November 17, 2008
Researchers Study How Floods Under Ice Speed Up Glaciers
Scientists say that enormous floods beneath the Antarctic ice sheet can now be linked directly to the speed at which that ice moves towards the ocean.
The giant Byrd Glacier in east Antarctica sped up just as two lakes under the ice overflowed, according to Leigh Stearns and colleagues. They say the floodwater acts as a lubricant, easing the ice over the bedrock.
Ice behavior in response to a warming Earth is one of the great uncertainties in projecting future ocean rise, world scientists said in their state of the climate assessment in 2007 (Intergovernmental Panel on Climate Change).
"Antarctica's under-ice plumbing system must now be an important consideration in ice dynamics," said Stearns, whose study was reported in Nature Geoscience.
Dr. Stearns from the Climate Change Institute at the University of Maine said previous work has shown that the water under the ice is moving around a lot, but what has been missing was the fact that this water is affecting ice flow."
"Not only is it moving, the addition of a little bit of water and a change of lubrication at the bed of a glacier can produce quite large-scale changes," she said.
For half a century scientists have known about more than 150 subglacial lakes in Antarctica. The largest, Lake Vostok, is the size of Lake Ontario in North America.
The lakes' contents stay liquid because of warm spots in the underlying rock despite being capped by several kilometers of ice.
However, it was always thought that these lakes were stagnant bodies, containing waters that were perhaps unaltered for millions of years.
But in 2005 scientists discovered that the lakes' levels could actually change rapidly and can fill and burst their rims under the ice sheet.
When they fill and flood, they actually lift the ice up by several meters, which can be seen by satellites that measure the height of the ice.
The researchers took a 48-year record of ice speeds recorded along Byrd Glacier and compared the data with satellite observations of ice surface elevation.
They then noticed a marked increase in ice flow speed between December 2005 and February 2007.
This coincided with rapid changes in ice surface elevation upstream, which they interpreted as the filling and draining of two subglacial lakes some two kilometers below the top of the ice.
In a normal year, Byrd Glacier would funnel around 20 billion tons of ice through a tight fjord towards the Ross Sea, with that ice stream moving at approximately 825m per year by the time it reaches the "grounding line", the point where it ceases to be a glacier and feeds into a floating ice shelf.
The glacier was seen to experience a 10 percent jump in speed when more than a cubic kilometer of water burst over the rims of these lakes and under Byrd.
The glacier dumped about 22 billion tons of ice a year into the Ross Sea between December 2005 and February 2007.
But Byrd began to return to its normal behavior once the floodwaters had dissipated under the glacier and out through the fjord.
"Previous studies had shown that a lot of resistance to the flow of Byrd Glacier was coming from sticky spots at the bed; that friction really does play a role in slowing down the glacier," Stearns said.
"The addition of a little bit more water probably flooded those bumps that were gripping the bed of the glacier; and once the water passed through, they stuck again."
Stearns's is the first research to show a direct link in Antarctica between the behavior of the lakes and the velocity of the ice moving overhead.
Science has witnessed a steady increase in the understanding of ice dynamics in the last decade. For example, it has been shown how polar glaciers speed up when the floating ice shelves that block their way to the ocean are removed.
Scientists suspect in Greenland that the melt waters that drain through holes, or moulins, in the ice cap to the bedrock may have contributed to the speed-up of glaciers in that region, too.
However, the events seen at Byrd are not of themselves climate-related. The lakes probably flood and drain on a regular basis that has nothing to do with atmospheric or ocean warming.
Scientists say the mechanisms involved need to be understood so the knowledge can be applied to those ice masses that are being exposed to warmer temperatures, such as in Greenland.
Dr. Helen Fricker from the Scripps Institution of Oceanography said these are processes they need to get right in the models so accurate predictions of sea-level rise are made over the next century.
"We discovered these active subglacial systems just in the last couple of years. Everybody was thinking this has got to make the ice flow faster, surely. This latest research now pins down the link."
Image Caption: Byrd Glacier, Antarctica from Landsat Byrd Glacier is a fifteen mile wide, one hundred mile long rock-floored ice stream located in southern Antarctica. This long, unique ice feature plunges through a deep valley in the Transatlantic Mountains and into the Ross Ice Shelf. This ice stream speeds as a river of ice at a rate of one half mile per year. Courtesy NASA/USGS
On the Net:
- Intergovernmental Panel on Climate Change
- Nature Geoscience
- Climate Change Institute, University of Maine