If you were looking down from space, it would appear that every winter the Antarctic continent more than doubles in size. That’s because the ocean around it freezes over creating a vast curtain of sea ice. It’s one of the planet’s most extraordinary seasonal cycles and it plays an important but little understood role in the global climate system.
In 2024, it was discovered that for the second year in a row there had been a big decline in the extent of the Antarctic sea ice. To find out what this means and how worried we should be about it, we spoke to Dr Will Hobbs, a sea ice researcher at the University of Tasmania, whose work for the Australian Antarctic Program Partnership first recorded this dramatic change.
How much has Antarctica’s sea ice decreased by?
Every September Antarctica’s sea ice reaches its annual maximum, but in the last two years however, Dr Hobbs’s team has seen a dramatic reduction in coverage.
‘Sea ice has always been strangely resilient. But last year, something pretty startling happened, which was that the winter growth season was very impeded for the second year in a row. The ice was lower than anything else on record by about half a million square kilometres (193,000 square miles).’ To understand just how much the winter ice has been reduced by, Dr Hobbs told me the sea ice loss is equivalent to an area twice the size of Texas, or seven times larger than the United Kingdom.
With two extreme winters back to back, the race is on to discover whether this change is part of a natural fluctuation, or whether this is the beginning of an abrupt regime shift in the way that sea ice forms.
Why is sea ice important?
To understand why this matters, it’s necessary to get a clearer picture of just what sea ice is and the role it plays – not just in Antarctica but in the global climate system.
At its simplest, sea ice is just frozen sea water that ebbs and flows in extent throughout the year. This makes it quite different from icebergs, which calves from glaciers formed snow falling in the Antarctic interior. Sea ice forms during the Antarctic winter, reaching thicknesses of more than a metre before naturally melting away in the spring.
‘Because it’s already part of the ocean, sea ice doesn’t directly contribute to sea level rise,’ Dr Hobbs explains, adding that it nevertheless plays an important role in regulating the Antarctic continent. Just like mangroves protect tropical shorelines from hurricanes by soaking up the energy of the battering waves, Antarctica’s sea ice performs the same role in protecting the enormous floating ice shelves that pour out from the continent. Anyone who has seen the remarkable carved shapes of icebergs can attest to the dramatic erosive powers of the sea, but for Dr Hobbs, the sea ice plays an even more important role – as a giant mirror.
‘[The sea ice] is a reflector which bounces sunlight back into space. The darker coloured ocean is really good at absorbing the sunlight, so when you remove that sea ice, the water in front of those ice shelves gets warmer, and that can increase melting – which can increase sea level rise.’
Sea ice and the deep ocean
Sea ice also has a complicated relationship with the deep ocean that researchers like Dr Hobbs are working hard to understand. The annual freezing and melting of the sea ice plays an important role in pumping water between the surface and the deep ocean.
The Southern Ocean around Antarctica is particularly important for the global climate because of its role as a heat and carbon pump. Vast amounts of atmospheric heat and carbon dioxide are absorbed by the sea and pumped deep into the ocean. Salty sea ice plays a major role in this as it freezes and melts, taking warmer salty water deep below the surface. The storage of heat and carbon dioxide in the deep ocean is vital for the way the planet regulates its temperature.
‘About 90% of the heat that we’re trapping on our planet through anthropogenic emissions is absorbed by the ocean. Of that, 70% is absorbed just by the Southern Ocean,’ says Dr Hobbs, putting sea ice in perspective. Simply put, the less sea ice there is, the less efficient the cycle becomes at taking heat and carbon from the surface of the sea and storing it out of harm’s way.
Sea ice models
Like anything related to the climate, understanding just how sea ice behaves is complex, but modelling this behaviour is a key part of Dr Hobbs’s work.
‘If you ever had the opportunity to go on a ship and icebreaker, you’ll see that sea ice moves around a lot. You’ve got all those processes between the wind, the atmosphere and the ocean currents merging into the sea ice cycle. That’s what makes my job so fascinating – but also really complicated,’ he said.
‘Historically, the atmosphere was always understood as the dominant driver of sea ice variability. We now know that the oceans are really important, but we don’t know if the warm ocean temperatures right now are natural variability and something we’ve never seen before or if it’s anthropogenic.’
To ensure that his models are as good as they can be, it’s essential to start with good data: a real challenge when you’re dealing with one of the most remote and under-observed places on the planet.
While much of the data comes from satellites, they can only record what’s happening on the ocean surface. To understand what’s happening with the heat cycle lower down in the ocean, Dr Hobbs and his team rely on a piece of robotic kit called an Argo float.
‘You can throw them in the ocean and they sink down to a few thousand metres of depth, recording temperature and salinity at different depths as they rise and sink in the water column,’ he told me. Whenever an Argo float surfaces, it uploads its data to a satellite. They can be active at sea for up to three years. On top of this, researchers travel into the field on icebreakers whenever they can to take measurements that even the best robotic tool can’t collect.
What next for Antarctic sea ice?
‘The pressing question at the moment is a physical mechanism for what’s happened this year and last year,’ says Dr Hobbs. The temperature of the ocean is clearly involved, but teasing out where that heat has come from, and how it’s interacting with water hundreds of metres is a slow process.
For the moment, the answers are opaque, but it’s clear that sea ice plays a more important role in global climate than just passively reflecting how hot or cold our planet is – and that something appears to be shifting in how much of it forms every year. As the spring thaw begins its natural cycle again in Antarctica and the ice melts back into the sea, Dr Hobbs and his team will continue to look at their satellites and their lonely floats deep in the Southern Oceans to better understand this crucial frozen part of this warming planet.
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