The world has been warned climate change may rob the planet of its greatest ally in reducing carbon emissions.

A team from Potsdam Institute for Climate Impact Research (PIK) has warned the shutdown of the Atlantic Meridional Overturning Circulation (AMOC) could trigger a substantial release of stored ocean carbon into the atmosphere over hundreds of years. Their new study simulated such a collapse under stable climate conditions and concluded such a collapse would add 0.2°C of extra global warming.

To assess how a collapse of the AMOC – a major Atlantic Ocean circulation system that transports warm water north and cold water south – would affect the carbon cycle and global temperatures, the authors simulated Earth’s climate stabilising at different atmospheric CO₂ levels and subsequently applied freshwater input to the Atlantic surface to induce an AMOC shutdown.

At pre-industrial atmospheric CO₂ concentrations, 280 ppm, even if the AMOC collapses under freshwater forcing, it fully recovers once the forcing ends. However, at CO₂ levels of 350ppm or higher – well below today’s level of around 430ppm – once the AMOC collapses it stays in the “off” state.

“Higher CO₂ concentrations fundamentally alter the AMOC’s stability, pushing the system into a bistable regime where the AMOC could weaken over hundreds of years before shifting to, and remaining in, a collapsed state. Once shutdown, we see it does not recover in the long run,” explained lead author Da Nian of PIK.

In all scenarios analysed in the paper, a shift of the AMOC into an off state would see additional warming of 0.17°C to 0.27°C.

“This change in temperatures is driven by a large release of carbon from the Southern Ocean, due to enhanced mixing that brings carbon-rich deep waters to the surface,” added co-author Matteo Willeit of PIK.

Regional temperature changes would be even more pronounced than global mean temperature change. In one scenario at CO₂ concentrations of 450ppm – last experienced by the Earth several million years ago, when polar ice was significantly reduced – Antarctic temperatures rise by 6°C while Arctic temperatures drop by 7°C due to AMOC collapse.

“The ocean has been our greatest ally, absorbing a quarter of human-made CO₂ emissions. Our study shows how an AMOC collapse could flip the Southern Ocean from a carbon sink into a carbon source, releasing vast amounts of CO₂ and fuelling further global warming. The more CO₂ in our atmosphere at the stage of shutdown, the higher the likelihood of additional warming. Put simply, rising emissions today increase the risk of a stronger climate response down the line,” concluded PIK director and co-author Johan Rockström.