Thwaites Glacier, infamous for its potentially outsized contribution to sea level rise, recently got its close-up. Researchers maneuvered a robot under Thwaites’s floating ice shelf and collected data about the Antarctic glacier’s so-called grounding line—the region where its ice first lifts off solid land. The new results revealed that enhanced melting occurs in places where Thwaites’s underbelly is particularly sloped and that water stratification helps to inhibit melting overall.
Reuniting with the Robots
In October 2019, Britney Schmidt, an Earth and planetary scientist then at the Georgia Institute of Technology in Atlanta, and her collaborators embarked on a multiday journey by plane to McMurdo Station, a research base in Antarctica operated by the United States. There, they met up with colleagues, tried their best to acclimate to the perpetual daylight of polar summer, and reunited with two very precious pieces of cargo.
The researchers had brought with them a pair of 3.5-meter-long robots. Known as Icefins, each bright yellow, pencil-shaped submersible was kitted out with instruments ranging from cameras to sonar to temperature and salinity sensors. Icefin is like a robotic oceanographer, said Schmidt, who led the sub’s development and is now at Cornell University in Ithaca, N.Y.
The submersible’s long, thin profile makes it well suited for maneuvering under ice, which is why Schmidt and her colleagues went to Antarctica to study Thwaites, a Florida-sized expanse of ice in West Antarctica that’s recently been melting at an alarming rate. The unstable nature of Thwaites, paired with its sheer volume—the water contained within it would raise global sea level by more than half a meter—has made it the target of a massive research effort that includes the project that brought Schmidt and her collaborators to Antarctica: the Melting at Thwaites grounding zone and its control on sea level (MELT) project.
After spending several months thoroughly testing the two Icefins on the McMurdo Ice Shelf, Schmidt and some of her colleagues departed for Thwaites with one of the robots in tow. Working in temperatures as low as −30°C, the researchers set up camp on the eastern part of the glacier’s ice shelf, roughly 2,000 kilometers (1,200 miles) from McMurdo. Their home away from home consisted of a line of brightly colored pyramidal tents—so-called Scott tents, named for the explorer Robert Falcon Scott—for sleeping in, a larger tent that could hold the entire group for meals and socializing, a drilling rig, and a dome-shaped tent that doubled as a scientific control room.
Like Hot Water Through Ice
Shortly after the New Year, the team began drilling. Using hot water, they bored through the full thickness of Thwaites’s ice shelf—587 meters (0.4 mile)—until they reached water. That process took roughly 24 hours. Schmidt and her colleagues then carefully lowered the Icefin down with just centimeters to spare around the robot. Roughly an hour later, when the Icefin’s sensors indicated that it had entered the Amundsen Sea, the team turned the robot toward the continent to seek uncharted territory.
For glaciers such as Thwaites that terminate in the ocean, some of their bulk rests on land, and some floats on the water. The transition is known as the grounding zone or grounding line. “That’s a really important place because it’s the place where the ice hits the water for the first time,” Schmidt said. Even water that’s only a few degrees above freezing can transfer enough heat to ice to kick-start melting, she explained.
“Until Thwaites, we had no data from the grounding line of any major glacier.”
But accessing the grounding line is notoriously tough—a glacier’s floating portion can extend for tens or even hundreds of kilometers beyond the grounding line, so exploring from the ocean side often isn’t practical. “It’s one of the hardest places to go look,” said Alastair Graham, a marine geophysicist at the University of South Florida who was not involved in the new research. Graham and his colleagues have studied how Thwaites’s grounding line has shifted position over time by analyzing the imprints it’s left behind in the seafloor.
“Until Thwaites, we had no data from the grounding line of any major glacier,” Schmidt said. “We were trying to get the very first data from this environment.”
Right up to the Edge
After maneuvering Icefin inland for a little over a kilometer (0.6 mile), the MELT team sent the robot to within a few centimeters of Thwaites’s grounding line. They discovered that the ice in the region was heavily scalloped and pitted with crevasses up to tens of meters deep. That was a big surprise, Graham said, because ice shelves long have been thought to be flat or gracefully sloping. “If you look at people’s drawings of grounding zones, they rarely have crevasses.”
The team furthermore found that ice at the grounding line was melting at different rates: Steeply sloped ice faces such as crevasse walls tended to melt much more rapidly than flatter ice faces. Only about 10% of Thwaites’s base is steeply sloped, but those regions account for 27% of ice loss, Schmidt and her colleagues reported in a paper published last month in Nature.
That difference likely arises because of water stratification, the researchers concluded. Colder, fresher water tends to linger above warmer, saltier water near the undersides of glaciers. Flatter ice faces are therefore mostly bathed in colder water, but steeply sloped ice faces are exposed to both colder and warmer waters.
Icefin also measured stronger water currents in crevasses, which might also play a role in transferring heat to the ice, the team suggested. That’s because stronger currents can disrupt water stratification. “You can imagine pouring your milk into your coffee and stirring it and seeing the billows and filaments,” said MELT team member Peter Davis, a physical oceanographer with the British Antarctic Survey in Cambridge, U.K. That mixing essentially removes the insulating layer of cold water that normally lingers near a glacier, explained Davis.
However, Davis and his colleagues calculated that overall, the underside of Thwaites is melting far less rapidly than predicted by models. That might sound like good news, Davis said, but the fact remains that the glacier is still retreating. “What this shows us is that the retreat is being driven by a lower rate of melting than perhaps we expected.” That observation sends scientists back to the drawing board, so to speak, to better understand what’s primarily responsible for Thwaites’s observed retreat, he said. Davis and his colleagues also reported their results last month in Nature.
It will be interesting to continue to monitor Thwaites, Graham said. There’s a chance that the glacier’s lower melt rate will translate into slower retreat in the future, he added, and Thwaites is definitely a glacier to watch. “I don’t think we should leave it alone.”
—Katherine Kornei (@KatherineKornei), Science Writer
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