West Antarctic Ice: Slip-sliding Away
BY BRUCE E. JOHANSEN
It’s the end of winter here, but summer along the coast of West Antarctica, with ice melting in ominous ways that may reshape coastlines around the world for coming generations.
Recent analysis by glaciologist Ted Scambos of the “National Snow and Ice Data Center” in Boulder, Colorado and several colleagues indicates that after major Antarctic ice shelves (“Larsen A” and parts of “Larsen B”) collapsed, remaining ice shelves have continued to lose ice for a decade or more. Warming ocean waters driven by strong winds seem to be eroding the ice on the Antarctic Peninsula and the edges of the “West Antarctic Ice Sheet.” Today, only parts of Larsen B and Larsen C remain.
The West Antarctica Ice Sheet’s “Pine Island Glacier” also is losing more than 19 cubic miles of ice per year. Using satellite images, scientists spotted a series of large surface undulations on the ice shelf. Next they matched the undulations with the timing of warm water pulses in the waters adjacent to the ice shelf. When surface winds are strong, they stir the Southern Ocean and lift the warm water onto the continental shelf where the additional heat contributes to melting. A channel of relatively warm water running below the Pine Island Glacier also may be accelerating the melting. The channel, according to an analysis posted by NASA’s Earth Observatory, conducts ocean water to the grounding line, melting the ice shelf from below.
How Much Sea-level Rise?
The stability of the West Antarctic Ice Sheet (which comprises about a quarter of the Earth’s largest mass of frozen water) has been a subject of intense scientific inquiry for many years. A vibrant debate has grown up regarding the future of the ice sheet, with assurances of stability on one side, and speculation of future collapse on the other.
The idea that global warming could provoke the disintegration of the West Antarctic Ice Sheet was aired as theory by glaciologists as early as 1979. J.H. Mercer has suggested that the West Antarctic Ice Sheet fell apart during an interglacial period about 125,000 years ago—without an added boost from the burning of carbon-based fuels. T.J. Hughes has examined the geophysical mechanisms which may cause the West Antarctic ice sheet to collapse.
An analysis by Jonathan L. Bamber of the United Kingdom’s “Bristol Glaciology Center” and colleagues estimated that collapse of the West Antarctic Ice Sheet could raise global sea level by 3.3 meters (about 11 feet), but with large regional variations. The analysis, published in the May 15, 2009 issue of Science, said that sea-level rise would vary around the world, with an extra foot and a half (25 percent more than the worldwide mean) on North America’s east and west coasts, because “the shift in a huge mass of ice away from the South Pole would subtly change the strength of gravity locally and the rotation of the Earth.”
Questions about the Pine Island Glacier
Two-thirds of the ice-mass loss from the West Antarctic Ice Sheet stem from the Pine Island Glacier and its environs, where release of ice doubled during the decade ending in 2008. Radar images from satellite observations of the Pine Island Glacier in Antarctica taken during the 1990s indicate that it has been shrinking rapidly. The shrinking of this glacier is important "because it could lead to a collapse of the West Antarctic Ice Sheet," said Eric Rignot, a computer-radar scientist at the “Jet Propulsion Laboratory” in California, who led the study.
"The continuing retreat of Pine Island Glacier could be a symptom of the WAIS [West Antarctic Ice Sheet] disintegration," said Craig Lingle, a glaciologist at the University of Alaska in Fairbanks, who is familiar with the study. By 2009, the Pine Island Glacier was losing ice four times as quickly as a decade previously, according to satellite imagery—a disclosure that provoked alarmed scientists to cut their estimates for the demise of this glacier from 600 years to one century.
A modeling study published in 2010 argues that the Pine Island Glacier has passed its tipping point, which could bring on collapse of parts of the West Antarctic Ice Sheet. The study by Richard Katz and colleagues at the University of Oxford projects that the glacier may lose half its mass in less than a century. Katz and M. Grae Worster wrote in Britain’s Proceedings of the Royal Society: “Our results indicate that unstable retreat of the grounding line over retrograde beds is a robust feature of models that evolve based on force balance at the grounding line. We conclude, based on our simplified model, that unstable grounding-line recession may already be occurring at the Pine Island glacier.”
Warming water in the “Amundsen Sea” is eroding the glacier from below, pushing the grounding line higher up the continental shelf. This model may understate the speed at which glacier’s grounding line is retreating, says Katz. "Ours is a simple model of an ice sheet that neglects some important physics," said Katz. "The take-home message is that we should be concerned about tipping points in West Antarctica and we should do a lot more work to investigate," he said.
The Pine Island Glacier is important because it is part of stream of ice that moves more rapidly than the ice cap surrounding it. This glacier is part of an ice stream which runs from the interior of the West Antarctic Ice Sheet into the surrounding ocean waters. If a glacier in this ice stream melts more quickly from the bottom than snow accumulates on its top, the net icemelt goes into the ocean, raising sea levels.
A "disaster scenario," as described by Richard Alley, a glaciologist at Pennsylvania State University, has the Pine Island Glacier retreating enough to "make a hole in the side of the ice sheet... The remaining ice would drain through that hole."
Bamber Jonathan L, Riccardo E. M. Riva, Bert L. A. Vermeersen, and Anne M. LeBrocq. “Reassessment of the Potential Sea-Level Rise from a Collapse of the West Antarctic Ice Sheet.” Science 324(May 15, 2009):901-903.
Hughes, T.J. , J.L. Fastook, and G.H. Denton. Climatic Warming and the Collapse of the West Antarctic Ice Sheet. Orono, Maine: University of Maine Press, 1979.
Bruce E. Johansen is Jacob J. Isaacson Professor at the University of Nebraska at Omaha and author of The Encyclopedia of Global Warming Science and Technology(2009).