SCIENCE:

Warm currents threaten to expand Antarctic melting

Another section of the vast Antarctic ice sheet is threatened by warm ocean waters, scientists reported yesterday.

Researchers have watched as warm, deep currents have carved away at the underside of ice shelves on the western Antarctic coast, accelerating ice loss by eroding the floating ice tongues that help slow glaciers' flow to the sea.

Now a pair of new studies suggest that, by the end of the century, the same process could begin thawing a portion of the vast Antarctic ice sheet that researchers considered to be relatively stable.

The Filchner-Ronne Ice Shelf covers more than 174,000 square miles in the Weddell Sea on the eastern side of the Antarctica Peninsula. The new research suggests that region, which has not experienced much ice loss so far, could begin a period of rapid change as rising air temperatures thin the region's sea ice.

A modeling study published online yesterday in Nature suggests that change in sea ice would alter ocean currents, sending pulses of warm water toward the Filchner-Ronne Ice Shelf.

That water would eat away at the ice shelf's underside, helped along by a quirk of topography described in the second new study, published online yesterday in Nature Geoscience.

Radar mapping shows the ice sheets that flow into the Filchner-Ronne Ice Shelf are perched over a massive basin that measures 60 miles long, 125 miles wide and up to 1.25 miles deep in some places.

The grounding line, where the ice shelf leaves bedrock and begins floating on water, lies just beyond the edge of the basin.

Potential victim of underwater geography

And because the basin slopes down toward Antarctica, scientists believe that once water carves past the grounding line, the underwater geography will aid its path under ice that now sits on bedrock. Eventually, that ice will begin to float, and the melting will accelerate rapidly.

"What we see in this region is currently a grounded ice sheet very nearly floating over a significant sub-glacial basin just at the point of the marine slope," said Martin Siegert, a professor of geosciences at the University of Edinburgh and a co-author of the Nature Geoscience paper. "When you add that to [the first] analysis, we think there is cause for concern."

The modeling study, conducted by researchers at Germany's Alfred Wegener Institute, predicts that air temperatures in the Weddell Sea will rise by about 7 degrees Fahrenheit by 2100.

That would warm ocean waters by 3.6 degrees Fahrenheit. And when those warm, deep currents eventually flood the subglacial basin, melting of the ice sheet could accelerate from the current rate of about 8 inches per year to more than 13 feet per year by the end of the century.

In some areas, annual ice loss could reach 164 feet by 2100.

"This region is on the threshold of change," Siegert said. "That's not to say the change is happening, but that it's on a physical threshold. 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 because of the reverse marine slope."

Angelika Humbert, a glaciologist at the Alfred Wegener Institute who did not contribute to either paper, called the new findings "robust."

"Not only does the climatic change lead to increased melt that itself accelerates the ice flow, additionally the bed is of a shape that supports that retreat," said Humbert. "Why is that a reason of concern? With increased flow speeds and retreating ground lines, the amount of Antarctic inland ice transported into the sea increases, and that raises the sea level."

Projected increase in sea-level rise

Because current projections of future sea-level rise do not take into account the newly discovered vulnerability of the Filchner-Ronne Ice Shelf, they should be considered low estimates, said Humbert, the author of a commentary on the two papers published online yesterday by Nature Geoscience.

The news comes two weeks after another study, also published in Nature, concluded that deep, warm ocean currents are driving ice loss in Antarctica. Researchers at the British Antarctic Survey said the warm water that is reaching Antarctica's western coast is carving out the bottoms of the floating ice shelves that help hold back the flow of glaciers in land (ClimateWire, April 26).

That the new papers suggest the same process could begin in the Weddell Sea by the end of the century is a surprise to researchers who have long considered the Filchner-Ronne Ice Shelf to be a relatively stable portion of the vast Antarctic ice sheet.

"This particular paper takes another part of West Antarctica that is generally thought of to be very safe and demonstrates that is not at all the case," said David Holland, an oceanographer at New York University.

"The word I would use myself is 'shocking,'" he said. "It doesn't mean this will happen this century, but these papers present the realistic possibility that it will."

Ted Scambos, senior scientist at the University of Colorado's National Snow and Ice Data Center, agreed. "I think it is a fairly profound discovery," he said. "What the study is showing is that new areas of Antarctica that we thought were slow and stable could be vulnerable by the end of the century."

Harder to determine is just how quickly the sea level would rise if warm water does begin to erode the ice shelf, experts said.

Because the ice shelf itself is already floating, its melt would not raise sea level. Thawing the portion of the ice sheet grounded on the newly discovered subglacial basin would raise the seas by centimeters, Siegert said.

But eventually, if the flow of ice from areas well-grounded on land begins to accelerate, that could have major consequences for sea level, he predicted, potentially causing ice streams in other areas to respond by speeding up or changing direction.

It is still a guessing game, said Scambos, who said looking that far into the future depends on a lot of "what-ifs."

Asked to hazard his own guess, he estimated that the process of melting and ice loss described in the new analyses could raise seas 2 to 3 meters (6.5 to 10 feet) over thousands of years.

"There's a lot of potential sea-level rise tied up in that area," Scambos said. "And once these things begin, they are very hard to undo."

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