The Southern Ocean’s layers – with cold, relatively fresh water sitting on top of warmer, saltier water below – act as a lid, trapping heat at depth.
But the new research shows this barrier has weakened.
As the feedback loop locked in the change, the ocean’s surface became saltier and less layered.
By analysing ocean data and using a high-resolution model, the research team found that warm ‘Circumpolar Deep Water’ has been rising closer to the surface, where it can directly melt sea ice.
They say loss of sea ice destabilises the world’s ocean current systems – warming the planet far quicker than expected.
“This is not just about warmer air temperatures,” says Professor Matthew England, an oceanographer at UNSW and co-author of the study.
“We’re seeing heat that has been stored in the ocean for decades now breaking through to the surface,” Prof. England says.
“Once that happens, it becomes very difficult for the system to return to its previous state.”
While the surface warming can fluctuate from year to year, the heat stored in the ocean represents a deeper, longer-term shift in the climate system.
A tale of two Antarcticas
The research also reveals a split across the continent.
While sea ice loss was driven mostly by this upward surge of deep ocean heat in East Antarctica, the story is different in West Antarctica.
Increased cloud cover, which is linked to warm air moving down from the subtropics, has trapped heat in the ocean – melting ice from above, most significantly during the summers of 2016 and 2019.
Together, these processes have reshaped the entire Antarctic sea ice system.
“This is a coupled atmosphere–ocean-ice event,” Prof. England says. “You’ve got changes in winds, clouds, sea-ice and ocean circulation all reinforcing each other.
“That’s why the response has been so abrupt.”