Blue Collar Scientist

A couple years ago, a large lake on the Greenland ice sheet ceased to be. The lake started out 2.2 square miles in size, and contained 4.391 × 10¹⁰ liters (11.6 billion gallons) of water, but in less than two hours, all the water had drained away.

What happened? A study published in Science has the answers, and although they aren’t surprising, they are pretty interesting.

The lake created a crack in the ice, and the water drained to the base of the ice sheet, more than a kilometer (0.6 miles or so) below. According to the story on LiveScience, the water drained faster than Niagara Falls.

Water rushes down a glacial moulin. Courtesy Science.

Once the water got down there, it spread out and lubricated the ice sheet, which started to move faster as a result. The investigators claim this is the first time that’s been observed to happen. I’m not sure if that is really so, but if it is, it is a good confirmation of an old and widely-accepted idea. This explanation from one of the co-investigators is one I’ve heard before:

“To influence flow, you have to change the conditions underneath the ice sheet, because what’s going on beneath the ice dictates how quickly the ice is flowing,” [Sarah] Das explained. “If the ice sheet is frozen to the bedrock or has very little water available, then it will flow much more slowly than if it has a lubricating and pressurized layer of water underneath to reduce friction.”

By the same token, you’re more likely to slip on a wet floor than a dry one.

Local easily-accessed glaciers, like the Knik, Matanuska, Portage, and Exit, as well as the Sargent and Harding Icefields (all within a couple hours drive of my house), understandably attract a lot of naturalist interpretive activity, and I’ve heard discussions similar to the above about all of these masses of ice. And I’ve witnessed part of the process described by the study authors.

Cracks or chutes in glacial ice - called moulins - are really weird to watch. Standing on a glacier in the rain, and watching the drainage from several acres or perhaps a square mile or two of glacial surface disappear down a moulin really makes you wonder what’s below you. What’s going on down there? How long it will take a seemingly narrow drain to fill up and start flooding the glacial surface? I’ve never had the patience to wait things out and see, and that’s partially because I’ve got a pretty good idea that the wait would be infinite¹. When you go to the toe of a glacier, there’s an awful lot of water coming out from underneath, and it has to come from somewhere.

The paper in Science deals not only with this lake and its effect on the ice sheet, but also how this event has influenced the rate of flow of the outlet glaciers - the tongue-like glaciers that deposit ice in the sea. While the lubrication of the ice sheet doubled its speed of flow, it had only a few percent effect on the speed of the outlet glaciers, whose flow is more regulated by the rate at which they calve.

More details are available from the Woods Hole press release, the University of Washington press release, and the National Science Foundation press release.

1. It is also true that, while standing in the rain on a glacier isn’t at all unpleasant if you are properly equipped, it still isn’t the most comfortable place to be. [↩]