Mysteries of Greenland's ice

Ice scientists are changing their thinking about the melting of the Greenland Ice Sheet, the two-mile deep block of ice that holds enough water to fill the Gulf of Mexico and put coastal regions awash in rising seas.

Watching surface ice melt in rising temperatures, researchers a few years ago surmised that the meltwater pouring through crevices in the ice sheet reached the base of the ice sheet, lubricating its movement over bedrock and causing outlet glaciers to flow more quickly into the sea.  Now the picture looks more complicated, they say, and less predictable.

"We've come to realize that sub glacial melt water is not responsible for the big acceleration that we've seen in the last ten years," Ian Howat, an Ohio State University researcher, told the annual meeting of the American Geophysical Union in San Francisco.  "Changes in the glacial fronts, where the ice meets the ocean, are the real key."

This image, courtesy of researcher Alberto Behar, at NASA's Jet Propulsion Laboratory, shows mist rising above the collapse of ice from the face of a west Greenland outlet glacier.

Meltwater increases through the summer, sending torrents thundering through pathways through the ice and slowing as the season shifts to winter, but the researchers found that the outlet glaciers in their study area of western Greenland do not exactly track this pattern.

To their surprise, they found that the outlet glaciers actually slowed their flow to the sea in the middle of summer, a time of maximum ice melt when they might have been expected to be flowing fastest.  Instead of responding directly to the meltwater flow, something else was at work.

The meltwater evidently overwhelms the system of tunnels through and beneath the ice sheet, causing them to collapse, and the glacier comes skidding to a stop.  "This was the equivalent of the pipes bursting on all that plumbing beneath the ice, releasing the pressure," said Howat.

So increasing melting does not automatically translate into faster glacier speed.  "The relationship is much more complex than that," he said, "and since the plumbing system evolves over time, it's especially hard to pin down."

What they do know, with more confidence now, is that Greenland ice sheet is losing mass much faster than it was 10 years ago, Howat said.  "Now we can confidently say that the Greenland Ice Sheet is losing mass at a rate of somewhere between 100 and 200 gigatons per year, a rate of one-half millimeter per year in sea level rise."