For decades, Antarctica seemed to defy global warming. Since satellites began monitoring the poles in the late 1970s, the seasonal growth and retreat of Antarctic sea ice – frozen seawater that expands around the continent each winter – appeared remarkably resilient. It was often described as the “heartbeat of the planet”.

Unlike the Arctic, where sea ice declined rapidly as the planet warmed, Antarctic sea ice showed little overall loss. It even expanded between 2007 and 2015. But that resilience has now broken.

Since 2015, Antarctic sea ice has declined sharply. In 2023, winter sea ice extent fell to record lows — so far below the long-term average that scientists considered it an event with roughly a one-in-3.5-million probability of occurring by chance.

Antarctica was long considered a part of the climate system expected to change slowly. The speed of the recent sea ice decline has therefore come as a shock.

Scientists did expect Antarctic sea ice to shrink as the planet warmed, but not this quickly. The downturn over the past decade was not predicted by the climate models used to understand how the continent responds to warming. This makes the recent decline especially concerning: it suggests things may be unfolding faster, or in different ways, than our models can fully capture.

This matters because sea ice reflects sunlight back into space and helps drive ocean currents that lock away heat and carbon deep underwater. Its decline will have consequences for the climate and for Antarctica’s unique ecosystems that rely on it.

A fundamental shift

In our new scientific study, we show that the ocean around Antarctica has undergone a fundamental shift. Heat that had been trapped deep below the surface is now rising upwards, where it can melt sea ice.

The chain of events that triggered this change began decades ago. Around Antarctica, winds strengthened as a result of the ozone hole and greenhouse gas emissions. These stronger winds acted like a pump, gradually drawing warm, salty deep water closer to the surface.

For years, the sea around Antarctica – the Southern Ocean – was strongly layered, with cold fresh water sitting on top of warmer, saltier water below. That layering stopped the heat from reaching the surface.

But eventually the barrier weakened. By 2015, warmer deep water had risen close enough to the surface for storms and strong winds to churn it upwards.

The waters around Antarctica have since become trapped in a self-reinforcing cycle. Rising deep water brings heat and salt to the surface. The heat melts sea ice, while the extra salt makes the surface waters denser and easier to mix with warmer waters below. That allows even more heat to rise upwards, making it harder for new sea ice to form, and so on.

The consequences are not only physical. Antarctic sea ice supports one of the world’s most distinctive ecosystems. Algae grow on and under the ice, feeding krill, which in turn sustain penguins, seals, whales and seabirds. Low sea ice has already been linked to mass drowning of emperor penguin chicks – putting the entire species at risk. A long-term shift to lower sea ice cover would therefore reshape not only the climate itself, but also the living Southern Ocean.

This is not just a regional story. Antarctic sea ice acts like a mirror, reflecting sunlight and helping keep the planet cool. As it shrinks, more heat is absorbed by the ocean. At the same time, changes in the Southern Ocean circulation could reduce the ocean’s ability to store heat and carbon.

In the past, Antarctica helped buffer global warming. Our results suggest it may now be shifting in the opposite direction.

Whether this marks a permanent change remains uncertain. But if low sea ice conditions persist, the Southern Ocean could start to accelerate global warming rather than limit it.

Aditya Narayanan received funding from the NERC DeCAdeS project (NE/T012714/1).

Alessandro Silvano receives funding from NERC (NE/V014285/1).

Alberto Naveira Garabato does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.