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There has been lots of talk lately about Antarctica and whether or not the continent's giant ice sheet is melting. One new paper 1, which states there’s less surface melting recently than in past years, has been cited as "proof" that there’s no global warming. Other evidence that the amount of sea ice around Antarctica seems to be increasing slightly 2-4 is being used in the same way. But both of these data points are misleading. Gravity data collected from space using NASA's Grace satellite show that Antarctica has been losing more than a hundred cubic kilometers (24 cubic miles) of ice each year since 2002. The latest data reveal that Antarctica is losing ice at an accelerating rate, too. How is it possible for surface melting to decrease, but for the continent to lose mass anyway? The answer boils down to the fact that ice can flow without melting.
West Antarctica is very different. Instead of a single continent, it is a series of islands covered by ice -- think of it as a frozen Hawaii, with penguins. Because it's a group of islands, much of the West Antarctic Ice Sheet (WAIS, in the jargon) is actually sitting on the floor of the Southern Ocean, not on dry land. Parts of it are more than 1.7 kilometer (1 mile) below sea level. Pine Island is the largest of these islands and the largest ice stream in West Antarctica is called Pine Island Glacier. The WAIS, if it melted completely, would raise sea level by 5 to 7 meter (16 to 23 feet). And the Pine Island Glacier would contribute about 10 percent of that.
Since the early 1990s, European and
Canadian satellites have been collecting radar data from West Antarctica.
These radar data can reveal ice motion and, by the late 1990s, there was
enough data for scientists to measure the annual motion of the Pine Island
Glacier. Using radar information collected between 1992 and 1996,
oceanographer Eric Rignot, based at NASA’s Jet Propulsion Laboratory
(JPL), found that the Pine Island Glacier's "grounding line" -- the line
between the glacier's floating section and the part of the glacier that
rests on the sea floor -- had retreated rapidly towards the land. That
meant that the glacier was losing mass. He attributed the retreat to the
warming waters around West Antarctica 6. But with only a few years of
data, he couldn't say whether the retreat was a temporary, natural anomaly
or a longer-term trend from global warming.
How the Ice Shelf Crumbles The retreat of West Antarctica's glaciers is being accelerated by ice shelf collapse. Ice shelves are the part of a glacier that extends past the grounding line towards the ocean they are the most vulnerable to warming seas. A longstanding theory in glaciology is that these ice shelves tend to buttress (support the end wall of) glaciers, with their mass slowing the ice movement towards the sea, and this was confirmed by the spectacular collapse of the Rhode Island-sized Larsen B shelf along the Eastern edge of the Antarctic Peninsula in 2002. The disintegration, which was caught on camera by NASA's Moderate Resolution Imaging Spectroradiometer (MODIS) imaging instruments on board its Terra and Aqua satellites, was dramatic: it took just three weeks to crumble a 12,000-year old ice shelf. Over the next few years, satellite radar data showed that some of the ice streams flowing behind Larsen B had accelerated significantly, while others, still supported by smaller ice shelves, had not 9. This dynamic process of ice flowing downhill to the sea is what enables Antarctica to continue losing mass even as surface melting declines. Michael Schodlok, a JPL scientist who models the way ice shelves and the ocean interact, says melting of the underside of the shelf is a pre-requisite to these collapses. Thinning of the ice shelf reduces its buttressing effect on the glacier behind it, allowing glacier flow to speed up. The thinner shelf is also more likely to crack. In the summer, meltwater ponds on the surface can drain into the cracks. Since liquid water is denser than solid ice, enough meltwater on the surface can open the cracks up deeper down into the ice, leading to disintegration of the shelf. The oceans surrounding Antarctica have been warming 10, so Schodlok doesn't doubt that the ice shelves are being undermined by warmer water being brought up from the depths. But he admits that it hasn't been proven rigorously, because satellites can’t measure underneath the ice. Glaciologist Robert Bindschadler of NASA's Goddard Space Flight Center intends to show just that. He's leading an expedition scheduled to start in 2011 to drill through the Pine Island Glacier and place an automated buoy into the water below it. According to Bindschadler, Pine Island Glacier "is the place to go because that is where the changes are the largest. If we want to understand how the ocean is impacting the ice sheet, go to where it's hitting the ice sheet with a sledgehammer, not with a little tack hammer." Meanwhile, measurements from the Grace satellites confirm that Antarctica is losing mass 11. Isabella Velicogna of JPL and the University of California, Irvine, uses Grace data to weigh the Antarctic ice sheet from space. Her work shows that the ice sheet is not only losing mass, but it is losing mass at an accelerating rate. "The important message is that it is not a linear trend. A linear trend means you have the same mass loss every year. The fact that it’s above linear, this is the important idea, that ice loss is increasing with time," she says. And she points out that it isn’t just the Grace data that show accelerating loss; the radar data do, too. "It isn't just one type of measurement. It's a series of independent measurements that are giving the same results, which makes it more robust." For more information about this topic, visit NASA's Global Climate Change website. References: 1 Marco Tedesco and Andrew J. Monaghan, "An updated Antarctic melt record through 2009 and its linkages to high-latitude and tropical climate variability," Geophys. Res. Lett., 36, L18502 (2009). 2 http://arctic.atmos.uiuc.edu/cryosphere/IMAGES/current.anom.south.jpg 3 http://www.sciencedaily.com/releases/2009/04/090421101629.htm 4 http://nsidc.org/seaice/characteristics/difference.html 5 J. L. Chen et al., "Accelerated Antarctic ice loss from satellite gravity measurements," Nat. Geosci. 2, 859-862 (2009). 6 E.J. Rignot, "Fast Recession of a West Antarctic Glacier", Science 281, 549-551 (1998) 7 P.A. Mayewski, et.al., "State of the Antarctic and Southern Ocean Climate System," Rev. Geophys., 47, 1-38 (2009). 8 D. J. Wingham et.al., "Spatial and Temporal Evolution of Pine Island Glacier thinning, 1995-2006," Geophys. Res.Lett. 36, L17501 (2009). 9 E. Rignot et.al., "Accelerated ice discharge from the Antarctic Peninsula following the collapse of Larsen B ice shelf," Geophys. Res. Lett. 31, L18401 (2004) 10 R. M. Robertson et al., "Long term temperature trends in the deep waters of the Weddell Sea", Deep Sea Research 49, 21, 4791-4806 (2002); http://condor.pems.adfa.edu.au/FD-Course/webpage/longterm.pdf. 11Isabella Velicogna, "Increasing rates of ice mass loss from the Greenland and Antarctic ice sheets revealed by GRACE," Geophys. Res. Lett. 36, L19503 (2009). 12 J. H. Mercer, "West Antarctic ice Sheet and CO2 Greenhouse Effect-Threat of Disaster," Nature, 271 (5643), 321-325 (1978). 13 R. Kwok & D.A. Rothrock, "Decline in Arctic sea ice thickness from submarine and ICESat records: 1958 - 2008," Geophys. Res. Lett., 36, L15501 (2009),
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Missing Children
Code Amber News Service (CANS) issued this Missing Endangered person Alert after the girl disappeared from her bedroom some time after 3 AM Tuesday morning in Satsuma. Satsuma is approximately 75 miles east of Gainsville. Haleigh Cummings a white female, 3 feet tall, weighs 39 pounds and has blond hair and brown eyes. She was last seen wearing a pink shirt and underwear. There are no suspects in this case at this time. However, due to the circumstances of the child's disappearance an abduction is strongly suspected. There is no suspect vehicle at this time. Anyone with information is asked to call the Putnam County Sheriff's office at (386) 329-0808 or dial 911. Download free video of missing Florida child Haleigh Cummings Email this alert to a friend in the area.
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