Antarctica’s climate-vulnerable hotspot is a remote and hostile place – a razor-thin strip of seawater, under a sheet of floating ice more than half a kilometer thick. Scientists finally explored it and discovered something surprising.
“The melting rate is much weaker than we thought, given how warm the ocean is,” said Peter Davis, an oceanographer at the British Antarctic Survey in Cambridge, who was part of the team that drilled and lowered the narrow opening into this corner. to launch the media. The discovery might seem like good news – but it’s not, he says. “Even though those melting rates are low, they’re still seeing a rapid retreat,” as the ice is disappearing faster than it’s being replenished.
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Davis and about 20 other scientists led this research at Thwaites Glacier, a huge conveyor belt of ice about 120 kilometers wide that runs off the coast of West Antarctica. Satellite measurements show Thwaites is losing ice faster than at any time in the last few thousand years (SN: 6/9/22). The flow into the ocean has accelerated by at least 30 percent since 2000, bleeding over 1,000 cubic kilometers of ice, accounting for nearly half of the ice lost from all of Antarctica.
Much of the current ice loss is driven by warm, salty ocean waters that erode the ice at its base zone — a crucial point, located about 500 meters below sea level in the Arctic, where the ice lifts its bed and floats.SN: 4/9/21).
Now, this first-ever view of the subsurface ice near the base zone shows that the ocean is attacking it in previously unknown and disturbing ways.
When researchers sent a remotely operated vehicle, or ROV, down the borehole and into the water below, they found a lot of melting in places where the ice was already under mechanical stress — inside large cracks called basal crevasses. These openings cut under the ice.
Even a small amount of melting at these weak spots could cause large-scale damage to structures in the ice, the researchers report in two papers published on February 15. nature.
These results are “somewhat of a surprise,” says Ted Scambos, a glaciologist at the University of Colorado Boulder, who was not part of the team. Thwaites and other glaciers are mainly monitored with satellites, which make it appear that thinning and melting occur uniformly under the ice.
As the world continues to warm due to human-made climate change, the shrinking of the ice sheet has the potential to raise global sea levels by 65 centimeters over the course of centuries. Its collapse will also destroy the remainder of the West Antarctic Ice Sheet, triggering an eventual three-meter global sea level rise.
With these new results, says Scambos, “we see that we are in a much more detailed process, which will be important in the models” of how the ice will respond to future warming, and how quickly sea levels will rise.
Part of the cold, thin shield part of the Thwaites Glacier
Simply taking these observations “is kind of like shooting the moon or even lead Mars,” Scambos says. Thwaites, like most of the West Antarctic Ice Sheet, rests on a bed that is hundreds of meters below sea level. A floating slope of ice, called an ice shelf, extends 15 kilometers into the ocean, creating an ice ceiling that makes this place almost inaccessible to humans. This, he says, could represent the pinnacle of exploration in Antarctica.
These new results stem from a $50 million effort — the International Thwaites Glacier Collaboration — by the United States’ National Science Foundation and the United Kingdom’s Natural Environment Research Council. The research team, one of eight taken from that collaboration, landed on the snow and plains of Thwaites in the last days of 2019.
The researchers used hot water drills to drill a narrow hole, not much wider than a basketball, through more than 500 meters of ice. Below the ice sat a column of water that was only 54 meters thick.
When Davis and his colleagues measured the temperature and salinity of that water, most were 2 degrees Celsius above freezing — potentially warm enough to melt 20 to 40 meters of ice per year. But the ice underneath appears to be melting at a rate of only 5 meters per year, researchers report in one piece. nature papers The team calculated the melting rate based on the salinity of the water, which shows the ratio of the water that flows out, which is salty, to the glacial meltwater, which is fresh.
The reason for that slow melting quickly emerged: Under the ice there was only a layer of cold water, only 2 meters thick, from the melted ice. “The water is much sweeter at the base of the ice,” says Davis, and this layer of cold ice is separated from the warmer water below.
These measurements provided a snapshot right into the borehole. A few days after the hole was opened, researchers began a wider exploration of the unbroken ocean cavity beneath the ice.
The thin, yellow-and-black cylinder workers descend into the borehole. This ROV, named Icefin, was led by Britney Schmidt, a glaciologist at Cornell University, during a seven-year career.
Schmidt and his team were controlling the craft from a nearby tent, monitoring the instruments while steering the craft with gentle switches at the 4-seat controller’s buttons. Grinding the glacial ceiling and silently watching the past on a computer monitor – a live film played over 3½ kilometers of fiber-optic cable.
As the Schmidt Icefin was lowered about 1.6 kilometers from the borehole, the water column gradually thinned until less than a meter of water separated the ice from the lower sea floor. A few fish and crustacean shrimps, which are called amphipods, hover among the rest of the barren piles of gravel.
This new section of the sea floor — which holds the ice, rises and floats inland — has been exposed for “less than a year,” says Schmidt.
At some point, Icefin crossed a dark, gaping glacial ceiling, a basal crevasse. Schmidt steered the craft into several of these gaps — often over 100 meters wide — and there he saw something remarkable.
Thwaites’ melting underbelly forced into deep crevasses
The walls of the crevasses were carved vertically rather than smooth, suggesting a higher rate of melting than the glacial plains. And in these places, I see that the earth is made so that the light is strongly refracted through the whirlpools of salt water and fresh water. That turbulent flow of warm ocean water and cold meltwater has loosened the cold layer that insulates the ice, drawing warm, salty water into contact with it, scientists think.
Schmidt’s team calculated that the walls of the caves are melting at rates of up to 43 meters per year, the researchers report in a second. nature paper Researchers have also found melting in other areas where the glacial ice is punctuated by short, steep sections.
Greater turbidity and deeper melting are also seen in oceanic rivers driven by caves. Each time the Schmidt Icefin was driven into the crevasse, the ROV detected streams of water flowing through it, as if the crevasse was inverted. These currents moved up to twice as fast as the currents outside the crevasses.
The fact that melting is contracting into crevasses has huge implications, says Peter Washam, an oceanographer in Schmidt’s team at Cornell: “The ocean relaxes these features by melting faster.”
This could greatly accelerate the years-long process by which some of these cracks propagate hundreds of meters through the ice until they break through at the top—the ice that falls. The floating ice shelf, which presses against the mountain under the sea, and fixes the ice behind it, could break off sooner than predicted. This in turn could cause the ice to pour into the ocean faster.SN: 12/13/21). “It will have an impact on the stability of the ice,” says Washam.
These new data will improve scientists’ ability to predict the retreat of Thwaites and other Antarctic ice sheets, said Eric Rignot, a glaciologist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., who helped the team by providing satellite measurements of ice changes. . “You just can’t imagine what the water structure might look like in these zones until you observe it,” he said.
But more is needed to fully understand Thwaites and how it will change as the world continues to warm. The glacier consists of two fast-moving ice lanes, one moving at 3 kilometers per year, the other at about 1 kilometer per year. Because of safety concerns, the team visited the lane more slowly – which was still very challenging. Rignot says that scientists sometimes visit a fast track, whose upper surface is cracked with more crevasses – making it even more difficult to land aircraft and land on the field.
The research reported today “is the most important step, but we need to follow the second step”, the research on the fast ice lane, he says. “It doesn’t matter how hard it is.”
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