Thursday, March 26, 2015

Antarctica is rapidly losing its edge

Antarctica’s icy fringe is dissolving in warming ocean waters—and the loss of ice has ramped up dramatically in the last decade, according to a new study.
 By piecing together an 18-year record of ice shelf thinning from three different sets of satellite data, the researchers found that some ice shelves in West Antarctica have lost as much as 18% of their volume in the last decade. But the story in East Antarctica is still murky, they report; although the volume of its ice shelves has fluctuated significantly, they found no clear trend of volume loss during that time period.

The Antarctic ice sheet, the thick layer of ice covering much of the continent, is anchored in place by its floating fringe, shelves of ice that jut out into the surrounding ocean. The shelves act as a buttress to the “grounded” ice, helping slow the flow of the ice sheet’s glaciers into the ocean. But warming ocean waters have been eating away at the underside of these ice shelves, thinning them in many places and reducing their ability to buttress the ice. This effect is particularly apparent in parts of the West Antarctic Ice Sheet (WAIS), long regarded as the more vulnerable part of the continent to climate change. Two regions of the WAIS, the Amundsen and Bellingshausen seas, have experienced especially dramatic losses of ice over the last couple of decades.

But even the far larger East Antarctic Ice Sheet (EAIS) is vulnerable: Some data suggest its fringing ice shelves may be thinning, too. A 2012 study in Nature, using satellite data from the ICESat mission from 2003 to 2008, sounded the alarm, reporting that ice shelves in the EAIS were now losing volume. In particular, the study noted volume loss at the Totten and Moscow University ice shelves, which help buttress a large section of the EAIS.

However, those 5 years of data are too few to fully identify trends in volume loss in many regions of Antarctica, says Fernando Paolo, a Ph.D. student at the Scripps Institution of Oceanography in San Diego, California. To get the biggest picture now available, Paolo and his colleagues stitched together satellite radar altimetry data from three consecutive and overlapping missions: the European Space Agency’s (ESA’s) ERS-1 and ERS-2 (which flew from 1991 to 2000 and 1995 to 2011, respectively), and ESA’s ENVISAT mission, which collected data from 2002 to 2012. Together, the three missions span nearly 20 years of observations.

And that matters, Paolo says, because in some parts of Antarctica, there have been significant fluctuations in the loss or gain of ice. While some regions—such as the Amundsen and Bellingshausen seas—are experiencing steady enough losses that the time period of observation might not influence the result, that’s not the case in other regions, particularly those in the EAIS, he says. “That’s where the long record is very important; you can start to see these fluctuations.”

Even so, the overall picture they report online today in Science is grim: The 18-year record suggests that the average loss in volume of Antarctica’s ice shelves across the entire continent has accelerated significantly in the last decade. From 1994 to 2003, the overall loss of ice shelf volume across the continent was negligible: about 25 cubic kilometers per year (plus or minus 64). But from 2003 to 2012, that number jumped to 310 cubic kilometers per year (plus or minus 74). That rapid acceleration was particularly apparent in the WAIS, where volume losses increased by 70% in the last decade. In the hot spots of the Amundsen and Bellinghausen seas, the ice shelves lost 18% of their thickness in less than 10 years.

But in the Totten and Moscow University ice shelves over on the eastern half of the ice sheet, the story was far less clear, Paolo says. “We still are not able to say with confidence whether that ice shelf is thinning or thickening—the data are very noisy, very difficult to interpret.” Although their data concurred with the earlier finding that the shelves had thinned from 2003 to 2008, he says, “before and after those 5 years the conditions changed. The ice shelves gained thickness.”

“It’s very important work,” says Martin Siegert, a glaciologist at Imperial College London. “These assessments of ice shelves need to be done regularly” to build up a time series of data—and ultimately to be able to separate a trend signal from the noise. But, he adds, identifying an overall trend is just one side of the equation. “You have to unpick that answer a little bit. Even the time series we’ve got isn’t enough to really understand what’s going on with this system.” For example, he says, although the new study found no overall change in the rate of melting at Totten Glacier over the last couple of decades, the large variability in melting rate within those decades warrants a much closer look at the region’s complex topography, among other factors, in order to anticipate how the system could actually change in the future.

To that end, Siegert is part of a team of scientists that has been examining the topography of the sea floor in the region, one piece of the puzzle. In a recent paper published in Nature Geoscience, his team identified two deep underwater cavities beneath the glacier that they note could be pathways for relatively warm ocean water to reach the underside of the glacier, enhancing its melting. But the topography is just one part of the story—coupling that with vastly improved satellite data as well as a better understanding of glacial processes and oceanographic and climate conditions is “probably what we need to do in all of these places,” Siegert says.

Paolo notes that his team’s findings don’t change the larger story of melting in Antarctica, even in East Antarctica. In fact, he says, they highlight the need for additional data to track its progress. To really understand what’s going on with the ice shelves and ice sheets, he says, scientists will need not only radar altimetry data but also continuous measurements of ocean properties near and under the ice shelves, and better bathymetry maps, such as Siegert’s team produced. “Now we can start thinking in a different way about how [the region] could respond in future to potential perturbations.”

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