Senin, 14 September 2020

Two major Antarctic glaciers are tearing loose from their restraints, scientists say - The Washington Post

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The new findings, published Monday in the Proceedings of the National Academy of Sciences, come from analysis of satellite images. They show that a naturally occurring buffer system that prevents the glaciers from flowing outward rapidly is breaking down, potentially unleashing far more ice into the sea in coming years.

The glaciers’ “shear margins,” where their floating ice shelves encounter high levels of friction that constrain the natural flow of ice, are progressively weakening and in some cases breaking into pieces.

“The stresses that slow down the glacier, they are no longer in place, so the glacier is speeding up,” said Stef Lhermitte, a satellite expert at Delft University of Technology in the Netherlands who led the new research along with colleagues from NASA and other research institutions in France, Belgium, Austria and the Netherlands.

While many of the images have been seen before, the new analysis suggests that they are a sign of further disintegration to come.

“We already knew that these were glaciers that might matter in the future, but these images to me indicate that these ice shelves are in a very bad state,” Lhermitte said.

It’s just the latest in a flurry of bad news about the planet’s ice.

Arctic sea ice is very close to — but likely to not quite reach — a record low for this time of year. Last month, Canada lost a large portion of its last major Arctic ice shelf.

And in Greenland, the largest still-intact ice shelf in the Northern Hemisphere, sometimes known as 79 North because of its latitude (its full name is Nioghalvfjerdsfjorden), just lost a large chunk of ice, equivalent in size to roughly two Manhattan islands, according to the Geological Survey of Denmark and Greenland. Experts there blamed the fracture on a strong general warming trend and temperatures that have been “incredibly” high in the northeast of Greenland in recent years.

Ice shelves are vast floating platforms that extend across the surface of the ocean at the outer edge of marine-based glaciers. As they flow over the water, these shelves freeze onto mountainsides and islands and anchor themselves to bumps in the seafloor. In this way, the shelves provide a braking mechanism on the natural outward flow of ice.

The buttressing effect occurs in the shear margins, where faster-flowing ice meets ice that is more static and stable, often because it is moored to some part of the landscape. In these places, the ice frequently crumples and contorts, a visible indication of the powerful stresses that it is under.

But when those stresses become too much, ice breaks. That’s what’s now happening in West Antarctica, the new research argues, suggesting that warm ocean water has thinned the ice shelves out enough from below that they became brittle.

At the same time, and for the same reason, the glaciers themselves began to flow outward faster. The resulting forces led the shear-margin ice to break into pieces — which means that the glacier, less constrained, will now be able to add ice to the ocean even faster.

For the Pine Island Glacier, the new study finds that while the cracking and fraying at the shear margin dates to 1999, it accelerated in 2016. Here’s a video based on images from the European Space Agency’s Sentinel satellite, showing the changes in the past four years:

Even more concerning is the Thwaites Glacier. Here, again, the breakdown of the shear margin has increased in recent years:

“This is important work,” Richard Alley, a glaciologist at Pennsylvania State University, said of the new study.

Alley noted that the processes playing out in Antarctica appear to have already reached their completion in parts of Greenland, where one of the largest glaciers, Jakobshavn, no longer has any significant ice shelf at all. When it lost that shelf around the year 2000, Jakobshavn’s rate of ice loss steeply increased.

The 79 North glacier still has a major ice shelf, as do some of Greenland’s other northernmost glaciers, but many of these have lost considerable size in recent decades.

“The new paper shows that the Amundsen Sea Embayment ice shelves have gone through most, but not all, of the Jakobshavn steps,” Alley said in an email. “[A] warming ocean thinned the ice shelves, this reduced buttressing, this let the non-floating ice move faster, contributing some to sea-level rise and also starting to break the sides of the ice shelves, but additional acceleration could occur if the rest of the steps (further fracture and ice-shelf loss) should occur.”

Multiple ice-shelf collapses have already been seen in Canada, Greenland and the warmer Antarctic Peninsula, where the onetime Larsen A and Larsen B ice shelves fractured and, today, no longer exist.

“When the ice shelves are damaged by climate change, as we saw in the Antarctic Peninsula in the last several decades, their buttressing effect is reduced and the ice streams speed up and raise sea levels,” said Isabella Velicogna, a glaciologist at the University of California at Irvine, commenting on the new study. “The speed-up increases damage, a positive feedback which is not good news.”

If a similar process plays out in the Amundsen Sea of West Antarctica, where Pine Island and Thwaites are, the sea-level consequences could be enormous.

Lhermitte provided calculations showing that over the past six years, the western and central parts of the Pine Island ice shelf have shrunk by about 30 percent, from about 1,500 square miles down to closer to 1,000 square miles. In other words, an area about the size of Los Angeles has been lost.

“This shear margin is so damaged we think it preconditions this ice shelf for destabilization on the longer term,” Lhermitte said. “These are the first signs we see that Pine Island ice shelf is disappearing. This damage is difficult to heal.”

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September 15, 2020 at 02:03AM
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Two major Antarctic glaciers are tearing loose from their restraints, scientists say - The Washington Post

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