Large hidden lakes beneath one of the planet's fastest-moving glaciers on the edge of West Antarctica are draining out at an unprecedented rate, a new study has found.
The ice mass, known as The Thwaites Glacier, is sliding unstoppably into the ocean, mainly due to warmer seawater lapping at its underside.
The details of its collapse may provide a timeline for when to expect a two feet of global sea level rise and destabilisation of the much larger West Antarctic Ice Sheet.
Recent efforts have used satellites to map the underlying terrain, which affects how quickly the ice mass will move, and measure the glacier's thickness and speed to understand the physics of its changes.
Researchers at the University of Washington (UW) in the US and the University of Edinburgh in the UK used data from the European Space Agency's CryoSat-2 to identify a sudden drainage of large pools below Thwaites Glacier, one of two fast-moving glaciers at the edge of the ice sheet.
The study finds four interconnected lakes drained in the eight months from June 2013 and January 2014.
The glacier sped up by about 10 per cent during that time, showing that the glacier's long-term movement is fairly oblivious to trickles at its underside.
"This was a big event, and it confirms that the long-term speed-up that we're observing for this glacier is probably driven by other factors, most likely in the ocean," said Ben Smith, a glaciologist at UW.
"The water flow at the bed is probably not controlling the speed," said Smith.
Other glaciers, like some in Alaska and Greenland, can be very susceptible to changes in meltwater flow. Water there can pond beneath the glacier until it lifts off parts of its bed, and suddenly surges forward.
This can increase a glacier's speed by several times and account for most of its motion. Researchers were not certain whether such an effect might be at play with Thwaites Glacier.
They used a new technique to find drops at the glacier's surface of up to 20 metres over a 20x40 kilometre area.
Calculations show it was likely due to the emptying of four interconnected lakes, the largest about the size of Lake Washington, far below.
The peak drainage rate was about 240 cubic metres per second - the largest meltwater outflow yet reported for subglacial lakes in this region.
"This lake drainage is the biggest water movement that you would expect to see in this area, and it didn't change the glacier's speed by that much," Smith said.
This may be because the Thwaites Glacier is moving quickly enough that friction is heating up its underside to ice's melting point, he said. The glacier's base is already wet and adding more water doesn't make it much more slippery.
The study supports previous research showing that Thwaites Glacier will likely collapse within 200 to 900 years to cause seas to rise by two feet.
The study was published in The Cryosphere journal.
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