Floods under Antarctic ice speed glaciers into sea: study

Yahoo News 17 Nov 08;

PARIS (AFP) – Scientists unveiled Sunday the first direct evidence that massive floods deep below Antarctica's ice cover are accelerating the flow of glaciers into the sea.

How quickly these huge bodies of ice slide off the Antarctic and Greenland land masses into the ocean help determine the speed at which sea levels rise.

The stakes are enormous: an increase measured in tens of centimetres (inches) could wreak havoc for hundreds of millions of people living in low-lying deltas and island nations around the world.

Researchers discovered only recently that inaccessible subglacial lakes in Antarctica periodically shed huge quantities of water.

Data collected by a satellite launched in 2003 -- the Ice, Cloud and Land Elevation Satellite, or ICESat -- revealed a complex network of subglacial plumbing in which water periodically cascades from one hidden reservoir to another.

But the new study, published online in the journal Nature Geoscience, is the first to measure the potential impact of this invisible flooding on sea-bound glaciers.

A trio of scientists led by Leigh Stearns of the Climate Change Institute at the University of Maine matched ICESat data against a nearly 50-year record of how fast the Byrd Glacier in East Antarctica has moved toward the sea.

They discovered that during the same 14-month period that 1.7 cubic kilometres (0.4 cubic miles) of water cascaded through subglacial waterways, the 75-kilometre (45-mile) long glacier downstream pick up speed, moving about 10 percent faster.

"Our findings provide direct evidence that an active lake drainage system can cause large and rapid changes in glacier dynamics," the researchers concluded.

"Water acts as a lubricant, reducing friction at the base of the ice and making ice flow faster," explained Helen Fricker of the Scripps Institution of Oceanography of California in a commentary, also in Nature Geoscience.

"The timing of the onset of speed up matched that of the lake drainage, and the slow-down coincided with the flood cessation," she noted.

The study adds to growing scientific concern about the pace at which glaciers are melting into the seas.

Two forces -- both driven by global warming -- cause sea levels to rise. One is thermal expansion of sea water.

The UN Intergovernmental Panel on Climate Change (IPCC) warned last year that thermal expansion will push sea levels up 18 to 59 centimetres (7.2 to 23.2 inches) by 2100, enough to wipe out several small island nations and severely disrupt low-lying mega deltas in Asia and Africa.

But the report failed to take into account the impact of the second force: additional water from melting sources of ice.

The ice sheet that sits atop Greenland, for example, contains enough water to raise world ocean levels by seven metres (23 feet).

Even the gloomiest global warming predictions do not include such a scenario.

But recent studies suggest that runoff from the Antarctic and Greenland ice sheets could drive sea levels higher than once thought, one reason the IPCC decided to remove the upward bracket from its forecast.

Under-ice flood speeds up glacier
Jonathan Amos, BBC News 17 Nov 08;

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.

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.

Ice acceleration

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.

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.

Friction release

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."