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Antarctica’s vast ice fields and the floating sea ice surrounding the continent are Earth’s largest heat shields, deflecting solar radiation off the planet, but two studies published today show how global warming is encroaching on even the coldest sunlight reflector region of the planet.

A study by scientists from the British Antarctic Survey focused on the staggering decline in sea ice last year. During the austral winter of 2023, Antarctic 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 such an extreme decline would have been a one-in-2,000-year event without climate change, “Which tells us the event was very extreme,” she said. “Anything less than one in 100 is considered extremely unlikely.”

In a separate paper, another team of scientists documented strong tides pushing seawater surprisingly far below the tongue of Thwaites Glacier in West Antarctica, raising concerns about glacier acceleration and sea-level rise.

“Pressurized seawater intrusion will cause vigorous melting of land ice kilometers away, making the glacier more vulnerable to ocean warming and increasing projections of ice mass loss,” the authors wrote in the paper published in the Proceedings of the National Academy of Sciences.

“The concern is that we are underestimating the rate at which the glacier is changing, which would be devastating to 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 surface of the ice rises by just a few inches. The newly recorded intrusion beneath the kilometer-wide ice may be why observed ice loss in recent decades has consistently been greater than predicted by ice sheet models, said lead author Eric Rignot, a glaciologist and climate ice researcher at the California University, Irvine.

“The important thing about Antarctic sea ice is that since 2016 it has changed its state,” he said.

Some papers explain last year’s decline in sea ice as a sudden event, but Rignot has a different view.

“I see it as Antarctic sea ice that has been developing very, very slowly for decades and has now reached a threshold,” he said. “This is an illustration that the Southern Ocean is changing. Remember, the loss of Antarctic sea ice since 2016 alone is greater than the loss of Arctic sea ice in all of the last 40 years combined. That’s a pretty big deal.

Antarctic sea ice area has not recovered much this year. February’s seasonal low was tied for 2022 as the second-lowest on record, marking three consecutive years of historic record lows, according to NASA, a possible sign of long-term change in the Southern Ocean.

The sea ice modeling study led by Diamond also supports the idea of ​​a large long-term change around Antarctica. She said their modeling shows that after an extreme decline like the one in 2023, sea ice does grow back, but not to what it was before.

“We found that the sea ice starts to recover a little bit in the coming years,” she said. “But even after 20 years, it’s still low. So I think that tells us that it may stay low for decades to come, compared to where we thought it would be.”

Profound global implications

Major changes in Antarctic sea ice will have profound effects on other parts of the planet. In addition to protecting the exposed edges of Antarctica’s continental ice shelves from waves, the formation of sea ice 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 adds to the overall warming of the planet and could potentially mean more comparatively warm water reaching and melting the floating ice shelves of Antarctica’s vast glaciers, which would accelerate sea level rise. The Southern Ocean is also one of Earth’s biggest carbon sinks, absorbing some of the carbon dioxide pollution from burning fossil fuels, but warmer water is slowing that process.

The fate of oceanic plankton, at the base of the marine food chain, is also closely related to the formation and extent of sea ice. Breaks in this cycle can ripple through ecosystems, making it much harder for nesting birds to find food for their chicks and reducing concentrations of krill, the tiny floating 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 such an exceptional year for sea ice,” Diamond said, explaining the impetus for his new research. “We’ve seen record lows in recent years, but 2023 was really off the scale compared to anything we’ve seen in the satellite record before.”

Diamond said her modeling study also shows the importance of reducing emissions now, because when greenhouse gas concentrations in the atmosphere are at lower levels, “you don’t see such an increase in the probabilities” of extreme sea ice loss.

There may be other trouble spots in Antarctica that lead Rigneault to take an exploratory trek to northeast Greenland, where there are Thwaites-like glaciers. Using the same types of satellite measurements will help determine whether glaciers on the other side of the planet from Antarctica are subject to the same type of interaction with seawater that can accelerate melting.

As the tide enters narrow gaps between the ice floor and the sea floor, the water it pushes up lifts the surface of the ice enough to make it visible from space, he said.

“It goes up by millimeters and we can choose that. Satellite technology is very powerful, it detects even the smallest changes.”

A few millimeters or centimeters may not sound like much, but Rignot said it can have a big effect, “Think about it if this was happening under your house,” he said. “You don’t need a meter of water to be worried. If it’s 5 or 10 centimeters of water coming into my house and shaking it regularly, 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 because it has the potential to cause sea levels to rise sharply if ice flowing into the ocean accelerates.

“We also need to study on the ground by immersing 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.