Antarctic ice is studied with ‘close’ from the depths – 02/16/2023 – Environment

By sending an underwater robot under an ice shelf in Antarctica that is melting at an accelerated rate, scientists have discovered new clues about how the melting is taking place. The findings will help them assess the risk created by this and other ice shelves for long-term sea level rise.

The researchers said that the overall melting of the lower part of a stretch of the Thwaites Ice Shelf in West Antarctica is less than predicted from estimates derived from computer models. But they also found that accelerated melting is happening in unexpected places: a series of terraces and crevasses that rise up into the ice.

The findings do not change the fact that Thwaites is one of Antarctica’s most rapidly retreating and least stable ice shelves, and one of the most worrying about sea level rise. Nor does it change predictions that the collapse of the shelf and the glacier it is part of will result in an approximate 60cm rise over a few centuries.

One of the scientists involved, Peter ED Davis, an oceanographer with the British Antarctic Survey, said the survey “is telling us a lot more about the processes that drive Thwaites’ retreat.” Published on Wednesday in the journal Nature, the findings will be used to improve models that predict Thwaites’ long-term future.

The survey is part of a larger effort, the International Thwaites Glacier Collaboration, sponsored by the US and UK, to gain a better understanding of what is happening at Thwaites.

The ice shelf is the floating tongue of the Thwaites Glacier, a river of ice the size of Florida that helps hold one of Antarctica’s two huge ice caps in place. The waters around Antarctica are warming due to climate change, and when this warm water flows under the ice shelf, the ice melts underneath and the shelf thins.

The so-called backfill line, the area where the floating ice meets the rocky bottom, has been receding as the shelf loses ice, having retreated about 13 km inland over the past two decades.

The new findings are contained in two papers published by Nature. Davis was lead author on one and Britney E. Schmidt, a geophysicist at Cornell University, on the other.

The researchers camped on the ice during the 2019-2020 Antarctic summer, often in extreme cold and windy conditions, and used hot water to drill several holes that cut through 610 meters of ice to reach the ocean below, just a short distance from the ice line. grounding.

Davis and his team lowered instruments into the water to measure its temperature, salinity and other characteristics. They found that the water was substantially above freezing temperature, but the slow current and layers of water of different salinity levels prevented mixing that could have carried more heat upward and melted more ice.

Oceanographer Alastair Graham of the University of South Florida, who studies historic Thwaites ice retreat but was not involved in those two studies, said the Davis team’s work showed that “there’s a lot of heat that’s coming up to the landing zone of the Thwaites ice sheet.” Thwaites. But not all of that ocean heat is converting to melt.”

The star of the research was the underwater robot, called Icefin, designed, built and operated by Schmidt and his team. Cylinder 23 cm in diameter and about 3.5 meters long, the robot carries cameras, sonar and other instruments, in addition to thrusters. Schmidt “steered” it through a long cable that transmitted signals from the surface.

Icefin explored steep crevasses and terraces on the underside of the ice and found accelerated melting there, as the nearly vertical orientation of the shelf’s side walls allowed layers of water to mix, bringing more heat into the ice.

In a few moments Icefin allowed the researchers to measure what was happening just a few centimeters from the ice. Seeing those ice faces and their orientation up close was confusing, Schmidt said, and “trying to understand all of this has been an important part of the work.”

Like Davis, Schmidt said the findings put what is happening on Thwaites Glacier in context. “It’s not ‘warm water equals x amount of melting,'” she said. “It’s ‘warm water plus process x leads to melting.”

Because there is less melting at the bottom but the Thwiates is still unstable, she said, “it actually takes a lot less than we thought to cause an imbalance.”

“It doesn’t mean the situation is worse,” Schit said. “It means it’s different.”

_{Translated by Clara Allain}