When a team of researchers started to put together a paper about the loss of Antarctic summer-time sea ice a few years ago, they never imagined that winter-time sea ice would also crash to record low extremes while they were writing it.

“It took the community by surprise”, says lead author Dr Edward Doddridge, a physical oceanographer with the Australian Antarctic Program Partnership.

“It underscores how rapidly the system is changing; at a pace too fast for our scientific capabilities to keep up with.”

“That's not a particularly reassuring place to be as an Antarctic researcher.”

Antarctica is surrounded by frozen seawater from the Southern Ocean – sea ice – that varies in extent between summer and winter, in one of the largest seasonal cycles on the planet.

In winter, Antarctic sea ice expands to about two-and-a-half times the size of Australia (an area of ~19 million km²), and in summer shrinks to about 2–3 million km².

At the interface between ocean and atmosphere – at the whim of waves, wind and currents – sea ice is highly sensitive to climate variability.

The environments around Antarctica are a complex tapestry of interconnected systems, finely attuned to the annual rhythms of sea ice.

Even relatively small changes in sea ice can have significant impacts.

Changes over the last decade, however, have been anything but small.

A small positive trend in sea-ice area from 1979-2016 abruptly reversed, with record summer lows in 2017, 2022, 2023, 2024 and 2025. Even winter, when the ocean should be freezing, saw consecutive extreme lows in 2023 and 2024. About a third of summer sea ice – around the size of South Australia – has disappeared in recent years.

For the first time, an international team of 25 including oceanographers, biologists and glaciologists has brought together existing research and new evidence from satellites, computer modelling and robotic ocean samplers to synthesise the global impacts of Antarctic summer sea-ice extremes on ocean currents, ice shelves, wildlife, and shipping access.

This is what they found.

Sea ice has a strong influence on global climate due its reflectivity or albedo. Bright white sea ice covered by snow reflects about 90 percent of the incoming energy from sunlight, while the darker ocean absorbs about 90 percent. If there’s less sea ice, the ocean absorbs much more heat.

“That warming causes you to lose a bit more ice, which causes a bit more sunlight to be absorbed, which causes the ocean to warm and you lose a bit more ice. That’s what we mean by a feedback loop, where the change ends up reinforcing itself and causing more change”, says Edward.

“Recent research suggests that the loss of sea ice in both the Arctic and the Antarctic has increased the heat energy trapped on the Earth by 12-15 percent of the impact of greenhouse gases.”

“Losing sea ice is making climate change up to 15 percent worse than it would otherwise be.”

Co-author Dr Will Hobbs, a sea-ice scientist with the Australian Antarctic Program Partnership, says their study finds that extreme summer sea-ice lows have a compounding effect.

“Historically in the Southern Ocean, the sea-ice system always resets in the midwinter. If you had a summer with low sea ice cover and the ocean would warm a bit, that extra heat would just mix out in the midwinter and the system resets. So, sea ice in one summer doesn't really tell you anything about the next summer. The ocean’s memory would be wiped every winter”, says Will.

“But what we wanted to test in our study was whether when you have a really extreme loss, not just a regular below average, but a really extreme loss like the 2016/2017 summer, does that create an outsize impact? And we found that it does.”

“The ocean surface temperature warms up much more in an extreme low year, like 2016, than you would expect if you just took a regular low year and scaled up the impact. But importantly, the ocean heat is able to jump across that winter thermal barrier and start affecting the next summer.”

“In our experiment, we found that it took the system about three years to fully recover from a 2016-type event. And in fact, although those experiments were run on a model, we see a very similar timescale of recovery in the satellite observation record”, Will says.

Their study shows the surface ocean warmed by up to 1.5 degrees Celsius in some places, and by 0.3 degrees on average around Antarctica — increased levels of warming that persist for up to three years and build with each consecutive summer of low sea-ice.

Many species of seals and penguins rely on sea ice, especially for breeding and moulting.

Entire colonies of emperor penguins experienced ‘catastrophic breeding failure’ in 2022, when sea ice melted before chicks were ready to go to sea.

After giving birth, crabeater seals need large, stable sea ice platforms for two to three weeks until their pups are weaned. The ice provides shelter and protection from predators. Less summer sea-ice cover makes large platforms harder to find.

Many seal and penguin species also take refuge on the sea ice when moulting. These species must avoid the icy water while their new feathers or fur grows, or risk dying of hypothermia.

Extreme sea-ice lows also adversely impact Antarctic krill, a high-biomass prey species for fish, seabirds, seals and whales.

Krill relies on sea ice for food and refuge. Krill recruitment (i.e. development into adulthood) depends on sea-ice conditions in the previous winter — reduced sea-ice extent compromises larval survival, and the population is reduced in the following summer, with potentially serious consequences for the food web.

'Fast ice' is immobile sea ice that’s attached to the coast or around icebergs. It comprises a relatively small fraction of the total ice cover in summer at about 13 percent, but plays an outsized influence because it provides a stable platform for wildlife and shipping.

“Historically, summer fast-ice cover hasn't been that well correlated with the total sea-ice area, but our study shows that in the last 10 to 15 years, it has actually become much more correlated, which means if you've got more sea ice area, you've got more fast ice and vice versa”, says Will.

“In recent years where we've had extreme summer sea ice loss, we're also seeing a loss of this really important fast ice.”

National Antarctic programs, such as Australia’s, use fast ice as a jetty to unload and resupply stations. Shrinking summer sea ice will narrow the time window during which Antarctic stations can be resupplied over the fast ice. These stations may soon need to be resupplied from different locations, or using more difficult methods such as small boats.

“We are only just beginning to understand the impacts of the recent extreme lows in sea-ice cover”, the authors conclude in the paper. “These impacts are diverse, complex, and pervasive. They occur across physical, ecological, and societal systems within and around Antarctica.”

“To fully understand the impact of Antarctic sea-ice extremes, we require significant advances in both our understanding of the physical and ecological systems, and in our ability to observe them.”

“Long-term observing is the only way to obtain the data required to assess the physical, ecological, and societal impacts of extreme events in Antarctic sea ice and to link these to changes in the global Earth system.”

“Without a well-observed baseline, it is not possible to assess changes. Many of our current observations are insufficient to assess the response of Antarctica and the Southern Ocean to extreme events”, the paper says.

But just days before their paper was published, the authors were informed by the US National Snow and Ice Data Center (NSIDC) that the satellite data they depend on will no longer be available. Daily sea-ice data from US military meteorological satellites have been shared since 1978.

“Having a long-term record is really key when you want to compare changes across time”, says Edward.

“If you want to understand how significant a low sea-ice event is now compared to the past, you need a consistent way of measuring sea ice across time. When this US Department of Defense data stops coming we lose that, we’ll have a break in our time-series and comparing across time becomes much much harder.”

“Observing the entire Earth is hard enough when we all work together. It’s going to be almost impossible if we don’t share our data.”

Researchers fear that loss of these data will hamper their efforts to track rapid and extreme changes happening in the Antarctic. A number of other satellites, for example from Japan and China, measure sea ice from space. While their high-quality data may still be available, it will be difficult to calibrate new data from their different sensors with the existing 47-year record from US satellites.