For several years, scientists have been worried about the retreat and eventual collapse of Thwaites Glacier, a Florida-sized plug that holds back the West Antarctic ice sheet from the Southern Ocean. If Thwaites goes kaput, the resulting catastrophe could raise global sea levels by more than two feet on its own, or by eight feet in combination with melting from nearby glaciers, according to NASA estimates.

That fear has driven a big push by international teams of researchers to understand what’s going on at Thwaites and nearby Pine Island Glacier. A group of researchers from the United States and the United Kingdom took advantage of an unusually calm period in Antarctica in January 2019 to explore the two glaciers and the ocean nearby with ships, unmanned submarines, and aircraft to find out what’s happening to the ice and how fast. The initial scientific fruits of this expedition, part of a five-year $50 million effort called the International Thwaites Glacier Collaboration, are now being published, and the results are worrying. Researchers operating special ship-mounted sonar gear found a series of 25-mile-wide channels in the seafloor that bring warm water to the base of the Thwaites and Pine Island Glaciers. When this warm flow meets the place where the glaciers rest on top of the edge of the Antarctic continent—known as the grounding line—the ice underneath the glacier melts and the whole glacier becomes more slippery. Think of an ice cube sliding across the counter on its own meltwater.

Marine geophysicist Kelly Hogan of the British Antarctic Survey mapped the seafloor in front of the glaciers. For two months in the winter of 2019, Hogan was part of a joint US-UK expedition to the region, a trip that began at Punta Arenas, Chile. After a five-day crossing to Antarctica on the US research vessel Nathaniel B. Palmer, Hogan arrived at the Thwaites study site and found herself staring at a massive wall of ice. “We approached Thwaites at night,” Hogan recalls. “It was dark and foggy. I went to the bridge to talk to the captain, and as we were talking this 25-meter cliff emerged out of the gloom.”

Over the next two months, the scientists traversed the 80-mile wide embayment in front of the glacier in a back-and-forth pattern known as “mowing the lawn.” The researchers used a multibeam echosounder mounted under the ship to collect sonar images of the seafloor that were assembled into a 3D map. Together, they revealed massive seafloor channels moving warm water to the base of the glacier.

“They are important because Thwaites is vulnerable to changing quickly under climate change,” Hogan says. “One of the drivers is warm water getting underneath the floating parts and increasing the melting. The fact we have these big underwater channels going all the way up to the base of the glacier—because they are deeper and larger, you get more of that warm water and would increase the ability to melt.”

The US/UK team published two scientific papers last week from the 2019 expedition. One, authored by Hogan in the journal Cryosphere, detailed the team’s new map of the seafloor at Thwaites using the ship-based sonar readings. The second paper contained new data from another group that flew over the glacier in a Twin Otter aircraft with ground-penetrating radar that was able to look through the ice. The researchers also used special equipment to detect gravitational changes in the glacier that revealed the density of the bedrock below the ice.

The crew flew over both the glacier and the bay where Thwaites meets the ocean, according to David Porter, an associate scientist at Lamont Doherty Earth Observatory at Columbia University and an author on the second study, also published in the journal Cryosphere. The two teams shared data sets from the aircraft and ship. “We used measurements of gravitational changes to make a new map of the glacier and seafloor shape,” Porter says. “In combination with seafloor bathymetry, the data has revealed the shape of the seafloor and that there are deep pathways to allow warm water to move onshore, across the continental shelf and go in contact with the ice.”

These underwater channels are up to 3,280 feet deep and between 12 miles to 25 miles wide, says Porter. “That’s one reason Thwaites has been changing,” he says.

Scientists say Thwaites and the other glaciers are not likely to collapse in the next century; they are simply too big to fail right now. At the same time, they are seeing troubling signs of increased melting that can still cause a slow rise in sea levels around the globe. One of the big questions is the speed of the melting, and whether it will reach some kind of tipping point where it will be unable to be reversed if society is able to contain carbon emissions and somehow slow climate change.

“The ice shelf is getting weaker,” says Stef Lhermitte, an assistant professor of geoscience and remote sensing at the Technical University of Delft in the Netherlands. “The ice shelf slows down the traffic behind. At the moment you lose the ice shelf, the glaciers are free to flow and discharge their ice into the ocean.”

Lhermitte led a separate study of Thwaites and Pine Island Glaciers with a group of Dutch, French, and US scientists, who used a 21-year dataset of satellite imagery to reveal the first signs of structural weakness—crevasses and open fractures in the ice shelves that could signal their disintegration in the future. Their results show that the damage is creating a positive feedback loop that triggers more damage and faster-moving ice flows.

That study, published today in the Proceedings of the National Academy of Sciences, concludes that understanding how the ice field ruptures as it moves across the bedrock is vital to understanding when this collapse might occur. In addition to identifying the weak points in the glacier, Lhermitte and colleagues created a computer model to predict how such cracking and buckling could affect other Antarctic glaciers in the future.

Lhermitte says the goal of this model was not to predict the exact date when Thwaites will collapse. That’s next to impossible right now, because there are too many other unknown factors to consider, such as the pace of climate change that is warming both the air and water temperature around the glaciers, as well as the movement of ocean currents around Antarctica. (A 2014 study published in the journal Science by University of Washington scientists used satellite data and numerical modeling to predict that the West Antarctic Ice Sheet, including Thwaites, may collapse in 200 to 1,000 years.)

Instead, Lhermitte’s model is an attempt to incorporate ice sheet damage into similar global climate models that predict both sea level rise and the future of Antarctica’s glaciers. “The understanding of how much and how fast these glaciers are going to change is still unknown,” Lhermitte says. “We don’t know all the process. What we have done with this study is look at this damage, the tearing apart of these ice shelves, and what their potential contribution to sea level rise could be.”

Predicting glacier ice movement is difficult because ice behaves as both a solid and as a liquid, says Penn State University professor of geosciences Richard Alley, who was not affiliated with any of these studies. Alley says the study about how glaciers fracture is both new and important because it gives more insight into how fast they might collapse. In an email to WIRED, Alley compared the science of studying how Antarctic glaciers move to the process of engineering a bridge.

“You do NOT want your bridge to break, and you do not want to need to predict exactly the conditions that will make it break, so you design with a large safety margin. We can’t ‘design’ Thwaites, so we face these large uncertainties. Quantifying parts of that is important, although remembering that this is still fracture mechanics, and it still might surprise us, one way or the other,” Alley wrote.

Lhermitte thinks his study results mean that Antarctic glaciers need to be closely watched in the coming years for any signs of rapid change that might lead to an environmental catastrophe. “They are these large sleeping giants,” Lhermitte says about Thwaites and Pine Island glaciers. “We start to be curious if they will stay sleeping or awake with large consequences, with sea level rise.”

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Source: https://www.wired.com/story/antarctic-glaciers-are-growing-unstable-above-and-below-water/

Glacier, Antarctica, Thwaites Glacier, Pine Island Glacier, Ice, Climate change, Sea level rise

World news – CA – Antarctic Glaciers Are Growing Unstable Above and Below Water

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