Collapse of key ocean current would cause carbon feedback
Seas around Antarctica could start releasing CO2
Nigel Killeen/Getty Images
Global warming caused by humanity’s carbon emissions has slowed the Atlantic Southern Overturning Circulation (AMOC), a system of currents including the Gulf Stream that warms Europe. If the AMOC completely collapsed, it could release massive amounts of carbon from the depths of the Southern Ocean into the atmosphere, a feedback that would warm the Earth even more.
Previous research has shown that closing the AMOC could cause colder winters in Europe, disrupt monsoons in Africa and Asia, and increase global temperatures. But new computer modeling showed it would also emit up to 640 billion tonnes of carbon dioxide near Antarctica, warming the planet by a further 0.2°C.
“The collapse of the AMOC could trigger (in) the Southern Ocean a great mixing and release the carbon stored in the deep waters,” says Da Nian of the Potsdam Institute for Climate Impact Research in Germany, who led the study. “This is a fairly novel result.”
“The key message is that a very serious event… could have even worse consequences than we previously thought,” says co-author Johan Rockström, also at the Potsdam Institute. “We have to be very careful because when one thing goes wrong it can have domino effects. »
Driven by differences in water density, the AMOC brings warm, salty water from the Gulf of Mexico to the North Atlantic, where it cools and sinks, returning south along the seafloor. But scientists believe that fresh meltwater from the Greenland ice sheet dilutes the AMOC and slows this sinking process.
Buoy measurements have recently shown that the southward return flow is weakening and that the AMOC has already decreased by about 15 percent. Model projections suggest it could collapse within decades or even centuries.
The new study modeled the collapse of the AMOC under different future climate scenarios. It has been found that when atmospheric CO2 concentrations are 350 parts per million or higher, the AMOC does not recover after it is shut down. Since CO2 is currently at 430 ppm, this suggests that the collapse of the AMOC would be irreversible.
The study also found that the closure of the AMOC, part of the global “conveyor belt” of currents extending across the Southern and Pacific Oceans, would trigger convection from deep water to the surface near Antarctica. The deep waters here, which are largely trapped beneath a layer of fresher surface water, have accumulated carbon from the atmosphere as well as from the sinking of dead plankton. The model suggests that much of this carbon would be released into the atmosphere.
Previous research on AMOC collapses in the distant past explains why convection would begin near Antarctica. This suggests that because less salty water flows and flows from the North Atlantic to the Southern Ocean, the seas around Antarctica are also becoming less salty. This breaks down the stratification of fresher surface waters on top of deeper, saltier waters and allows deeper waters to reach the surface.
“To see this happening in a warmer climate like this, and with such a significant increase in CO2 emissions, is quite striking,” says Jonathan Baker of the UK Met Office. “It’s an interesting study, but it depends on whether Southern Ocean convection strengthens or not, and that remains quite uncertain, with different models showing different responses.”
The collapse of the AMOC would cool the Arctic by 7°C, freezing Canada, Scandinavia and Russia, the study also found. At the same time, it would warm Antarctica by 6°C. While the West Antarctic Ice Sheet is still at risk of passing a tipping point today, this temperature increase could also trigger the collapse of the much larger East Antarctic Ice Sheet, causing sea levels to rise by tens of meters.
Even though the impact of CO2 releases will be felt more than 1,000 years or more after the AMOC closes, humanity’s greenhouse gas emissions could potentially block the future collapse of the AMOC in the coming decades, Rockström warns.
“That commitment period could be…25 to 50 years from now. It’s literally now,” he says. “What matters is not the time of impact, it is the time of commitment, because what right have we bequeathed to all future generations a planet that is less and less livable?”
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