A steep decline of Antarctic sea ice may mark a long-term transformation in the Southern Ocean, and seawater intrusions beneath the Thwaites Glacier could explain its melting outpacing projections.
Antarctica’s vast ice fields and the floating sea ice surrounding the continent are Earth’s biggest heat shields, bouncing solar radiation away from the planet, but two studies released today show how global warming is encroaching even on the sunlight reflector in the coldest region on the planet.
Research by scientists with the British Antarctic Survey focused on last year’s dizzying sea ice decline. During the austral winter of 2023, Antarctic sea ice extent was about 770,000 square miles below average, an area bigger than Alaska.
Lead author Rachel Diamond said the modeling study showed that such an extreme decline would be a one-in-2,000-years event without climate change, “which tells us that the event was very extreme,” she said. “Anything less than one-in-100 is considered exceptionally unlikely.”
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In a separate paper, another team of scientists documented how strong tides push seawater surprisingly far beneath the tongue of the Thwaites Glacier in West Antarctica, reinforcing concerns about the glacier’s melt speeding up and adding to sea level rise.
“Pressurized seawater intrusions will induce vigorous melt of grounded ice over kilometers, making the glacier more vulnerable to ocean warming, and increasing the projections of ice mass loss,” the authors wrote in the paper, published in the Proceedings of the National Academy of Sciences.
“The worry is that we are underestimating the speed that the glacier is changing, which would be devastating for coastal communities around the world,” said coauthor Christine Dow, professor in the Faculty of Environment at the University of Waterloo in Ontario, Canada.
In that study, scientists traced the course of the seawater by analyzing data from sensitive radar instruments that can detect when the surface of the ice raises just a couple of inches. The newly recorded kilometer-wide intrusion beneath the ice may be the reason that observed ice loss in recent decades has consistently been greater than that projected 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 since 2016, it has changed state,” he said.
Some papers have explained last year’s sea ice decline as an abrupt event, but Rignot has a different perspective.
“I see it as Antarctic sea ice evolving very, very slowly for decades, and now it has reached a threshold,” he said. “It’s an illustration that the Southern Ocean is changing. Remember that the loss of Antarctic sea ice just since 2016 is bigger than the loss of Arctic sea ice in all of the last 40 years combined. It’s kind of a big deal.”
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Antarctic sea ice extent did not recover much this year. The seasonal minimum in February was tied with 2022 as the second-lowest on record, marking three consecutive years of historic record low 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 their modeling showed that, after an extreme decline like that in 2023, the sea ice does grow back, but not to what it was before that.
“We found that sea ice does begin to recover a little over subsequent years,” she said. “But even after 20 years, it is still low. So I think that tells us it might stay low for the coming decades, relative to where we thought it would be.”
Profound Global Consequences
Big changes to Antarctic sea ice will have a profound effect 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 an engine for ocean currents and influences weather patterns in the Southern Hemisphere, the researchers said.
And since open ocean water is considerably darker than ice, it absorbs a lot more heat. That adds to the overall heating of the planet, and can potentially mean more relatively warm water reaching and melting the floating ice shelves of Antarctica’s vast glaciers, which would speed up 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 slows that process.
The fate of ocean plankton, at the base of the marine food chain, is also closely tied with sea ice formation and extent. Disruptions to that 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 nourish whales in the Southern Ocean. In recent years, scientists have also documented catastrophic breeding failures of emperor penguin colonies because of low sea ice.
“2023 was just such an exceptional year for sea ice,” Diamond said, explaining the impetus for her new study. “We’ve seen record lows in recent years, but 2023 was really so off the scale compared to anything that we’ve seen in the satellite record before.”
Diamond said her modeling research also showed the importance of cutting emissions now, because when greenhouse gas concentrations in the atmosphere are at lower levels, “you don’t see such an increase in the probabilities” for extreme sea ice loss.
There may also be other trouble spots in Antarctica, which are leading Rignot to take a research trek to northeastern Greenland, where there are glaciers similar to Thwaites. Using the same types of satellite measurements will help determine if the 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 tidal surge pushes into narrow gaps between the bottom of the ice and seafloor, the water it pressurizes raises the surface of the ice enough to make it visible from space, he said.
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“It goes up by millimeters and we can pick that out. The satellite technique is very powerful, it detects the tiniest changes,” Rignot said.
A few millimeters or centimeters may not sound like a lot, but he said it can have a big effect, “Think about this if this was happening beneath your house,” he said. “You don’t need a meter of water to feel worried. If it’s 5 or 10 centimeters of water coming into my house and regularly shaking it, 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 a surge of sea level rise if the ice flowing toward the ocean speeds up.
“We need to study also on the ground, dipping instruments into 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 to be able to observe it at depth. We cannot model things that we can’t observe.”
NOTE – This article was originally published in insideclimatenews and can be viewed here
