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West Antarctic Ice Sheet (changes)

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Idea

The West Antarctic Ice Sheet or WAIS is the ice sheet that covers Antarctica on the Western Hemisphere side of the Transantarctic Mountains. The bed of the this WAIS ice sheet lies well below sea level, level. and The its ice edges gradually flow flows into floating ice shelves such as the Ross Ice Shelf: Shelf and Ronne Ice Shelf, and also glaciers that dump ice into the Amundsen Sea.

The West Antarctic Ice Sheet contains about 2.2 million cubic kilometers of ice, enough to raise the world’s oceans about 4.8 meters if it all melted. For more on this, see Sea level rise: Antarctica.

The An melting important of question is whether the WAIS may is cause unstable in a significant way that makes a large amount of sea melting level likely rise to in occur. In 2014 two papers were published suggesting that this century. is If the entire case: WAIS melted, it would make sea levels rise 4.8 meters. For more, seeSea level rise: Antarctica.

Instability

The West Antarctic Ice Sheet sits in a bowl-shaped depression in the earth, with the bottom of the ice below sea level. Warm ocean water is causing the ice sitting along the rim of the bowl to thin and retreat. As the edge of the ice moves away from the rim and enters deeper water, it can retreat faster.

So, there could be a kind of tipping point, where the West Antarctic Ice Sheet melts faster and faster as its bottom becomes exposed to more water. On YouTube you can watch a video that illustrates the process.

Scientists have been concerned about this for decades. But now two teams of scientists claim that tipping point has been passed. Here is a long quote from a short ‘news and analysis’ article by Thomas Sumner in the 16 May 2014 issue of Science:

A disaster may be unfolding—in slow motion. Earlier this week, two teams of scientists reported that Thwaites Glacier, a keystone holding the massive West Antarctic Ice Sheet together, is starting to collapse. In the long run, they say, the entire ice sheet is doomed. Its meltwater would raise sea levels by more than 3 meters.

One team combined data on the recent retreat of the 182,000-square-kilometer Thwaites Glacier with a model of the glacier’s dynamics to forecast its future. In a paper on page 735, they report that in as few as 2 centuries Thwaites Glacier’s edge will recede past an underwater ridge now stalling its retreat. Their models suggest that the glacier will then cascade into rapid collapse. The second team, writing in Geophysical Research Letters, describes recent radar mapping of West Antarctica’s glaciers and confirms that the 600-meter-deep ridge is the final obstacle before the bedrock underlying the glacier dips into a deep basin.

Because inland basins connect Thwaites Glacier to other major glaciers in the region, both research teams say its collapse would flood West Antarctica with seawater, prompting a near-complete loss of ice in the area over hundreds of years.

“The next stable state for the West Antarctic Ice Sheet might be no ice sheet at all,” says the Science paper’s lead author, glaciologist Ian Joughin of the University of Washington, Seattle. “Very crudely, we are now committed to global sea level rise equivalent to a permanent Hurricane Sandy storm surge,” says glaciologist Richard Alley of Pennsylvania State University, University Park, referring to the storm that ravaged the Caribbean and the U.S. East Coast in 2012. Alley was not involved in either study.

Where Thwaites Glacier meets the Amundsen Sea, deep warm water burrows under the ice sheet’s base, forming an ice shelf from which icebergs break off. When melt and iceberg creation outpace fresh snowfall farther inland, the glacier shrinks. According to the radar mapping released this week in Geophysical Research Letters from the European Remote Sensing satellite, from 1992 to 2011 Thwaites Glacier retreated 14 kilometers. “Nowhere else in Antarctica is changing this fast,” says University of Washington Seattle glaciologist Benjamin Smith, co-author of the Science paper.

To forecast Thwaites Glacier’s fate, the team plugged satellite and aircraft radar maps of the glacier’s ice and underlying bedrock into a computer model. In simulations that assumed various melting trends, the model accurately reproduced recent ice-loss measurements and churned out a disturbing result: In all but the most conservative melt scenarios, a glacial collapse has already started. In 200 to 500 years, once the glacier’s “grounding line”—the point at which the ice begins to float—retreats past the ridge, the glacier’s face will become taller and, like a tower of blocks, more prone to collapse. The retreat will then accelerate to more than 5 kilometers per year, the team says. “On a glacial timescale, 200 to 500 years is the blink of an eye,” Joughin says.

And once Thwaites is gone, the rest of West Antarctica would be at risk.

Eric Rignot, a climate scientist at the University of California, Irvine, and the lead author of the GRL study, is skeptical of Joughin’s timeline because the computer model used estimates of future melting rates instead of calculations based on physical processes such as changing sea temperatures. “These simulations ought to go to the next stage and include realistic ocean forcing,” he says. If they do, he says, they might predict an even more rapid retreat.

Here are the relevant papers:

• Eric Rignot, J. Mouginot, M. Morlighem, H. Seroussi and B. Scheuchl, Widespread, rapid grounding line retreat of Pine Island, Thwaites, Smith and Kohler glaciers, West Antarctica from 1992 to 2011, Geophysical Research Letters, accepted 12 May 2014.

• Ian Joughin, Benjamin E. Smith and Brooke Medley, Marine ice sheet collapse potentially underway for the Thwaites glacier basin, West Antarctica, Science, 344 (2014), 735–738.

On YouTube you can watch a video that shows Pine Island Glacier, Thwaites Glacier and some other nearby glaciers.

Pine Island Glacier

In 2011, scientists working in Antarctica discovered a massive crack across the Pine Island Glacier, a major glacier in the West Antarctic Ice Sheet. The crack was 30 kilometers long, 80 meters wide and 60 meters deep. The pictures above and below show this crack - the top one is from NASA, the bottom one was taken by an explorer named Forrest McCarthy.

By July 2013, the crack expanded to the point where a slab of ice 720 square kilometers in size broke off and moved into the Amundsen Sea.

References

category: oceans