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Antarctic Glacier Thinned as Rapidly in the Past

The Pine Island Glacier in West Antarctica thinned at modern rates some 8,000 years ago
Pine Island Glacier Ice Shelf


NASA's DC-8 flew over the Pine Island Glacier Ice Shelf on Oct. 14, 2011 as part of the agency's Operation IceBridge. A large, long-running crack was plainly visible across the ice shelf.
Credit: NASA ICE/Flickr

The Pine Island Glacier, which sits on part of west Antarctica, is the single largest contributor to global sea-level rise.

That is because the enormous glacier, which constitutes 10 percent of the West Antarctic Ice Sheet, is thinning rapidly, allowing more and more of its land-based ice to reach the sea. How fast this rapid thinning goes on, and for how long, will determine how quickly sea levels rise in the future.

Now, a group of researchers from the British Antarctic Survey, working with scientists at Columbia University's Lamont-Doherty Earth Observatory, have discovered important information about how the Pine Island Glacier thinned in the past, which could provide clues to the pace it will melt in the future.

"The paper is kind of saying, look, this is what this particular glacier is capable of," said Mike Bentley, a researcher at Durham University in England who co-led the paper.

In recent years, the Pine Island Glacier has experienced a high-speed reduction. Between 2002 and 2007, satellite measurements showed that ice from the glacier's grounding line, the spot where it transitions from being on the land to in the sea, thinned at a rate of 1.2 meters to 6 meters per year.

But scientists don't know whether that pace will last a few years, a few decades or a few centuries. To understand this, they turned to the past, to see what glaciers in that region had previously done.

If clues about the ancient behavior of these glaciers are any guide, the Pine Island Glacier may very likely continue thinning at a high rate for many years to come.

"What we showed for the first time is, 8,000 years ago, this glacier was capable of thinning at the sorts of rates we are seeing now, and then the second finding was, that thinning went on for several decades, possibly centuries," Bentley said.

Clues in the rocky debris
To understand the thinning rate of a glacier from 8,000 years ago that no longer exists, the scientists turned to the mountains that jut up from the vast ice sheet. They are adjacent to the Pine Island Glacier and hold an unnamed glacier that flows into it.

These sorts of mountains are called nunataks, after the Inuit word for the exposed part of a mountain or ridge that is otherwise covered in snow.

The higher levels of these nunataks were once glacier-covered but are now bare rock. And some of those rocks, left behind by long-ago glaciers, also contain clues about the glaciers from whence they came.

A glacier flowing down a mountain carries with it all kinds of debris, Bentley explained. But some of the debris, like boulders, drops out and is left behind as the glacier retreats. Prior to being left behind, the boulders were covered by the glacier and so were not exposed to sunlight. As soon as the glacier retreated, however, they were exposed to the sun.

This leaves a time stamp of a sort. "If you have an ice sheet covering the rock, it also covers the rocks from cosmic rays," said Joerg Schaefer, a geochemist at Lamont-Doherty whose lab analyzed the rocks the glacier left behind.

The cosmic rays produce rare chemicals in the rocks, and by analyzing the concentrations of those chemical isotopes, researchers at Schaefer's lab are able to determine just how long ago the rocks were first exposed to the sun -- and thus when they were left behind by the glacier.

The researchers took a sampling of boulders left behind on two nunataks near the Pine Island Glacier. The boulders were as far apart in elevation as 100 meters (328 feet). If the glacier had moved slowly, there would be a big difference between the exposure date of the higher-elevation rocks and that of the lower-elevation rocks.

What the researchers found, however, was that the exposure date of both high- and low-elevation rocks was essentially the same -- 7,900 years ago. This means the glacier thinned and retreated so quickly that it lost 100 meters of elevation in very little time.

"The glacier here had to have been thinning at about a meter per year for several decades, possibly centuries," Bentley said.

If it has done it in the past, the thinking goes, it can do it again in the future -- which means the Pine Island Glacier could continue to thin significantly for a long time to come.

Swift before, but future still uncertain
"What we see in this moment looks very similar to the beginning of what we saw 8,000 years ago," Schaefer said. "With the unfortunate exception that the climate forcing is much, much faster than it was 8,000 years ago."

The researchers also tried to explain the mechanism behind why the glacier thinned so fast 8,000 years past. They posed two reasons.

One scenario is that an underwater geologic feature keeping the glacier and its ice shelf in place shifted, allowing the glacier to flow more rapidly to the sea. The researchers could not find evidence for this, but they cannot rule it out, either.

The other possibility they listed is that the glacier's ice shelf portion was being melted from below by a warm ocean, similar to what is happening to ice shelves today.

If that scenario is eventually proved true, the parallels between what happened 8,000 years ago and what is happening now are even stronger.

In an email, the University of Washington's Eric Steig, a leading Antarctic ice researcher, said the second scenario "is a very plausible explanation, but would be difficult to prove conclusively (since this happened 8,000 years ago!)"

Steig called the paper "excellent, very interesting and innovative," but also cautioned about making too many connections between past events and future ones.

"A very important point here is that the new findings show that Pine Island Glacier was once much bigger, and thinned rapidly to something like its current size. That tells us very little about what will happen as it thins further over the coming decades," Steig said.

While some may see evidence of rapid glacier thinning in the past and again today as evidence that the West Antarctic Ice Sheet is nearing a collapse driven by human-caused climate change, Steig said at this point, scientists just don't know whether that is the case.

"Time will tell," he said.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500

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