For the first time, scientists have visited the northwestern face of the vast Thwaites Ice Shelf, in Antarctica, and discovered that it is inundated with warm water from multiple directions, which could speed its melting.
When the researchers used an autonomous underwater vehicle to collect oceanographic measurements, they were surprised to find a mass of warm water flowing westward from Pine Island Bay, suggesting that Thwaites Ice Shelf may be unexpectedly vulnerable to conditions in this distant region. The team reported the findings April 9 in Science Advances.
The ice shelf juts out from Thwaites Glacier, which is part of the West Antarctic Ice Sheet and currently covers an area larger than the state of Florida. However, the glacier is losing about 50 billion tons of ice more than it receives in snowfall annually.
"Thwaites Glacier is one of the largest glaciers in Antarctica, and it holds a lot of fresh water," said Anna Wåhlin, a professor of physical oceanography at the University of Gothenburg, in Sweden, and first author of the paper. "If all that water were to enter the ocean, it would raise global sea levels quite significantly."
As ice flows down to the coastline in a slow river and meets the sea, it forms a floating platform. Until now, researchers have been unable to access the water below this ice shelf, and only some parts of the remote area have been explored by icebreakers.
"The northwestern face of the ice shelf — that has never been reached before by any vessel," Wåhlin said.
However, in February 2019, the ice unexpectedly opened up enough to allow the ship carrying Wåhlin and her colleagues to explore previously uncharted waters along the central and western parts of the Thwaites Ice Shelf front.
"It was quite a moment to stand there on the bridge with everyone [in] dead silence and just see it appear out of the mist," she said.
Wåhlin and her colleagues used instruments aboard the ship and sent an uncrewed vehicle beneath the shelf to collect information on depth, temperature, salinity, oxygen levels and speed of water currents.
Researchers had predicted that three deep troughs in the seafloor were directing warm water flowing southward into the cavity beneath the ice shelf, where it meets the ice, melting some of it. The uncrewed vehicle explored two of these troughs and observed two distinctively different masses of water flowing into the area. One was relatively less salty and had more oxygen, similar to measurements for water from farther east, in Pine Island Bay.
The fact that this water can still be distinguished about 100 kilometers from its source could indicate that water is flowing steadily along two paths, one to the north and west and one to the east, Wåhlin and her colleagues wrote in the study.
Scientists thought that the way from Pine Island Bay to the ice shelf was blocked by an underwater ridge, but the findings suggest that this ridge may lie deeper or be less extensive than expected.
"That tells us that there is a deep connection that no one knew about," Wåhlin said.
The eastern part of Thwaites Ice Shelf has a roughly triangular shape, with the tip pointing north. Here, the ice shelf rests on a raised area of the seafloor; this so-called pinning point holds back some of the ice flowing out to sea. Wåhlin and her team found that the warm, salty water masses entered the cavity beneath the ice shelf from both sides of this pinning point.
The researchers estimated that the heat being transported in through just one of the underwater troughs was sufficient to melt ice at a rate of 85 gigatons per year, similar to estimates for melt beneath the entire ice shelf between 2010 and 2018.
"It could be that this is an abnormal situation that we observed, or it could be that melting has sped up quite significantly from 2010 to 2018," Wåhlin said.
This would spell trouble for the pinning point.
"The fact that it is open for those warm waters [from] all over, it tells us that this pinnacle is probably not sustainable in the long run," Wåhlin said. "And if the pinning point goes, that means that the ice on land will speed up and move faster out into the ocean."
But this wouldn't necessarily be disastrous for the ice shelf as a whole. It's possible that the influx of ice would cool the surrounding waters.
"It could be that that effect is overcoming the effect of melting the ice, so it could be that it might self-regulate," Wåhlin said. "We have to see what happens in the coming years."
She and her colleagues will return to Thwaites Ice Shelf in January, and they hope to send their autonomous underwater vehicle farther beneath the shelf.
"One thing we would like to know is what happens when this warm water comes into contact with the ice," Wåhlin said. "And also, how far does this warm water mass go underneath the ice shelf?"
The study, "Pathways and modification of warm water flowing beneath Thwaites Ice Shelf, West Antarctica," published April 9 in Science Advances, was authored by A. K. Wåhlin and B. Y. Queste, University of Gothenburg; A. G. C. Graham, University of South Florida; K. A. Hogan and R. D. Larter, British Antarctic Survey; L. Boehme, University of St Andrews; E. C. Pettit, Oregon State University; J. Wellner, University of Houston; and K. J. Heywood, University of East Anglia.