An Antarctic area previously thought stable may be at risk as ocean currents undermine a floating frozen shelf and threaten to push part of the West Antarctic Ice Sheet into retreat, according to papers in Nature journals.
Rising temperatures will thin the sea ice on the Weddell Sea, sending warm currents under the Filchner-Ronne Ice Shelf, where melting is seen growing to an annual 4 meters (13 feet) by 2100 from 0.2 meters now, according to one of the two papers. The second study found land under the ice sheet is below sea- level and slopes into the interior, forming a smooth basin the size of Wales that may make it easier for ocean water to enter.
The studies in Nature and Nature Geoscience show the threat posed to that part of the Antarctic by higher temperatures, risking an increase in the southern continent’s contribution to rising sea levels. The West Antarctic Ice Sheet contains enough water to lift sea levels more than 3 meters, though that isn’t likely for thousands of years, according to the United Nations.
“What we see in this region is a grounded ice sheet very nearly floating over a significant sub-glacial basin,” Martin Siegert, a co-author of the second paper, said on a conference call with reporters. Combined with the findings of the first paper, “we believe it’s at significant risk,” he said.
Siegert is professor of geosciences at the University of Edinburgh in Scotland. The study on Antarctic currents was led by Hartmut Hellmer, a scientist at the Alfred Wegener Institute of Polar and Marine Research in Bremerhaven, Germany.
Rising Sea Levels
The Wedell Sea lies to the east of the Antarctic Peninsula, a spit of land that points toward South America. Researchers have already documented the collapse of the Larsen B ice shelf to the north of Ronne-Filchner, as well as the retreat of the Pine Island glacier and threats to the Wilkins ice shelf, both to the west of the peninsula.
While neither of the studies quantified the potential effect on sea levels, the findings show the region may be on the brink of “fast change,” said Angelika Humbert, a professor of ice modeling at the Alfred Wegener Institute.
“With increased flow speed and a retreating grounding line, the amount of Antarctic inland ice transported into the sea increases, and that raises the sea level,” Humbert said on the call. She wasn’t an author of either paper.
Humbert wrote in an article in Nature Geoscience that the two studies show the Filchner-Ronne Ice Shelf, which has “so far seemed stable,” may be susceptible to changes such as those seen on the other side of the Antarctic Peninsula.
Melting Ice Shelves
The research adds to a study last month by the British Antarctic Survey that showed upwellings of warm water are melting the base of 20 of the 54 ice shelves surrounding the southern continent that were studied.
The melting of ice shelves doesn’t boost sea levels because they rest on the ocean, displacing water. In contrast, the loss of ice from the grounded ice sheet does raise the oceans.
The UN Intergovernmental Panel on Climate Change in 2007 estimated the smaller West Antarctica ice sheet is shrinking, while the larger East sheet is growing or stable, concluding that the balance for the continent ranged from an annual gain of 50 gigatons of ice to a loss of 200 gigatons in the decade through 2003. The resulting sea-level change ranged from an annual drop of 0.14 millimeters to a gain of 0.55 millimeters.
Hellmer’s team used computer models to simulate the effects of rising temperatures in the Weddell Sea. They found it melted sea ice, redirecting warm currents under the Filchner-Ronne Ice Shelf. They projected average thinning of 4 meters a year by the end of the century, with a portion melting by as much as 50 meters a year near the point that the shelf is grounded.
“If we assume that grounding lines retreat into deeper basins, our melt rates have to be considered as lower bounds,” the researchers wrote.
Siegert’s team, led by Neil Ross at the University of Edinburgh, used radio echo-sounding data from flights over Antarctica to find evidence that the bed under the ice in West Antarctica is at points more than 1.5 kilometers (1 mile) below sea-level. The smooth surface indicates it had been covered in marine sediment and so was once clear of ice, they wrote. Such deglaciation is possible again, Siegert said.
“We believe this region is on the threshold of change,” he said. “It needs some push to get over that, but we don’t believe that push has to be very hard to deliver a lot of deglaciation.”
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