by Tom Yulsman, CE Journal, October 23, 2010

The Jakobshavn Glacier in Greenland already flows so fast you can stand on a ridge above it and watch it move. In fact, it’s the fastest moving glacier on Earth, flowing from the land to the sea at more than 14 kilometers per year.

But we may not have seen anything yet. The map above shows why it could be primed to move even faster, and thereby dump even more ice into the sea.

And if the same process plays out with glaciers farther north — and there is growing evidence that it may do just that (see the map and discussion lower in this post) — there would be significant implications for global sea level.

The image above is a kind of topographic map showing the elevation of the bedrock below and around the Jakobshavn Glacier — as if Greenland’s overlying ice had been removed. Warm colors show areas of the bedrock that are above sea level. The green and blue colors show areas where it is below sea level. The fjord through which Jakobshavn flows out to sea on Greenland’s western coast is visible as the green and blue trough at the left side of the image. As is evident from the map, the fjord actually continues inland for another 50 miles, and to a depth of 1.6 km below sea level. Why is this significant?

The calving front of the glacier marked on the map shows the current edge of the glacier. This is where icebergs calve from Jakobshaven and float out to sea. Since 2001, the calving front has retreated about 6 miles (10 kilometers). During July 6 and 7 this past summer, it pulled back an entire mile. Now, the calving front is poised right at the western edge of the fjord’s continuation into the ice sheet.

“If the calving front retreats from where it is now, it can draw out a lot of ice,” says Konrad Steffen, director of the Cooperative Institute for Research in Environmental Sciences at the University of Colorado.

The effect will be like pulling the plug from the drain of a bathtub. It will mean that much more ice will be able to drain from the Greenland Ice Sheet through Jakobshavn into the sea.

“You can actually pull out a lot of ice with a fjord below sea level like this,” Steffen says. “And there are about four or five others like this.”

The image at left is from a presentation Steffen gave last Thursday at the Center for Environmental Journalism. Like the image at the top of this post, it is a topographic map of bedrock. This is the topography of Greenland with all the ice removed.

The blue areas on the map show regions where the enormous weight of the ice sheet — which is almost 2 miles thick at its maximum — pushes the bedrock below sea level. And the numbers mark the major fjords through which glaciers drain into the sea.

Because the ice must pass through such narrow outlets, these fjords are the bathtub plugs. But if the calving fronts of the glaciers were to retreat far enough into the interior, the plugs would be removed, and the interior of the ice sheet could then drain much more quickly.

Jakobshavn Glacier on the southwestern coast is marked number 4. And as the map shows, there are several more farther north. What’s happening to them?

The graphic below addresses this question. It is based on data collected by NASA’s Grace satellites. (The satellites, which fly in formation together and use gravity measurements to detect changes in mass below them, are seen in the illustration to the right.)

In the map on the left, pink and blue colors indicate where Greenland was losing ice mass between 2003 and 2007. Most of the loss was concentrated on the southeast coast. As the map to the right indicates, by this past March loss of ice had extended all the way up to the northwestern coast — where at least two of the big fjords of concern are located. One of these is the Petermann Glacier (marked number 2 on the bedrock topography map).

In early August, an ice island four times the size of Manhattan Island calved from the front of the Petermann glacier. With this, the glacier lost about one-quarter of its 43-mile long floating ice-shelf. “The freshwater stored in this ice island could keep the Delaware or Hudson rivers flowing for more than two years. It could also keep all U.S. public tap water flowing for 120 days,” says Andreas Muenchow of the University of Delaware’s College of Earth,Ocean and Environment (quoted in a press release).

Petermann and the other more northerly glaciers “go farther inland, are much broader, and go much lower below sea level than Jakobshavn,” Steffen says. (In fact, whereas Jakobshavn is about 7.5 miles across, Petermann is about 60 miles wide.) Right now, these larger rivers of ice are not moving as quickly as Jakobshavn. But if their ice fronts were to retreat more rapidly, and the glaciers were to speed up enough, they could “drain most of northern Greenland in decades or a century.”

And that could have a very significant impact on global sea level. How significant? For insight into that question, please see my previous post, which was also based on Steffen’s presentation to our group.

Link:  http://www.cejournal.net/?p=4225#more-4225