By Jonathan Amos
Science reporter, BBC News
The Byrd Glacier is about 135km long and 24km wide
Great floods beneath the Antarctic ice sheet can now be linked directly to the speed at which that ice moves towards the ocean, scientists say.
Leigh Stearns and colleagues have been able show how the giant Byrd Glacier in east Antarctica sped up just as two lakes under the ice overflowed.
The flood water acts as a lubricant, easing the ice over the bedrock.
The observation is described as critical because of how it informs our understanding of future sea levels.
The more ice the polar regions dump in the ocean, the higher the waters will rise.
But when world scientists released their state of the climate assessment in 2007 (Intergovernmental Panel on Climate Change), they said ice behaviour in response to a warming Earth was one of the great uncertainties in projecting future ocean rise.
The work of Dr Stearns and colleagues, reported in Nature Geoscience, indicates that Antarctica's under-ice "plumbing system" must now be an important consideration in "ice dynamics".
"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," said Dr Stearns from the Climate Change Institute at the University of Maine, US.
"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 told BBC News.
BYRD GLACIER SPEEDS UP AS SUBGLACIAL LAKES FLOOD
(1) The upstream lake (UL) begins to flood, sending water cascading downhill
(2) The downstream lake (DL) fills and floods, accelerating Byrd Glacier
(3) After 1.7 cu km of water has gone through the fjord, Byrd slows down
(4) The event saw a 10% speed up in the major outlet glacier
Scientists have known about subglacial lakes in Antarctica for half a century. There are more than 150; and the biggest, Lake Vostok, is the size of Lake Ontario in North America.
Despite being capped by, in some cases, several kilometres of ice, the lakes' contents stay liquid because of warm spots in the underlying rock.
It was always thought, however, that these lakes were stagnant bodies, containing waters that were perhaps unaltered for millions of years.
Only in 2005 did scientists discover that the lakes' levels could actually change rapidly - they can fill and burst their rims under the ice sheet.
When they fill and flood, they actually lift the ice up by several metres - something which can be seen by overflying satellites that measure the height of the ice.
Dr Stearns - working with Ben Smith and Gordon Hamilton - took a 48-year record of ice speeds recorded along Byrd Glacier and compared the data with satellite observations of ice surface elevation.
The group found a marked increase in ice flow speed between December 2005 and February 2007.
This coincided with rapid changes in ice surface elevation about 200km upstream, which the team interprets as the filling and draining of two subglacial lakes some two kilometres below the top of the ice.
Nasa's Icesat spacecraft tracks changes in the height of the ice sheet
The numbers involved are remarkable. In a normal year, Byrd Glacier would funnel something on the order of 20 billion tonnes 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.
When more than a cubic kilometre of water burst over the rims of these lakes and under Byrd, the glacier was seen to experience a jump in speed of 10%.
Between December 2005 and February 2007, the glacier dumped about 22 billion tonnes of ice a year into the Ross Sea.
Once the flood waters had dissipated under the glacier and out through the fjord, Byrd was seen to return to its normal behaviour.
"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," explained Dr Stearns.
"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."
The research is the first to show a direct link in Antarctica between the behaviour of the lakes and the velocity of the ice moving overhead.
The past decade has seen a steady increase in the understanding of ice dynamics. 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.
And in Greenland, scientists suspect 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.
Cause and effect
It should be stressed 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.
However, the scientists say the mechanisms involved need to be understood so the knowledge can be applied to those ice masses which are being exposed to warmer temperatures, such as in Greenland.
"These are all processes we need to get right in the models so we can make accurate predictions of sea-level rise over the next century," commented Dr Helen Fricker from the Scripps Institution of Oceanography, US.
"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."