Warmer surface temperatures during summers can cause more ice on Antarctica ice shelves to melt into standing water ponds, then leak into cracks and increase the odds of collapse, according to a new study published by an American team of scientists.
Led by Ted Scambos of the University of Colorado at Boulder, the team focused on the Larsen Ice Sheet on the Antarctic Peninsula.
The Larsen Ice Sheet experienced major retreats in 1995 and 1998, including more than 775 square miles that disintegrated during a January 1995 storm.
The team used satellite images of meltwater on the ice surface and a sophisticated computer simulation of the motions and forces within ice shelves. The results indicated that added pressure from surface water filling up the cracks and crevasses can completely crack ice shelves, causing portions to float away and eventually melt.
A paper on the subject by Scambos and Jennifer Bohlander of the University of Colorado-headquartered National Snow and Ice Data Center, Mark Fahnestock of the University of Maryland and Christine Hulbe of NASA's Goddard Space Flight Center in Greenbelt, Md., appeared in the January issue of the Journal of Glaciology.
"The result implies that other ice shelves are closer to the breaking point than we previously thought," said Scambos. "The shelf retreats that have occurred so far have had few consequences for sea-level rise, but breakups in some other areas like the Ross Ice Shelf could lead to increases in ice flow off the Antarctic and cause sea level to rise."
Floating ice shelves, which account for about 2 percent of Antarctic ice, typically undergo cycles of advance and retreat over many decades. While scientists have known that meltwater fills crevasses and enlarges the cracks, this is the first study to explain the physics linking ice-shelf viability and meltwater ponds.
The extra outward pressure of the water counteracts the internal pressure holding the ice together, according to the scientists' conclusions. Crevasses routinely form on the landward side of the shelf.
Satellite observations of melted water on the ice surfaces provide important clues to the water pressure theory, said Fannestock. After analyzing images of the Larsen Ice Shelf over the past two decades, he deduced the years with the longest duration of surface meltwater corresponded to the years of major shelf breakup events. The melt season during the 1995 Larsen Ice Sheet retreat was more than 80 days long - about 20 days longer than average.
NASA's Hulbe used a computer model to simulate the thermodynamics of part of the Larsen Ice sheet before and after the 1990 retreat events to assess if meltwater "wedges" could split a crevasse to the bottom of the ice sheet. She found that depending on the internal strength of the ice, water-filled crevasses as shallow as 15 feet could fracture through a 660-foot-thick ice shelf.
The researchers believe the splintered remains are likely held together by bridges between crevasses until a combination of winds, tides and another season of melting lead to a breakup. "The findings provide a solid link between climate warming and the recent extensive disintegration of some Antarctic ice shelves," said Scambos. "The process can be expected to be more widespread if Antarctic summer temperatures continue to increase."
In the past, researchers thought Antarctica was very cold and stable, said Scambos. But the recent research shows the summertime temperatures on the Larsen Ice Sheet are just a few degrees below what the researchers believe is the threshold for surface "ponding" and subsequent ice-cracking events.
Warmer summer temperatures on the much larger Ross Ice Shelf in Antarctica could have severe repercussions because that ice shelf is part of the "braking system" for some very large glaciers, Scambos said. "If we begin to get significant water ponding there, and the shelf is eventually destroyed, we would likely have ice pouring off the Antarctic at a much faster rate. That would increase sea level significantly."
"We need to monitor the summertime temperatures to see what the future holds for these ice sheets," said Hulbe. While some areas of the Antarctic have warmed by as much as 4.5 Fahrenheit in the past 50 years, few records have been kept of seasonal temperatures over ice shelves, she said.
The National Snow and Ice Data Center is part of the Cooperative Institute for Research in Environmental Sciences, a joint institute of CU-Boulder and the National Oceanic and Atmospheric Administration located on the CU-Boulder campus.