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.

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