A vast lake containing 11.6billion gallons of meltwater has drained from the top of the Greenland ice sheet to its base in only 90 minutes, at a rate faster than that of Niagara Falls.

Observations of the event in July 2006 have provided fresh insights into the dynamics of Greenland's land ice, which would add 7 metres (23ft) to global sea levels if it were to melt completely under the impact of global warming.

The research reveals a “plumbing system” for the ice sheet, by which meltwater can force its way through up to a kilometre of ice to the underlying bedrock.

While the rapid drainage of the meltwater was dramatic, analysis has indicated that such events appear to have less impact on the stability of the ice sheet than some scientists had feared.

Every summer, meltwater accumulates on the surface of the Greenland ice sheet in huge lakes and some of this water sinks underneath the ice to reach the bedrock. This water provides a layer of lubrication, which has been suggested as an important factor in the accelerating advance of inland glaciers towards the sea.

The process has raised concern that global warming could trigger a feedback effect that could speed up the disintegration of Greenland's ice. As rising temperatures melt more surface water in summer, greater quantities are expected to flow to the bedrock. This could lubricate ice flow still further, increasing the rate at which glaciers discharge into the sea.

The new research, led by Ian Joughin, of the University of Washington, and Sarah Das, of the Woods Hole Oceanographic Institution, suggests that while this process is important, it will not increase ice flow as much as had been feared.

“The new findings indicate that while surface melt plays a substantial role in ice sheet dynamics, it may not produce large instabilities leading to sea level rise,” Dr Joughin said.

Richard Alley, Professor of Geosciences at Pennsylvania State University, commented on the research: “What Joughin, Das and their co-authors confirm is that iceflow speed-up with meltwater is a widespread occurrence, not restricted to the one site where previously observed.

“But, they also show that the really fast-moving ice doesn't speed up very much with this. So we can expect the ice sheet in a warming world to shrink somewhat faster than previously expected, but this mechanism will not cause greatly faster shrinkage.”

In the study, which is published in the journal Science, the researchers document the fate of a lake of meltwater that formed and disappeared in July 2006. It once covered 5.6sq km (2.2sq miles), and emptied from the bottom in 24 hours, with most of the water draining in just 90 minutes.

The scientists said that they were relieved not to have been on the lake in their 10ft boat at the time - and they now plan to deploy instruments only when surface lakes are dry.

“We found clear evidence that supraglacial lakes - the pools of meltwater that form on the surface in summer - can actually drive a crack through the ice sheet in a process called hydrofracture,” Dr Das said.

“If there is a crack or defect in the surface that is large enough, and a sufficient reservoir of water to keep that crack filled, it can create a conduit all the way down to the bed of the ice sheet.”

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