AMOC collapse could turn the Southern Ocean into a CO₂ source — simulations show 0.2°C of extra warming

Researchers at the Potsdam Institute for Climate Impact Research (PIK) simulated what happens if the Atlantic Meridional Overturning Circulation (AMOC) collapses. The AMOC is the Atlantic's vast circulation system that transports warm water north and cold water south.

The simulations showed that at atmospheric CO₂ levels of 350 ppm or higher, a collapsed AMOC does not recover. Current CO₂ concentration is around 430 ppm, already well above this threshold.

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

— Da Nian (PIK, lead author)

The Southern Ocean flips from sink to source

In all scenarios, AMOC collapse leads to 0.17–0.27°C of additional warming. This is driven by massive CO₂ release from the Southern Ocean as enhanced mixing brings carbon-rich deep waters to the surface.

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

— Matteo Willeit (PIK, co-author)

+6°C in Antarctica, -7°C in the Arctic

Regional temperature shifts are even more dramatic. At 450 ppm CO₂, AMOC collapse raises Antarctic temperatures by 6°C while Arctic temperatures drop by 7°C.

The ocean was our greatest ally

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

— Johan Rockström (PIK Director, co-author)

For more on the ocean environment, see 'Ocean warming alters archaeal nitrogen cycling.'

A note from the author: 'The ocean was our greatest ally' — that phrase hits hard when the study is about that ally potentially turning against us.

Irreversible at 350 ppm; we're at 430. The numbers alone tell you there's no going back.