“Keystone” Ice Shelf Cracking Up

December 13, 2021

Science:

An alarming crackup has begun at the foot of Antarctica’s vulnerable Thwaites Glacier, whose meltwater is already responsible for about 4% of global sea level rise. An ice sheet the size of Florida, Thwaites ends its slide into the ocean as a floating ledge of ice 45 kilometers wide. But now, this ice shelf, riven by newly detected fissures on its surface and underside, is likely to break apart in the next 5 years or so, scientists reported today at a meeting of the American Geophysical Union.

The most dramatic sign of impending failure is a set of diagonal fractures that nearly span the entire shelf. Last month, satellites spotted accelerating movement of ice along the fractures, says Erin Pettit, a glaciologist at Oregon State University, Corvallis, who is part of a multiyear expedition studying the glacier. The shelf is a bit like a windshield with a series of slowly opening cracks, she says. “You’re like, I should get a new windshield. And one day, bang—there are a million other cracks there.”

Once the ice shelf shatters, large sections of the glacier now restrained by it are likely to speed up, says Ted Scambos, a glaciologist at the University of Colorado, Boulder, and a leader of the Thwaites expedition. In a worst case, this part of Thwaites could triple in speed, increasing the glacier’s contribution to global sea level in the short term to 5%, Pettit says.

Even more worrisome is the process that has weakened the ice shelf: incursions of warm ocean water beneath the shelf, which expedition scientists detected with a robotic submersible. Because Thwaites sits below sea level on ground that dips away from the coast, the warm water is likely to melt its way inland, beneath the glacier itself, freeing its underbelly from bedrock. A collapse of the entire glacier, which some researchers think is only centuries away, would raise global sea level by 65 centimeters. And because Thwaites occupies a deep basin into which neighboring glaciers would flow, its demise could eventually lead to the loss of the entire West Antarctic Ice Sheet, which locks up 3.3 meters of global sea level rise. “That would be a global change,” says Robert DeConto, a glaciologist at the University of Massachusetts, Amherst. “Our coastlines will look different from space.”

Although it is unclear whether the shelf will fall apart in 1 year or 10 years, Pettit and her colleagues are pursuing important work, adds DeConto, who is unaffiliated with the Thwaites team. The oceans are simply getting too warm for these marine ice sheets, which formed in conditions much cooler than today, he says. “This marine-based ice is not going to come back.”

Exploring the future of this keystone of the West Antarctic Ice Sheet is the aim of the International Thwaites Glacier Collaboration (ITGC), a multiyear, more than $50 million expedition funded by the United States and United Kingdom. The glacier, far from any research stations, is challenging to reach under the best circumstances, and ITGC’s first scientific campaign on the ice, in the Antarctic summer of 2019–20, contended with severe storms. But the team managed to erect several temporary camps, including one in the middle of the ice shelf and another farther upstream, near the grounding line where the glacier detaches from the continent.

On top of the 300-meter-thick shelf, the researchers used ground-penetrating radar to image the underside of the ice. They were surprised to find it was not flat and smooth, but carved into a series of upside-down valleys, some 50 meters deep. These undulations stress the ice shelf, and the team saw signs of that stress: Fractures had formed at the apex of each valley, Pettit says. “They’re just waiting to be activated in a new way.”

Meanwhile, at the upstream camp, researchers led by Britney Schmidt, a planetary scientist at Cornell University, dug a borehole and sent an instrument-studded robot called Icefin plunging through it to the ocean hidden below. Schmidt then navigated Icefin to the point where the ice and rock met. Nearly everywhere—even at the grounding line itself—the water was 1°C or 2°C above freezing. Although not unexpected, given the 10-kilometer retreat of the grounding line in the past decade, it was a sure sign of the extended reach of warm waters ushered in by climate change.

During its surveys, Icefin also scanned the underside of the ice with a laser and found valleys similar to those seen downstream. Local variations in water temperature suggested the valleys create turbulence that draws in warmer waters, which deepen them, says Peter Washam, an oceanographer at Cornell. “They’re really hot spots of melting.”

The researchers have plugged many of these observations into computer models of the ice shelf, as detailed in one of three papers about Thwaites in the journal Cryosphere. The models suggest the extensive surface cracks seen in the past 5 years have opened as ice thinned by melting grinds into an offshore, undersea mountain, which had long helped hold the ice shelf back. Several of these cracks, including one nicknamed “the dagger,” are now extending toward the middle of the shelf. Once there, they may trigger the incipient cracks in the valleys underneath to grow and weaken the shelf further, Pettit says.

The newest wrinkle is the growth of the diagonal fractures, which stretch more than 40 kilometers from the grounding line all the way to the offshore mountain. Although the ice directly behind the mountain still seems stuck, GPS stations placed during the first field season show slippage along the fracture zone is allowing other ice to maneuver around the mountain, which is likely to speed up the crack up. “It’s got enough freedom now that it can reroute itself around,” Pettit says.

With several seasons left in the ITGC campaign, researchers will be able to watch as the shelf disintegrates—and they’ll have to retrieve their instruments before the ice cracks, with several fissures only 3 kilometers away from their former campsite. The ice shelf failure will be a warning that Thwaites, and the rest of the West Antarctic Ice Sheet, could begin to see significant losses within decades, especially if carbon emissions don’t start to come down, Pettit says. “We’ll start to see some of that before I leave this Earth.”

2 Responses to ““Keystone” Ice Shelf Cracking Up”

  1. Gingerbaker Says:

    Really interesting article on the various contributors to today’s sea level rise here:

    https://tamino.wordpress.com/2021/12/10/big-change-in-sea-level-rise/

    • rhymeswithgoalie Says:

      Lovely chart! (OK, I like pictures.)

      [“Steric” is primarily thermal expansion, and maybe some salt change; TWS is terrestrial water storage.]


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