Antarctica’s vast ice fields and the floating sea ice that surrounds the continent are Earth’s largest heat shields, reflecting solar radiation from the planet. But two studies published today show how global warming is even spreading to the sunlight reflector in the coldest region of the planet.
Research by scientists at the British Antarctic Survey focused on the staggering decline in sea ice over the past year. During the austral winter of 2023, Antarctica’s sea ice extent was about 770,000 square miles below average, an area larger than Alaska.
Lead author Rachel Diamond said the modeling study showed that such an extreme decline would be a 2,000-year event without climate change. “This shows us that the event was very extreme,” she said. “Anything less than one in 100 is considered extremely unlikely.”
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In a separate paper, another team of scientists documented that strong tides are pushing seawater surprisingly far below the tongue of the Thwaites Glacier in West Antarctica, adding to fears that the glacier is accelerating and amplifying sea level rise.
“The intrusion of pressurized seawater will cause violent melting of land-based ice over several kilometers, making the glacier more vulnerable to ocean warming and increasing projections for ice mass loss,” the authors write in the Proceedings of the National Academy of Sciences published articles.
“The concern is that we are underestimating the rate at which the glacier is changing, which would have devastating consequences for coastal communities around the world,” said co-author Christine Dow, a professor in the Faculty of Environment at the University of Waterloo in Ontario , Canada.
In this study, scientists tracked the movement of seawater by analyzing data from sensitive radar instruments that can detect when the ice surface rises just a few centimeters. The newly recorded miles of penetration beneath the ice may be why observed ice loss in recent decades has been consistently greater than predicted by ice sheet models, said lead author Eric Rignot, a glaciologist and climate ice researcher at the University of California, Irvine.
“The important thing about Antarctic sea ice is that it has changed state since 2016,” he said.
Some publications explained last year’s sea ice decline as an abrupt event, but Rignot takes a different view.
“The way I see it is that Antarctic sea ice has been developing very, very slowly over decades and has now reached a threshold,” he said. “It is an example of the Southern Ocean changing. Keep in mind that the loss of Antarctic sea ice since 2016 alone is greater than the loss of Arctic sea ice in the last 40 years combined. That’s a big deal.”
The extent of Antarctic sea ice has not recovered significantly this year. February’s seasonal minimum was tied with that of 2022 for the second lowest ice on record and marked three consecutive years of historically low ice extents, according to NASA, a possible sign of a long-term shift in the Southern Ocean.
The sea ice modeling study led by Diamond also supports the idea of a major long-term shift around Antarctica. She said her modeling showed that after an extreme decline like this, sea ice will grow back in 2023, but not to the level before.
“We found that the sea ice actually started to recover a little bit in the following years,” she said. “But even after 20 years it is still low. I think that tells us that it could remain low in the coming decades compared to what we expected.”
Profound global consequences
Major changes to Antarctic sea ice will have profound impacts on other parts of the planet. The formation of sea ice not only protects the exposed edges of Antarctic continental ice shelves from waves, but also acts as a driver for ocean currents and influences weather patterns in the Southern Hemisphere, the researchers said.
And because open ocean water is significantly darker than ice, it absorbs much more heat. This contributes to the overall warming of the planet and can potentially cause more relatively warm water to reach and melt the floating ice shelves of Antarctica’s vast glaciers, which would accelerate sea level rise. The Southern Ocean is also one of Earth’s largest carbon sinks, absorbing some of the carbon dioxide pollution from fossil fuel combustion, but warmer water slows this process.
The fate of marine plankton, which forms the base of the marine food chain, is also closely linked to the formation and extent of sea ice. Disruptions to this cycle can impact ecosystems, making it much harder for breeding birds to find food for their chicks and reducing concentrations of krill, the tiny swimming crustaceans that feed whales in the Southern Ocean. In recent years, scientists have also documented catastrophic breeding failures of emperor penguin colonies due to low sea ice.
“2023 was simply an extraordinary year for sea ice,” said Diamond, explaining the impetus for her new study. “We’ve seen record lows in recent years, but 2023 was really so far away compared to anything we’ve seen before in the satellite records.”
Diamond said her modeling research also showed the importance of reducing emissions now, because when greenhouse gas concentrations in the atmosphere are lower, “you don’t see such an increase in the probabilities” of extreme sea ice loss.
There could also be other problem areas in Antarctica, prompting Rignot to make a research trip to northeast Greenland, where there are glaciers similar to Thwaites. Using the same type of satellite measurements will help determine whether glaciers on the other side of the planet in Antarctica are experiencing the same type of interaction with seawater that can accelerate melting.
As the tidal wave pushes into narrow gaps between the bottom of the ice and the ocean floor, the water it pressurizes raises the ice surface enough to be visible from space, he said.
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“It’s millimeters up and we can see that. Satellite technology is very powerful; it detects the smallest changes.”
A few millimeters or centimeters may not sound like much, but Rignot said it can have a big impact: “Think about it when that happens under your house,” he said. “You don’t need a meter of water to worry. If I have 5 or 10 centimeters of water coming into my house and shaking it on a regular basis, I don’t like it.”
He said the new seawater intrusion study should prompt a return to the remote Thwaites Glacier to study it more closely because it has the potential to trigger sea level rise as ice flowing toward the ocean accelerates.
“We also need to study on the ground and put instruments in the ocean to see how these things work,” he said. “We use satellites to see the surface, but the key to understanding these processes is being able to observe them at depth. We can’t model things we can’t observe.”
Bob Berwyn
Reporter, Austria
Bob Berwyn is an Austria-based reporter who has been covering climate science and international climate policy for more than a decade. He previously covered the environment, endangered species and public lands for several Colorado newspapers and also worked as an editor and editorial assistant at community newspapers in the Colorado Rockies.