London, July 5 : Climatologists have determined that nature has given "brakes" to the base of Greenland's ice sheet, which was earlier sliding into the oceans because of lubrication by water.
According to a report in New Scientist, Greenland's mighty ice sheet has enough water locked away to raise global sea level 6.5 metres were it to melt. Each summer, vast lakes of meltwater form on its surface.
The water seeps through cracks in the kilometer-thick ice to bedrock, where it acts as a lubricant. The sheet can move up to twice as fast in the summer, when meltwater is flowing, as when it is not.
Many fear a positive feedback loop, whereby the accelerating flow will bring more ice down out of the mountains and toward warmer temperatures near sea level.
Roderik Van De Waal and colleagues at Utrecht University in the Netherlands now say there is no evidence this will happen.
According to the research team, who has studied the longest available record of ice and water flow in the region, since 1991, the western edge of Greenland's ice sheet has actually slowed its ocean-bound progress by 10%.
They looked at how meltwater has correlated with the speed of ice flow at the western edge of the sheet, just north of the Arctic Circle, since 1991.
They found that meltwater pouring down holes in the ice - called "moulins" - did indeed cause ice velocities to skyrocket, from their typical 100m per year to up to 400m per year, within days or weeks.
But the acceleration was short-lived, and ice velocities usually returned to normal within a week after the waters began draining.
Over the course of the 17 years, the flow of the ice sheet actually decreased slightly, in some parts by as much as 10%.
"For some time, glaciologists believed that more meltwater equaled higher ice speeds," said Van de Waal. "This would be kind of disastrous, but apparently it is not happening," he added.
According to Van de Waal, the channels that carry the meltwater out to sea freeze up during the winter months.
In summer, pulses of water rushing down the moulins to the bedrock overwhelm the narrowed channels, and the increased pressure lifts the ice sheet off the rock, enabling it to move faster.
However, after a few days the channels are forced open by the water, and it drains away from the glacier. As a result, the ice grinds back down against the bedrock and the lubricant effect is lost.
Van De Waal said that this indicates that, overall, meltwater has a negligible effect on the rate at which the ice sheet moves.