U.S. satellite observations show that Greenland's glaciers are dumping ice into the Atlantic Ocean at a quickening pace because they are moving faster. The scientists who have measured the changes attribute them to warming air and say they will cause sea levels to rise faster than predicted.
U.S. researchers say the movement of glaciers in the southern half of the Greenland ice sheet is accelerating, causing them to lose mass to the sea. Eric Rignot of the California Institute of Technology told a scientific meeting in St. Louis that the amount of ice being lost annually has more than doubled since 1996. The loss last year was 150 cubic kilometers.
"The mass loss resulting from this glacier acceleration in Greenland is very significant. These are very active glaciers. They all end up in the ocean, discharge icebergs and are very dynamic. One you push them a little bit out of equilibrium, they start retreating very fast," he said.
His calculations are based on satellite observations of the moving glaciers taken in 1996, 2000, and last year. He and a colleague combined the movement data with ice thickness estimates to calculate Greenland's annual ice loss over the last decade. Their new paper in the journal Science is the first to include glacier velocity as a factor in the loss.
They say that rising air temperatures appear to be triggering the glacier speedup. They believe the base of the glaciers are lubricated by melting caused by the warming.
Rignot says the glacier movement is responsible for two-thirds of the ice loss, and is therefore a much more important factor than once thought. What this means, he says, is that Greenland is contributing more to sea level rise, about half a millimeter per year, than current estimates show.
"You should view predictions of what it is going to do in the future as conservative because they did not include some of the processes that we are seeing. If I had to say something about the next 10 years, I would say it could be two to three times more than what has been predicted," he said.
Observations of South American glaciers show the same trend, according to Gino Casassa of the Center for Scientific Studies in Valdivia, Chile. He told the science meeting in St. Louis that the volume lost is smaller than in Greenland, but the melt still diminishes the amount of fresh water available while increasing glacier flooding and ice and rock falls.
"In Venezuela, for instance, there are practically no glaciers left. There is less than one square kilometer of glaciers. In the rest of South America, there is a similar situation. There are very few glaciers that are advancing in Patagonia and Tierra del Fuego basically, but the vast majority are retreating and thinning," he said.
Concerning Greenland, Eric Rignot says that if warming continues, the recent trend of faster moving glaciers in southern Greenland may reach those in the northwestern part.