The walk under the ice of an autonomous underwater vehicle is giving scientists their first direct evidence of how and where the warm waters of the ocean threaten the stability of Antarctica’s Thwaites Glacier. These new data will ultimately help scientists more accurately project the fate of the glacier: how quickly it melts and retreats inland and how far it can be from complete collapse, the team reports on April 9 in Science Advances
“We know there’s a sick patient out there and he’s not able to tell us where it hurts,” says Eric Rignot, a glaciologist at the University of California, Irvine, who did not participate in the new study. "So this is the first diagnosis."
Scientists have observed the Florida-sized Thwaites Glacier with growing concern for two decades. Satellite images reveal that it is retreating at an alarming rate of 0.6 to 0.8 kilometers per year on average since 2001, prompting some to call it the "final trial glacier." But estimates of how fast the glacier recedes, based on computer simulations, vary widely from one place to another on the glacier, Rignot and other researchers at Science Advances reported in 2019. Such uncertainty is the biggest difficulty when it comes to future projections. of the sea. level rise (SN: 1/7/20).
The main culprit is known for the rapid retreat of Thwaites and other Antarctic glaciers: the relatively warm ocean waters seep beneath the floating ice shelves, the strips of glaciers jutting out into the ocean (SN: 9/9/20). This water eats the supports of the ice shelves, points where the ice is anchored to the bottom of the sea which reinforces the rest of the glacier against slipping into the sea.
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Scientists used satellite data to trace approximately what lies beneath Thwaites' ice shelf. Three deep canals carved into the seabed snake under a vast cavity filled with water 120 kilometers in diameter. But without direct measurements of the chemistry and the path that water takes to reach the belly of Thwaites, it was impossible to know where the threatening, hot water is really coming from and where the ice is attacking, says Anna Wåhlin, an oceanographer physicist at the University of Gothenburg in Sweden.
In February and March 2019, Wåhlin and his colleagues sent the AUV Ran to cross two of the deep canals. Gliding about 50 meters above the seabed, the AUV collected the first direct measurements of temperature, salinity and oxygen levels in the water. From those measurements, the team was able to track the origins of different plots of water mixing under Thwaites.
Based on its chemical composition, some of the warm water came from neighboring Pine Island Bay. “We were very surprised,” because Pine Island Bay was not previously thought to be a major player in the future of Thwaites, Wåhlin says. The body of water from there was near the bottom of the cavity, about 500 feet deep, and was less salty than the surrounding sea water and several degrees Celsius warmer than the freezing point. That is an unstable situation, likely to create turbulence and increase the potential for ice erosion, says Wåhlin.
The finding also suggests that what happens in Pine Island Bay does not necessarily remain in Pine Island Bay and that the fate of the Thwaites may be closely intertwined with that of the Pine Island Glacier, another rapidly melting ice river, says Wåhlin. Together, the two glaciers are responsible for most of the ice and water that Antarctica is currently pouring. But while Thwaites is still fixed to the bottom of the sea in some places, which slows its glide into the sea, those bases are not on Pine Island.
In April, scientists identified three turning points for the precarious Pine Island Glacier, thresholds that could cross as weather conditions evolve that will lead to phases of rapid and irreversible retreat. The third and final threshold, caused by an increase of about 1.2 degrees Celsius in ocean water temperature compared to current ocean temperatures, would lead the glacier to full collapse, the team discovered.
An upcoming expedition that Wåhlin and others plan for January 2022 will use two AUVs to explore much further into the cavity below Thwaites. Ideally, the AUVS will approach several hundred miles from the shore, to the shoreline, where the base of the glacier rests on land.
“That’s the key to the line,” Rignot says. Observing how water masses interact with the glacier’s landline will be crucial to understanding the future of the glacier, he says. "That's where the melting makes the biggest difference in glacier stability."
And there’s a lot that researchers still don’t know about the vast water cavity under Thwaites ’ice shelf, including its precise dimensions and the best places to explore AUVs, he adds. "We're just getting started."