this finding is still based on a single model. Rather than work from a steady increase in CO2 over a period of decades, he and his coauthors used a simple doubling of C02 all at once, after which it remains constant in the atmosphere.
That’s a rougher model, but its results are significant enough to point the way toward future research. Liu said the next step for researchers is to run similar experiments across a diverse range of complex models. More model data will shine a brighter light on just how likely a total AMOC collapse is, what regions would be impacted, and how soon it could happen.