Many glacial lakes atop the Greenland Ice Sheet disappear completely within hours when large cracks form below them, draining the lakes and sending torrents of water to the base of the ice sheet thousands of feet below. Now a group of researchers led by Laura Stevens of MIT/Woods Hole Oceanographic Institution Joint Program in Oceanography has found a surprising mechanism that triggers this process.
Thousands of supraglacial lakes form each spring and summer on top of the Greenland Ice Sheet. When they drain, they send torrents of water to the base of the ice sheet, lubricating the interface between rock and ice. That allows the ice sheet to flow faster to the ocean and discharge ice into ocean, which causes sea levels to rise faster. Image credit: Laura Stevens / Woods Hole Oceanographic Institution.
“Our discovery will help us predict more accurately how glacial lakes will affect ice sheet flow and sea level rise as the region warms in the future,” Stevens said.
To find out what triggers sudden lake drainages, the scientists deployed a network of 16 GPS units around North Lake, a 1.5-mile-long supraglacial lake in southwest Greenland.
They used these instruments to record movements of the ice before, during, and after three rapid lake drainages in the summers of 2011, 2012, and 2013.
Their study, published online in the journal Nature, showed that in the 6 to 12 hours before the lake cracked and drained, the ice around the lake moved upward and slipped horizontally.
“Meltwater had begun to drain through a nearby system of moulins (vertical conduits through the ice), which connected the surface to the base of the ice sheet 3,215 feet below. The accumulating water creates a bulge that floats the entire ice sheet, creating tension at the surface underneath the lake. The stress builds up until it is relieved by a sudden large crack in the ice below the lake,” the scientists explained.
“In some ways, ice behaves like Silly Putty – if you push up on it slowly, it will stretch; if you do it with enough force, it will crack,” Stevens added.
This illustration shows the part of the process where tensional stress builds up until it is relieved by a sudden large crack in the ice that extends below the lake. The huge volume of water in the lake surges into the opening, widening and extending it, and keeping it filled with water all the way to base of the ice sheet. Image credit: Jack Cook / Woods Hole Oceanographic Institution.
“Ordinarily, pressure at the ice sheet surface is directed into the lake basin, compressing the ice together. But, essentially, if you push up on the ice sheet and create a dome instead of a bowl, you get tension that stretches the ice surface apart. You change the stress state of the surface ice from compressional to tensional, which promotes crack formation.”
Once the tension initiates the crack, the volume of water in the lake does play a critical role, surging into the opening, widening and extending it, and keeping it filled with water all the way to base of the thick ice sheet.
These are called hydrofractures, and the team has documented how they can drain more than 11 billion gallons of water out of North Lake in about 90 minutes.
At times, water flowed out of the lake bottom faster than the water goes over Niagara Falls, they estimated.
“You need both conditions – tension to initiate the crack and the large volume of water to amplify it—for hydrofractures to form,” Stevens said.
The key finding of this study is that without the former, even large supraglacial lakes will retain their water.
At the base of the ice sheet, the water that drains from the lake lubricates the interface between ice and rock, allowing the ice sheet to slide faster toward the coast. That in turn accelerates the outflow of ice from land to sea and causes sea levels to rise faster.
So understanding the mechanisms that trigger the drainages will help scientists predict more precisely how supraglacial lakes will affect sea level rise as climate conditions shift in the future.
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Laura A. Stevens et al. 2015. Greenland supraglacial lake drainages triggered by hydrologically induced basal slip. Nature 522, 73-76; doi: 10.1038/nature14480
