Getting to the Bottom of the Greenland Ice Sheet

16 March 2009 | 05:17 Code : 18878 Geoscience events
the world’s largest island—is also home to one of the world’s largest ice sheets...

The world’s largest island—is also home to one of the world’s largest ice sheets (after Antarctica). If Greenland’s two-mile-thick ice sheet melts completely, it would ultimately raise global sea level by 23 feet, drowning significant portions of coastal regions under water.It would also add large quantities of fresh water to the ocean, with the potential to change ocean circulation patterns that could, in turn, affect rainfall patterns, fisheries, and climateIn July 2008, a team including scientists Sarah Das and Mark Behn from Woods Hole Oceanographic Institution (WHOI) and Ian Joughin and graduate student Kristin Poinar from the University of Washington, along with photographer Chris Linder and writer Amy Nevala from WHOI, returned to Greenland for a third summer to learn how the ice sheet is flowing faster toward the coast and discharging more ice to the seaThey investigated glacial lakes, which form atop the ice sheet each spring and summer as returning sunlight melts ice and snow. They have found that as lakes grow, large cracks can open suddenly in the lakes’ bottoms, allowing water to drain in a dramatic waterfall more than a half-mile down to the bedrock beneath the ice sheet. The water lubricates the base of the glacier, like grease on a railroad track, allowing glaciers to flow faster. As global temperatures rise, more lakes and cracks may form, accelerating the flow of ice to the ocean.The life cycle of Greenland’s lakes“It’s hard to envision how a trickle or a pool of meltwater on the ice sheet’s surface could eventually lead to a process that cuts through a thick, cold ice sheet all the way to the bottom,” Das said. But it’s easy to see why such a large-scale phenomenon could remain unknown in the 21st century. Like the proverbial tree falling in the forest, there had never been anyone there to see it.During two field seasons on the Greenland Ice Sheet in 2006 and 2007, however, the research team set up and left behind instruments near two of the thousands of lakes that form atop the ice sheet each summer, to record what happens while they were not there. They used Global Positioning System receivers and seismometers to measure ice movement, water pressure loggers to detect changes in lake levels, and meteorological sensors to precisely measure surface melting.Their observations, published May 9, 2008, in the journal Science, confirmed a long speculated, but never detected plumbing system for ice sheets.The scientists proved that water filling large glacial lakes can build up enough pressure to crack their bottoms, creating conduits that penetrate the ice sheet and could send torrents of water all the way to the bedrock. The water acts as lubrication between ice and ground, speeding up the glacier’s flow toward the ocean. Evidence collected from their instruments showed that cracks, reaching the bedrock, had suddenly drained the lakes. But, still, no one had ever actually witnessed such an eventNoisy birthday giftTwo months after their journal article appeared, Das and her colleagues headed back to their ice camp in Greenland for a third summer. A week before her 35th birthday, Das received a glaciologist’s perfect gift. That day, fog had forced the researchers to work close to their camp near a glacial lake. Suddenly, cracks and booms echoed in the distance. The team was stunned, and a little fearful.“You got the sense that the ice sheet was crumbling under your feet,” Das said. “You looked at each other, and looked at your camp, and hoped that a large, gaping hole didn’t open up underneath you.”After a couple of hours, all the noise stopped. “Everything seemed to go quiet again,” Das said. “So we decided, ‘Well, let’s go for a hike.’ ”The lake was drained of water. Soon they found the source of the noise and the drainage: A monstrous crack more than two miles in length had split the center of the lakebed from edge to edge. The former lakebed was littered with huge blocks of ice that had broken off, and tumbled and thundered, as the billions of gallons of water that had filled the lake poured through the crack and down into the ice sheet.

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