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Disappearing Lakes

Summertime, and the living is, while not exactly easy in Greenland it certainly is warmer. In fact it’s warmer than ever as global warming increasingly causes glaciers to move more quickly to the sea, and for more and more lakes and streams to form on top of the glaciers.

Summertime is also when scientists are able to take advantage of the break in the weather to study the mysteries of why the glaciers are speeding up and why lakes, some as big as several miles long, sometimes disappear in mere minutes. They wondered if one might have to do with the other.

Greenland’s glaciers have ten percent of the world’s ice. Their melting due to global warming could have a significant impact on sea levels. For scientists to understand what might happen in the future, they need to figure out the mechanics of what is speeding up the glaciers. With some of Greenland’s glaciers, the speed has nearly doubled.

Ian Joughin, a glaciologist at the University of Washington, says there were two questions: “How much can the melt speed up the flow of ice? Which would, of course, carry the ice more quickly to the ocean…And two, how does that water get to the bed?”

He and Sarah Das of the Woods Hole Oceanographic Institute are working together to research Greenland’s glaciers. Scientists have theorized that the lakes find cracks in the ice, expand those cracks, and disappear down them to the bedrock beneath the ice. Joughin says, “The basic idea is that…the water in the summer getting down there lubricating the flow would speed up the glacier and move the ice toward the ocean more quickly.”

The researchers place pressure sensors at the bottom of these lakes. As the water drops, the pressure also drops as the weight of the water goes away. They also have very precise global positioning sensors on the ice, measuring both forward movement and any up and down changes.

Researchers already knew some lakes had disappeared relatively quickly. Satellite photos would show a lake but the next time the satellite passed, about 24 days later, the lake would be gone. By using the sensors Joughin’s and Das’ team would learn more precisely how quickly a lake could disappear and what happened to the surrounding ice. The equipment would record what happened until the researchers retrieved the information a year later.

What they found, Joughin says was “quite a surprise.” He explains that one lake that was 2.5 miles long and up to 40 feet deep “basically drained in the space of about 90 minutes.” He says, “In the middle of this lake a big crack opened about two miles long through half a kilometer (three-tenths of a mile) of ice, cracked open (and) started draining the water.” The rate at which the lake drained averaged 8,700 cubic meters per second, which exceeds the average flow of water over Niagara Falls.

The water tunnels, which are called moulins, carried the water under the glacier to bedrock. The global positioning devices recorded what happened next. Joughin says, “Not only did it (the GPS device) measure some speed up of the ice, but it also measured that the surface of the ice sheet had lifted up by about three feet.”

So one question had been answered. Joughin explains they demonstrated that “the surface of the ice sheet is very well connected to the bed of the ice sheet by this sort of fracturing process.”

But the change wasn’t enough. The ice sheet had moved forward about half a meter (19-20 inches). While that represents a 50 to 100 percent speed up for the main ice sheet, it was not so big when compared with the speeds of what scientists call the outlet glaciers. A 20-inch jump is less than a 15 percent speed increase for these glaciers, and outlet glaciers are what Joughin calls, “the workhorses that really discharge all that ice to the ocean.”

Joughin says although the glaciers have doubled their speed, “it’s not because of this surface melt lubrication effect.”

Joughin and other researchers will be back this summer, again retrieving information from their instruments, and looking for new clues to the mechanics of why Greenland’s glaciers are moving faster than ever.

This research was published in the May 9, 2008 issue of the Journal Science, and was funded by the National Science Foundation.

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1. Faster Sea Level Rise | ScienCentral | Science Videos | Science News says:
   

   September 30th, 2008 at 11:46 am (#)

   [...] After studying the last great North American ice sheet, the Laurentide, which encompassed large parts of Canada and the United States, researchers at the University of Wisconsin-Madison calculated unsettling numbers about the melting rate of today’s largest Arctic ice mass, the Greenland ice sheet. [...]

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