Warmer surface temperatures over just a few months in the Antarctic can splinter an ice shelf and prime it for a major collapse, NASA and university scientists report in the latest issue of the Journal of Glaciology.
Using satellite images of tell-tale melt water on the ice surface and a sophisticated computer simulation of the motions and forces within an ice shelf, the scientists demonstrated that added pressure from surface water filling crevasses can crack the ice entirely through. The process can be expected to become more widespread if Antarctic summer temperatures increase.
"The importance of melt water implies that ice shelf stability may not be limited by the mean annual temperature, as has long been thought, but by the mean summer temperature," says co-author Christina Hulbe of the University of Maryland and NASA's Goddard Earth Science & Technology Center. "As the mean summer temperature exceeds 0 degrees Celsius, surface melting is likely to promote ice-shelf retreat."
The team of scientists - Ted Scambos and Jennifer Bohlander of the National Snow and Ice Data Center at the University of Colorado, Mark Fahnestock of the University of Maryland, and Hulbe - focused on the Larsen Ice Shelf on the Antarctic Peninsula, which experienced major retreats in 1995 and 1998. Over 775 square miles (2000 square kilometers) of the northern section of this ice shelf disintegrated in January 1995 during a storm.
"This result implies that other ice shelves are closer to the breaking point than we previously thought," says Scambos. "The shelf retreats that have occurred so far have had few consequences for sea-level rise, but breakups in some other areas, such as the Ross Ice Shelf, could lead to increases in ice flow off the Antarctic and cause sea level to rise."
The floating ice shelves, which account for about 2 percent of all Antarctic ice, typically undergo cycles of advance and retreat over many decades. It has long been recognized that melt water filling of crevasses could enlarge the cracks, but this study is the first to connect the basic physics of that process with significant effects of surface melt ponds on ice-shelf viability. The extra outward pressure of the water counteracts the internal pressure holding the ice together.
Crevasses routinely form at the landward side of the shelf as glacial ice pushes past coastal features and flows into the floating ice. The crevasses slowly travel seaward as the ice shelf grows.
Satellite observations of melted water on the ice surface provided an important clue to the water-pressure theory. Analyzing images of the Larsen Ice Shelf over the past 20 years, Fahnestock found that the years with the longest duration of surface melt water were also the years of major shelf breakup events. The "melt season" during the major retreat year of 1995, for example, was over 80 days long, about 20 days longer than average.
To find out if the accumulated melt water "wedge" could split a crevasse to the bottom of the ice, Hulbe used a computer model to simulate the thermodynamics of a northern section of the Larsen Ice Shelf before and after the major retreats of the 1990s. Depending on the internal strength of the ice, a water-filled crevasse just 15-50 feet (5-15 meters) deep could fracture through the 220-yard (200-meter) thick ice shelf. The splintered remains are probably held together by bridges between crevasses until a combination of winds, tides, and another season of melting lead to a breakup.
"We need to monitor the summertime temperatures to see what the future holds for these ice shelves," says Hulbe. While some areas of the Antarctic have warmed by as much as 2.5 degrees Celsius in the last 50 years, few records have been kept of seasonal temperatures over ice shelves.
"In the past, researchers thought the Rose Ice Shelf area was up to 20 degrees below the climatic limit and therefore very stable," says Scambos. "Our initial look at summertime temperatures there shows it is just a few degrees below what we think is the threshold for surface ponding."
More information can be found at:
"The Link Between Climate Warming and Break-up of Ice Shelves in the Antarctic Peninsula," Scambos, T.A., Hulbe, C., Fahnestock, M., and Bohlander, J. Journal of Glaciology, vol. 46, no. 154, pp. 516-530.
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