Greenland's many supraglacial lakes have been seen suddenly and mysteriously draining as climate change causes this region to warm, and now scientists have finally explained the mechanism behind this phenomenon. 
 
 The findings were published in the journal Nature. 
 
 The Greenland Ice Sheet, the second largest ice sheet in the world, has long been of concern to scientists as rising temperatures continue to melt its glaciers. Consequently, this meltwater forms thousands of lakes atop the ice sheet each spring and summer. And yet, these vast bodies of water can empty in a matter of just a few hours. 
 
 When they drain, they send torrents of water to the base of the ice sheet thousands of feet below, 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. 
 
 So what exactly is causing this sudden drainage? 
 
 In 2008, scientists from Woods Hole Oceanographic Institution (WHOI) and the University of Washington documented for the first time how the icy bottoms of lakes atop the Greenland Ice Sheet can crack open suddenly. Initially, they thought the sheer weight of water was putting too much pressure on the ice to the point that it cracked. However, this theory did not explain why some lake bottoms cracked while others did not. 
 
 "Our discovery will help us predict more accurately how supraglacial lakes will affect ice sheet flow and sea level rise as the region warms in the future," lead author Laura Stevens from WHOI said in a press release. 
 
 Stevens and her colleagues deployed a network of 16 GPS units around North Lake, a 1.5-mile-long supraglacial lake in southwest Greenland. This lake was among those seen suddenly cracking and draining. They recorded ice movement before, during and after three rapid lake drainages in summer 2011-2013. 
 
 It turns out that tension from below the ice sheet, and not above it from the lakes, was causing cracks in the ice. (Scroll to read on...) 
 
 According to the data, in the 6 to 12 hours before the lake cracked and drained, the ice around the lake moved upward and slipped horizontally. The scientists say that meltwater had begun to drain through a nearby system of moulins - vertical channels 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. 
 
 "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 explained. "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." 
 
 Though once this tension initiates the crack, the volume of water in the lake does play a role. Lakewater pours into the crack and widens it even more, keeping it filled with water all the way to base of the thick ice sheet and forming what are called hydrofractures. 
 
 "You need both conditions - tension to initiate the crack and the large volume of water to amplify it - for hydrofractures to form," Stevens said. Meaning, the weight from meltwater alone is not enough to cause supraglacial lakes to suddenly drain. 
 
 This entire underlying mechanism is so impactful, it can reportedly drain more than 11 billion gallons of water out of North Lake in about 90 minutes. That's even faster than the water goes over Niagara Falls. 
 
 By better understanding these sudden drainages scientists hope to determine how supraglacial lakes will affect sea level rise as climate conditions shift in the future. 
 
 For more great nature science stories and general news, please visit our sister site, Headlines and Global News (HNGN). ||||| In 2008 scientists from Woods Hole Oceanographic Institution (WHOI) and the University of Washington documented for the first time how the icy bottoms of lakes atop the Greenland Ice Sheet can crack open suddenly--draining the lakes completely within hours and sending torrents of water to the base of the ice sheet thousands of feet below. Now they have found a surprising mechanism that triggers the cracks. 
 
 Scientists had theorized that the sheer weight of the water in these supraglacial lakes applied pressure that eventually cracked the ice, but they could not explain why some lake bottoms cracked while others did not. 
 
 "Our discovery will help us predict more accurately how supraglacial lakes will affect ice sheet flow and sea level rise as the region warms in the future," said lead author Laura Stevens, a graduate student in the Massachusetts Institute of Technology-Woods Hole Oceanographic Institution (MIT/WHOI) Joint Program in Oceanography. 
 
 The research was published June 4 in the journal Nature. 
 
 To find out what triggers sudden lake drainages, a research team, including Stevens and colleagues from WHOI, the University of Washington, MIT, and the University of Tasmania, deployed a network of 16 GPS units around North Lake, a 1.5-mile-long supraglacial lake in southwest Greenland, where the scientists first documented large-scale cracks and lake drainages. 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 showed that in the 6 to 12 hours before the lake cracked and drained, the ice around the lake moved upward and slipped horizontally. The scientists say that 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. 
 
 "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," said Stevens. "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 scientists have 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, the scientists 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. 
 
 The GPS network also recorded the more sudden and momentous movements of the ice sheet surface at the time of the hydrofracture, showing that portions of the ice sheet bed beneath the lake can slip up to a foot and a half. That is equivalent to the movement caused by a magnitude-5.5 earthquake. 
 
 "It's just a different type of solid crystals--ice instead of rock--breaking due to stress," said Jeff McGuire, a co-author on the study and a seismologist at WHOI. The research team spanned scientific disciplines, including McGuire, a seismologist; Mark Behn, a geophysicist at WHOI who studies faults in Earth's crust; glaciologists Sarah Das of WHOI, and Ian Joughin and David Shean of the University of Washington; Tom Herring, a GPS expert at MIT, and Matt King, who studies geodesy, Antarctic ice sheets, and sea level at the University of Tasmania. 
 
 Thousands of supraglacial lakes form each spring and summer on top of the Greenland Ice Sheet as sunlight returns to the region. The heat melts snow and ice into water that pools in depressions in the ice sheet to form lakes. As the region becomes warmer, more lakes will likely form, leading to a first-order prediction of more hydrofractures, more lubrication, more ice sheet slippage, and faster-rising sea levels. 
 
 However, discovering the new trigger mechanism changes the equation, because the trigger is less likely to occur at lakes at higher elevations on the ice sheet--even though water volumes in those lakes can be large. Stevens explained that the ice sheet further inland is thicker and moves more slowly. The ice deeper down flows viscously, dampening impacts on the surface topography. That results in a flatter surface where fewer lake basins and crevasses form. Fewer crevasses mean less water leakage to the base, which reduces bulging that increases surface stresses. 
 
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 The Woods Hole Oceanographic Institution is a private, non-profit organization on Cape Cod, Mass., dedicated to marine research, engineering, and higher education. Established in 1930 on a recommendation from the National Academy of Sciences, its primary mission is to understand the ocean and its interaction with the Earth as a whole, and to communicate a basic understanding of the ocean's role in the changing global environment. For more information, please visit http://www. whoi. edu . 
 
 Funding for this research was provided by the National Science Foundation's Office of Polar Programs and National Aeronautics and Space Administration's Cryospheric Sciences Program ||||| Geoscientists have solved a decade-long mystery of how some of the large lakes that sit atop the Greenland ice sheet can completely drain billions of gallons of water in a matter of hours. 
 
 In 2006, Greenland's North Lake, a 2.2 square-mile (5.6 square kilometers) supraglacial meltwater lake, drained almost 12 billion gallons of water in less than two hours. In a study published two years later, researchers determined that this astonishing phenomenon is possible because giant hydro-fractures(water-driven cracks) can form directly beneath the lake basin and stretch down to the bed of the ice sheet, emptying the lake of water. But just how these fractures developed has been unknown — until now. 
 
 In the new study, published today (June 3) in the journal Nature, scientists using GPS technology discovered that the hydro-fractures form from tension-related stress caused by movements of the ice sheet. These movements are, in turn, triggered by the trickling meltwater. [See Gorgeous Images of Greenland's Supraglacial Lakes] 
 
 The new research may help scientists better understand how much the ice sheet is contributing to sea level rise, researchers say. 
 
 Most of Greenland's supraglacial lakes drain slowly when superficial streams route water into nearby, permanent crevasses or moulins (vertical conduits or shafts in a glacier). Fairly recently, however, satellite images showed that about 13 percent of the lakes drain quickly, emptying completely within 24 hours. 
 
 "The images would show the lake there one day, and gone the next day," said first author of the new study, Laura Stevens, a glaciology doctoral candidate with the Massachusetts Institute of Technology/Woods Hole Oceanographic Institution (MIT-WHOI) Joint Program. "So we've known for the last 10 to 15 years that the water could disappear quickly." 
 
 A moulin, or crack, formed along the hydro-fracture through Greenland's North Lake basin continues to drain meltwater to the bed after the 2013 North Lake rapid drainage. Credit: Ian Joughin 
 
 The 2008 research, led by Stevens' co-author Sarah Das, a WHOI geological scientist, showed that temporary hydro-fractures could cause rapid lake drainage on an unprecedented scale. But that research was unable to determine what triggered the cracks in the first place. Two other, similar studies of different rapidly draining supraglacial lakes were also unable to identify what caused the hydro-fractures. 
 
 "The coverage of GPS stations was not dense enough," Stevens told Live Science. "This study goes beyond previous studies on the lakes, because we have 16 GPS stations, as opposed to one or four." 
 
 When meltwater in the summer drains to the bed from the ice surface through crevasses or moulins, it can cause the area within and around the lake basin to be "jacked up," Stevens said. Additionally, it can decrease the surface area of the ice-sheet bed that's in contact with the underlying bedrock, lubricating the bed and making it easier for the bed to move horizontally. 
 
 Placed around North Lake, the team's 16 GPS stations recorded these two types of movements — called uplift and slip — between 2011 and 2013. This provided an in-depth perspective on the meltwater injected into the bed before, during and after the yearly drainages. 
 
 "We found that before we get the main expression of the lake drainage, there is a period of time (about six to 12 hours) where uplift and slip increase," Stevens said. "That motion is enough to take the surface of the ice sheet and put portions of it in high tension that allows cracks to start forming." 
 
 The study provides a clearer picture of the amount and location of the meltwater that travels down to the ice-sheet bed, which could help scientists better understand how fast the ice sheet flows during the summer, Stevens said. 
 
 This is important because the inland ice that moves toward the coast gets funneled to so-called outlet glaciers, which are tonguelike protrusions that can break off to form icebergs that may drift off into the ocean and eventually melt. 
 
 "It's half of the equation of how the Greenland ice sheet contributes to sea level rise, with the other half being the years when the ice sheet melts quicker than the snow is deposited," Stevens said. 
 
 Follow Joseph Castroon Twitter. Follow us @livescience, Facebook& Google+. Original article on Live Science.
– For years, scientists have known glacial lakes can rapidly empty themselves of billions of gallons of water—in at least one case, faster than the speed at which water flows over Niagara Falls. Now, they finally know how it's done. Researchers had guessed that the weight of the water caused cracks to form in the lake's icy bottoms that let water drain thousands of feet to the ice sheet’s bed, but that didn't explain why some lakes cracked and others didn't. To solve the mystery, researchers arranged 16 GPS units around Greenland's supraglacial North Lake and recorded ice movements over three summers, in 2011 to 2013, per a press release. They found that tension comes from below: Movements during three lake drainages showed ice was "jacked up" six to 12 hours before a lake bottom cracked, lead author Laura Stevens tells LiveScience. What's happening: Meltwater drains through vertical channels to the ice sheet's base, where it accumulates between the bedrock and ice sheet, causing a bulge. The bulge floats the ice sheet, placing tension on the lake bottom. Then, snap, a crack forms. "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 says. But volume does come into play. "You need both conditions—tension to initiate the crack and the large volume of water to amplify it—for hydrofractures to form," Stevens says. In the case of North Lake, a hydrofracture drained 11 billion gallons of water in 90 minutes. That draining water lubricates the base of the ice sheet, allowing the sheet to move faster toward the ocean, where it "discharges" ice, spurring sea-level rise, per Nature World News. Stevens says the research "will help us predict more accurately how supraglacial lakes will affect ice sheet flow and sea level rise as the region warms in the future." (Read about what lies under the Greenland Ice Sheet.)