A new study warns that a shutdown of the Atlantic Meridional Overturning Circulation (AMOC) would trigger the release of up to 640 billion tonnes of carbon dioxide from the deep Southern Ocean near Antarctica. This feedback effect could raise global temperatures by an additional 0.2°C. Researchers at the Potsdam Institute for Climate Impact Research highlight the risk as humanity's emissions continue to weaken the key ocean current.
Global warming has already slowed the AMOC by an estimated 15%, buoy measurements show. This system, which includes the Gulf Stream, carries warm, salty water northward from the Gulf of Mexico, where it cools, sinks, and flows back south along the seafloor. Fresh meltwater from Greenland's ice sheet is diluting the water, hindering the sinking process and risking a full collapse within decades to centuries, model projections indicate. At current CO2 levels of 430 parts per million, any shutdown would be irreversible, the study finds, as concentrations above 350 ppm prevent recovery in simulations. The research, published in Nature Communications Earth & Environment, models scenarios where AMOC collapse disrupts the global ocean conveyor belt, extending to the Southern and Pacific oceans. This would unleash convection near Antarctica, mixing deep waters rich in carbon—accumulated from past atmospheric absorption and sinking plankton—to the surface. Da Nian, who led the study at the Potsdam Institute, stated: “AMOC collapse could trigger (in the) Southern Ocean big mixing and release the carbon stored in the deep water.” Co-author Johan Rockström added: “The key message is that a very bad occurrence… could have even worse implications than we previously thought.” Impacts would include colder Arctic temperatures by 7°C, freezing parts of Canada, Scandinavia, and Russia, while warming Antarctica by 6°C and threatening the East Antarctic Ice Sheet. The CO2 release would unfold over 1,000 years or more, but Rockström warns emissions could lock in collapse within 25 to 50 years. “It’s literally now,” he said. Jonathan Baker at the UK Met Office called the findings striking but noted uncertainty in Southern Ocean convection across models.