´óÏó´«Ã½

Greenland's glaciers are not speeding up as much as previously thought, researchers have estimated.

As a result, the ice rivers may be contributing "significantly less" to sea-level rise than had been thought.

Previous studies had estimated that the nation's glaciers would double their flow by 2010 and continue to maintain that speed, they explained.

But the team, <link> <caption>writing in Science</caption> <url href="http://www.sciencemag.org/content/336/6081/576" platform="highweb"/> </link> , said the glaciers could eventually flow faster than earlier studies estimated.

The team of US researchers based their findings on data stretching back to 2000-2001, collected from more than 200 outlet glaciers.

"So far, on average, we are seeing about a 30% speed-up in 10 years," observed lead author Twila Moon from the University of Washington, Seattle.

This is less than earlier projections, one of which estimated that glacial flow would increase by 100% between 2000 and 2010 before stabilising at that new velocity.

<bold>Uncertain future</bold>

The volume of ice and meltwater from land being deposited in the sea has a direct impact on global sea level.

Glaciers are the main transportation mechanism that moves this material from the interior of land masses such as Greenland and Antarctica to the oceans.

The faster ice-river flow results in an increase in the volume of ice and meltwater ending up in seawater.

To produce the findings of the glaciers' velocity rates, the researchers used data collected by Canadian, German and Japanese satellites.

"There is the caveat that this 10-year time series is too short to really understand long-term behaviour," said co-author Ian Howat from Ohio State University.

"There still may be future events - tipping points - that could cause large increases in glacier speeds to continue.

"Perhaps some of the big glaciers in the north of Greenland that have not yet exhibited any changes may begin to speed up, which would greatly increase the rate of sea-level rise," Prof Howat suggested.

In contrast to earlier research that glacier flow would increase before maintaining a fixed velocity, the scientists said they found no indication that the glaciers would stop gaining speed during the rest of this century.

This meant that, ultimately, the glaciers could make a greater contribution to global sea-level rise than the earlier studies had projected.

Jeff Kargel, a glaciologist at the University of Arizona, said the team had "done much to extend [the] knowledge of Greenland glacier flow speed oscillations and trends".

But Dr Kargel, who was not involved in the study, added that the 10 years' worth of data was very brief.

"So short, in fact, it verges towards being the glaciological equivalent of weather," he observed.

"All said, I think this is valuable analysis, and it probably does mean that Greenland is undergoing a gentler acceleration of mass loss than hitherto considered likely."