The collapse of a key ocean current would cause a carbon feedback

Global warming caused by human carbon emissions slows the Atlantic Meridional Overturning Circulation (AMOC), a system of currents including the Gulf Stream that warms Europe. If the AMOC were to collapse completely, it could release huge amounts of carbon from the deep Southern Ocean into the atmosphere, a feedback loop that would warm Earth even more.

Previous research has shown that shutting down the AMOC can cause cooler winters in Europe, disrupt monsoons in Africa and Asia and increase global temperatures. But new computer modeling showed that it would also release up to 640 billion tonnes of carbon dioxide off Antarctica, warming the planet by an additional 0.2°C.

“A collapse of the AMOC could trigger (in) the Southern Ocean major mixing and release carbon stored in deep water,” he says From Nian at the Potsdam Institute for Climate Impact Research in Germany, who led the study. “It’s a fairly new result.”

“The key message is that a very bad event … can have even worse consequences than we previously thought,” says co-author Johan Rockströmalso at the Potsdam Institute. “We have to be very careful because if one thing goes wrong, it can have these domino effects.”

Driven by differences in water density, the AMOC brings warm, salty water from the Gulf of Mexico into the North Atlantic, where it cools and sinks, returning southward along the sea floor. But scientists think fresh meltwater from the Greenland ice sheet is diluting the AMOC and slowing this sinking process.

Buoy measurements recently showed that the return flow to the south is weakening and the AMOC has already dropped by an estimated 15 percent. Model projections suggest it could collapse anywhere from decades to centuries.

The new study modeled the collapse of the AMOC under various future climate scenarios. It found that when atmospheric CO2 concentrations are 350 ppm or higher, the AMOC does not recover after shutdown. As CO2 is currently at 430 ppm, this suggests that the collapse of the AMOC would be irreversible.

The study also found that shutting down the AMOC, which is part of a global “conveyor belt” of currents extending into the Southern and Pacific oceans, would free up the flow of deep water to the surface near Antarctica. The deep water here, which is largely trapped under a layer of fresher surface water, has accumulated carbon from the atmosphere and from the sinking of dead plankton. The model suggests that much of this carbon would be released into the atmosphere.

Previous research the collapse of the AMOC in the distant past explains why convection would start near Antarctica. This suggests that as less salty water sinks and flows from the North Atlantic to the Southern Ocean, the seas around Antarctica are also becoming less salty. This breaks up the layering of fresher surface water above the saltier deep water and allows the deep water to reach the surface.

“To see this play out in a warmer climate like this, and with such a large increase in CO2, is quite striking,” he says Jonathan Baker at the UK Met Office. “It’s an interesting study, but it depends on whether Southern Ocean convection gets stronger, and that’s still quite uncertain because different models show different responses.”

A collapse of the AMOC would cool the Arctic by 7°C and freeze Canada, Scandinavia and Russia, the study also found. At the same time, Antarctica would warm by 6°C. While the West Antarctic Ice Sheet is at risk of breaking the tipping point even today, this increase in temperature could also trigger the collapse of the much larger East Antarctic Ice Sheet, causing tens of meters of sea level rise.

While the impact of the CO2 release would be felt over 1,000 years or more after the AMOC shuts down, humanity’s greenhouse gas emissions could potentially block the AMOC’s future collapse over the next few decades, Rockström warns.

“That time of commitment could be … in the next 25 to 50 years. It’s literally now,” he says. “It’s not the time of impact that matters, but the time of commitment, because what right do we have to pass on to all future generations on a planet that is becoming less and less viable?”

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