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Thursday, April 18, 2024

A Robot Finds More Trouble Under the Doomsday Glacier

Icefin the robot is designed to go where no human can, swimming off the coast of Antarctica under 2,000 feet of ice. Lowered through a borehole drilled with hot water, the torpedo-shaped machine takes readings and—most strikingly—video of Thwaites Glacier’s vulnerable underbelly. This Florida-sized chunk of ice is also known as the Doomsday Glacier, and for good reason: It’s rapidly deteriorating, and if it collapses, global sea levels could rise over a foot. It could also tug on surrounding glaciers as it dies, which would add another 10 feet to rising seas.

In a pair of papers published today in the journal Nature, scientists describe what Icefin and other instruments have discovered underneath all that ice. Simply put: trouble. Models of future sea-level rise characterize the bit of Thwaites that’s floating on the ocean—known as an ice shelf—as having a fairly simple, flat underside, but the robot found that 10 percent of it is way more complex. There are terraces, for instance, of vertical walls over 30 feet high where melting is happening much faster than in flat areas. That small portion is “contributing 25 percent of the melting that we see,” says Britney Schmidt, an Earth and planetary scientist at Cornell University, who leads the Icefin project. (She’s the lead author of one of the papers and coauthor on the other.) “So it's a really outsized impact.”

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As those features melt, they may be sending shocks through the system. “What we know about Thwaites is that it's falling apart,” says Schmidt. “We've been looking at it for the last 30 years, watching rifts and crevasses propagating across the system and destabilizing the whole ice shelf. And what we're showing here is the way that the ocean kind of works into these weak spots, and in a sense makes it worse.”

To deploy Icefin and other instruments, Schmidt and her colleagues drilled down near the glacier’s grounding line, the point where the ice lifts off the Antarctic land mass and starts floating on the sea. Thwaites’ risk of melting isn’t due to rising atmospheric temperatures above, but from rising ocean temperatures below. Its grounding line has retreated 10 miles inland since the late 1990s, which means that now more of the glacier’s ice is making contact with warm saltwater. A phenomenon known as tidal pumping is not helping: The ice heaves up when the tide comes in, allowing yet more water to rush underneath. 

Scientists have good estimates of where the retreating grounding line is, thanks to satellites watching for tiny changes in the ice’s elevation. But they haven’t had a good picture of what the glacier’s belly looks like at the grounding line, because it’s under thousands of feet of ice. “These data are really exciting because we're getting a look into a hidden system,” says University of Waterloo glaciologist Christine Dow, who studies Antarctic glaciers but wasn’t involved in the research. 

With Icefin, the researchers could remotely pilot a camera while measuring the salinity, temperature, and oxygen content of the water. “We saw that the ice base itself was very complex in its topography, so there's lots of staircases, terraces, rifts, and crevasses,” says British Antarctic Survey physical oceanographer Peter Davis, the lead author of one of the papers and coauthor on the other. “The rate of melting on different surfaces was very different.”

Where the glacier’s underside (or basal ice, in the scientific parlance) is smoother, melting is definitely happening, but at a much slower rate than where the topography is jagged. That’s because a layer of cold water rests where the ice is flat, insulating it from warmer ocean water like a liquid blanket. But where the topography is sloped and irregular, there are more vertical surfaces where warm water can attack the ice, including making incursions from the side. This melting creates a peculiar “scalloped” look, like the surface of a golf ball. 

These complex, expanding basal features could then influence the rest of the ice. “If you open up features underneath the ice, you also get similar reflections of them on the surface, because of the way that the ice is floating,” says Davis. “So there's a fear that if you're widening these rifts and crevices under the ice, you can destabilize the ice shelf, which could lead to greater disintegration over time.”

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If you’re feeling relieved that the flatter bits of basal ice are insulated against melting to a certain degree—don’t be. “It sounds like what we're saying is that there's less melting than there was before, and that's not true,” says Schmidt. Instead, they’re showing that the dramatic deterioration of Thwaites has been happening under conditions that are milder than models previously estimated. “That's important,” she continues. “That means that it takes less to get this degree of change.”

Put another way: Thwaites’ underside may be much more sensitive than previously believed. “What it shows us is that it's easier, perhaps, to knock these systems out of equilibrium in the first place,” says Davis. “In the past, we have associated rapid retreat with rapid melting. And I think what the results are showing us is that you don't need rapid melting to drive retreat. What you do need, though, is a change in melting. So you need something to shift the system away from a balance.”

That’s especially troubling because it means that the retreat of the grounding line can’t be explained by sky-high rates of basal melt, says Alexander Robel, head of the Ice and Climate Group at Georgia Tech, who wasn’t involved in the new papers. And other factors could set off further melt. “If ocean temperature or ocean circulation were to change in the future,” says Robel, “we could potentially get even higher basal melt rates that would produce even faster grounding line retreat rates.”

Better understanding how Thwaites is crumbling is critical for projecting how quickly it’ll add to sea-level rise. Typically, forecasts are based on simplified models that represent the underside of ice sheets as flat or sloped—partly because instruments like Icefin are only just beginning to map them in detail, partly because of the computing power needed to parse such complexity over vast areas. 

But the complex features that Icefin has discovered could be essential for modeling the glacier in much finer detail. “This is such a key region for Antarctic stability,” says Dow. “Any data we're getting from there is going to be hugely valuable for trying to figure out what that system will do in the future.”

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