A new study suggests some of Antarctica's ice sheet grows from the bottom up, adding a new wrinkle to efforts to predict how the continent's glaciers will respond to climate change.
Radar images show that water under the base of the ice sheet refreezes into ice, creating a new bottom layer that accounts for up to half the total thickness of the ice sheet in some locations. That base layer can warp and lift the ice above it, which is formed by snow deposited on the ice sheet's surface.
The findings, published yesterday in the journal Science, suggest scientists still have much to learn about the factors that govern the behavior of ice sheets -- knowledge that is crucial to developing more accurate projections of future sea level rise.
The discovery is the fruit of an international research effort to map a massive mountain range buried under miles-thick ice in a remote section of East Antarctica.
Researchers from seven nations traveled to the site in 2008, bringing with them a pair of Twin Otter aircraft equipped with ice-penetrating radar and other scientific instruments. The data they collected during low-altitude flights sketched a picture of the ice sheet's internal structure and the Gamburtsev Mountains below.
What they found surprised them, said the study's lead author, geophysicist Robin Bell of Columbia University's Lamont-Doherty Earth Observatory.
"We could see the mountain," Bell said. "We could also see these funny bulbous features on top, warping the layers on top of that." The portion of East Antarctica that Bell's team studied, known as "Dome A," is home to ice roughly 1 to 2 miles thick.
Eventually, the scientists realized those strange, bell-shaped pockets of ice had formed when water below the ice sheet froze.
"They were all going in the same direction," Bell said. "They all started in places where the bottom of the ice sheet is wet. That allowed us to make the linkage between these funny features and the water."
Scientists have long known that liquid water forms under Antarctic ice. Heat from the Earth's core, along with friction from the movement of ice over bedrock, helps melt the base of the ice sheet. Meanwhile, the mass of ice above acts like an insulating blanket.
Mountains of new ice underneath the ice sheet
Researchers have also observed water in lakes underneath glaciers that refreezes into thin layers of ice. What is new, experts said, is the idea that water at the ice sheet's base refreezes in huge amounts that can reshape the surface of the ice sheet.
At the Dome A site in East Antarctica -- roughly the size of the state of California -- the base layer of refrozen ice accounted for up to half the total thickness of the ice sheet, and 24 percent of the area covered by ice.
"The paper reports a fascinating result," said Richard Alley, a glaciologist at Pennsylvania State University, "that melting beneath deep ice produces water that flows beneath thinner ice and refreezes, and that this has been going on long enough to make a big refrozen layer."
Said Alley, "We knew previously that some of this occurs, but not nearly to this extent." He pointed to an ice core taken from East Antarctica's Lake Vostok, which lies underneath the ice sheet. That bottom part of that core is frozen lake water, Alley said.
Sasha Carter, a postdoctoral researcher at the Scripps Institution of Oceanography who studies the Antarctic ice sheet, said the new research "shows that subglacial water transport and the refreezing of that meltwater is reshaping the interior of East Antarctica at a scale previously not believed to be possible."
Bell said she and her colleagues believe that water under the ice sheet refreezes in two ways. Water that collects in valleys underneath the ice sheet, in the Gamburtsev Mountains, refreezes when it passes under thinner parts of the ice sheet that are less insulated from cool surface temperatures.
Water also refreezes when it is forced up valley walls, experiencing a rapid change in pressure that triggers a freezing process known as "supercooling."
The question now, experts said, is what the discovery of the bottoms-up ice formation means for efforts to understand ice sheet behavior and to investigate past climate by extracting ice cores.
"We know a huge amount about the ice sheets," said Alley. "We can be confident that making the air over Greenland warmer, or making the air over parts of Antarctica a lot warmer, or making the water under the ice shelves of either sheet even a little warmer, will tend to raise sea level. But how much, how rapidly?"
Answering that question requires a solid understanding of the factors that affect the Antarctic ice sheet's behavior -- how it adds and loses ice. The new study is a step toward that understanding, he said.
Meanwhile, Bell said, the findings' impact on efforts to drill new ice cores is still unclear. The layer of ice formed by refreezing water may help preserve the oldest ice, by lifting it off the bedrock and preventing it from being destroyed by melting.
But the bottoms-up ice formation could make finding that old ice harder, since the new layer of ice scrambles the older layers of ice above it, or it could make it easier, because the older ice will be closer to the surface of the ice sheet. That means scientists wouldn't have to drill as far to reach the oldest ice.
Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500