For decades, Antarctic sea ice appeared resistant to global warming. Now, scientists say a cascade of ocean and atmospheric changes has ‘flipped the system’ – and the consequences could accelerate climate change worldwide.
For many years, as Arctic sea ice thinned and shrank in step with a warming planet, the frozen waters surrounding Antarctica appeared stable. The sea ice even expanded in the late 2000s.
While scientists were actively looking for answers, testing how Antarctica had escaped the consequences of a warming planet, sea ice levels then took a U-turn and dropped to record lows.
A new international study published in Science Advances shows the reasons why – and the consequences could be profound.
Lead author Dr Aditya Narayanan from the University of Southampton and a visiting Research Fellow based at the Centre for Marine Science and Innovation at UNSW Sydney says a ‘triple whammy’ of interacting climate processes tipped the Southern Ocean into a new state – which triggered a sudden and dramatic decline in Antarctic sea ice after 2015.
“First, greenhouse gas emissions and the ozone hole strengthened the winds surrounding Antarctica,” Dr Narayanan says.
“This hauled warm, salty water up to the surface. Then the mixing of that water unleashed its heat,” he says.
“And this resulted in a feedback loop that has since kept sea ice in a prolonged low state.”
A world out of kilter
When Antarctic sea ice levels fell to record lows in 2023, it was one of the least understood and extreme events in the modern climate record.
Dr Narayanan says Antarctica may now be shifting its state from a buffer against global warming to an amplifier of it.
“Antarctic sea ice in the Southern Ocean helps drive the planet's ocean overturning circulation,” he says.
This is the global system of surface and deep-water currents that regulates Earth’s climate – bringing warm surface water toward the poles and sinking cold, dense water to the ocean floor.
The impacts are local as well as global. Sea ice supports one of the planet’s most productive ecosystems – from microscopic algae, to krill, whales and penguins.
Emperor penguins are entirely dependent on stable Antarctic sea ice for breeding, moulting and resting. They rely on stable sea ice levels from April to December. The rapid declines in sea ice, particularly during 2022–2024, caused catastrophic breeding failures. Many chicks drowned or froze before they could develop their waterproof feathers.
“The system flipped,” Dr Narayanan says.
“What started as a slow build-up of deep-sea heat under the Antarctic sea ice was followed by a strong mixing of water, ending in a vicious feedback cycle where it’s too warm to let ice recover.”
Rising heat
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.”
What happens in Antarctica doesn’t stay in Antarctica
Sea ice plays a central role in regulating Earth’s climate. Its bright surface reflects sunlight back into space, so the ocean absorbs less solar energy.
And, when driving the global ocean circulation, it keeps only heat but also carbon locked deep below the surface.
“Antarctica has historically helped slow the pace of climate change,” Dr Narayanan says. “If that starts to reverse, it has global consequences.”
Co-author Dr Alessandro Silvano, from the University of Southampton, says there are further local effects with global impact.
“It could also destabilise ice shelves that prevent glaciers from sliding into the sea, raising global sea levels.”
A new ‘normal’
The researchers cannot yet say whether this shift is temporary, or the beginning of a new normal.
While climate models long predicted that Antarctic sea ice would decline under global warming, they had failed to capture the timing, speed and regional complexity of what has now occurred.
Dr Narayanan says if the processes continue, this “would fundamentally change how the climate system behaves.”
Future changes will depend on the balance between warming, winds and freshwater input from melting ice.
“What happens at the bottom of the world shapes what happens everywhere else,” says Dr Narayanan.
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Melissa Lyne, UNSW news & content