by Jonathan Fildes
Science and technology reporter, BBC News, San Francisco

from BBCNews Website

Satellite images reveal the changes in volume of water

Scientists "stacked" multiple images of on top of each other and then

subtracted some to show up changes in the surface slope in different years.

The appearance of a footprint-shaped depression

clearly reveals the changes in the volume of water under the ice.

Giant "blisters" containing water that rapidly expand and contract have been mapped beneath the Antarctic ice sheet. Fed by a complex network of rivers, the subglacial reservoirs force the overlying ice to rise and fall. By tracking these changes with NASA's Ice Cloud and land Elevation Satellite (ICESat) scientists were able to map the extent of the subglacial plumbing.

The results, published in the journal Science, show that some areas fell by up to 9m (30ft) over just two years.

"We didn't realize that the water under these ice streams was moving in such large quantities, and on such short time scales," said Dr Helen Fricker of the Scripps Institution of Oceanography, University of California, and one of the authors of the paper.

"We thought these changes took place over years and decades, but we are seeing large changes over months."

The results are important for understanding how the Antarctic Ice sheet, which contains nearly 90% of the world's ice, may respond to global warming and how much it may contribute to sea level rise.

Climate response

Nearly 150 subglacial lakes have been identified beneath the vast Antarctic ice sheet. The new ones were found under the fast-flowing Whillans and Mercer Ice Streams that carry ice from the interior of the West Antarctic Ice Sheet to the floating Ross Ice Shelf.

These streams of ice move many meters every day and are of particular interest to climate scientists.

"It's the fast-moving ice that determines how the ice sheet responds to climate change on a short timescale," said Professor Robert Bindschadler of NASA's Goddard Space Flight Center in Maryland, and one of the researchers on the study.

As atmospheric temperatures rise, melting the ice shelves, their ability to hold back the ice streams on land would be reduced.

"We aren't yet able to predict what these ice streams are going to do."

However, understanding how much water flows beneath the ice is critical because it is one of the factors that determine how fast they flow. More water could speed up the flow of ice into the sea, raising sea levels.

"It's essentially the grease on the wheel," said Professor Bindschadler.

Ebb and flow

Using elevation data from NASA's ICESat, cross-checked with other NASA satellites, the team was able to map the rise and fall of the overlying ice, and hence areas where water pooled or flowed away.

Launched in 2003, ICESat can measure changes in elevation as small as 1.5cm (0.6ins) from its orbit 645km (400 miles) above the Earth. The study revealed a complex network of ponds and rivers, the largest of which occurred under the Whillans ice stream and covered an area of 500 sq km (190 sq miles). It also showed that water was constantly moving between different reservoirs.

For example, a feature known as Lake Englehardt took just under three years to empty two cubic kilometers (2 trillion liters) of water. In the same period, Lake Conway filled with an additional 1.2 cubic kilometers (1.2 trillion liters) of water. Not all of the water was the same as some escaped to the ocean or was refrozen on to the base of the glacier.

Observations like these were only possible using the new satellite technique.

"Until now, we've had just a few glimpses into what's going on down there," said Professor Bindschadler. "This is the most complete picture to date what's going on beneath fast flowing ice."

The findings were presented at the American Association for the Advancement of Science (AAAS) annual meeting here in San Francisco, US.