Greenland Meltdown? Not Necessarily
Discover.com, Jan. 12, 2009
As the world warms, Greenland's dwindling glaciers may actually slow in their retreat, according to new research.
Greenland is covered in an ice sheet big enough to raise the planet's sea level by 6.6 meters (21.5 feet) were it to ever melt. Even as the world warms, though, most of the ice remains safe in the island's cold interior, for now.
But since the year 2000 scientists have watched with alarm as its edges -- huge glaciers that stream down out of the mountains and onto the surface of deep fjords have been sliding into the ocean at ever-increasing rates.
Now new research by Andreas Vieli of Durham University in the United Kingdom and a team of scientists suggests a strange paradox: As the planet heats up through the middle of the 21st century, many of the glaciers may break up more slowly.
The team's conclusion stems from running computer simulations of Helheim glacier, a massive tongue of ice spilling onto the sea in East Greenland.
Between 2001 and 2005 the glacier went from losing 28 cubic kilometers of ice per year, which is considered stable, to approximately 40 cubic kilometers per year.
Most of the loss was due to warming ocean waters, which heat the floating parts of glaciers from below. As the floating ice melts it breaks up faster, or calves off the glacier, allowing more ice to slide down in its place.
"The floating tongues have a kind of buttressing effect on the rest of the glacier," Vieli said. "If you increase how fast you remove the floating tongue you remove the buttressing, and the glacier's sliding will increase, too."
Across Greenland the story appears the same -- glaciers whose tongues terminate in fjords speed up rapidly from just small increases in ocean temperature.
But once the floating tongue disintegrates, the model showed, it is no longer subject to warm waters, and it's a whole new ball game.
The ice will retreat up out of the fjord's deep bed and onto dry land.
"Surface melting could become the dominant process then," Vieli said, due to warming air temperatures, which can't break up the ice as quickly. The glacier's frozen bed may also become wet and lubricated, which could hasten sliding somewhat, but not to the same extent as calving in the fjord.
"The retreat can continue up the deep bed, but probably not much beyond," Richard Alley of Pennsylvania State University said. "After that, surface melting and surface melting thawing the bed to speed sliding are probably the two big issues, and neither is likely to be a run-for-the-hill-fast issue."
Glaciers' recession onto land could temper Greenland's contribution to sea level rise. But both Alley and Vieli noted that the situation is much different in the ice sheets of Antarctica. There, the continent's massive ice streams stay below sea level for hundreds of miles, so as they retreat, there will be no escaping the warming ocean's corrosive influence.