In July 2006, a team from Woods Hole Oceanographic Institution, US, the University of Washington, US, and Newcastle University, UK, saw around 0.044 cubic km (11.6 billion gallons) of water from a roughly 2 km-diameter meltwater lake create cracks to flow through 980 m of ice in 24 hours. Most of the water drained away in just 90 minutes, creating a maximum drainage rate of 8700 cubic m/s – faster than the average flow over Niagara Falls. In one location the surface of the ice sheet rose by 1.2 m, while a block of ice from the middle of the lake bottom rose by 6 m. And the horizontal speed of the ice sheet reached twice its average daily rate.
“We found clear evidence that supraglacial lakes – the pools of meltwater that form on the surface in summer – can actually drive a crack through the ice sheet in a process called hydrofracture,” said Sarah Das of Woods Hole Oceanographic Institution. “If there is a crack or defect in the surface that is large enough, and a sufficient reservoir of water to keep that crack filled, it can create a conduit all the way down to the bed of the ice sheet.”
The team believes that as cracks form and become filled with water, the greater weight and density of the water forces the crack to open. This results in the the formation of moulins through the ice sheet that are likely to remain open for the rest of the melt season.
Previous satellite observations have shown that supraglacial lakes could disappear in as little as a day but scientists did not know where the water was going, how fast or the impact on ice flow.
“It’s hard to envision how a trickle or a pool of meltwater from the surface could cut through thick, cold ice all the way to the bed,” said Das. “For that reason, there has been a debate as to whether such processes could exist, even though some theoretical work has hypothesized this for decades.”
Das and colleagues say the meltwater is responsible for 50–100% of the summer speed-up in ice flow in slower-moving areas of the ice sheet. That figure is slightly larger than earlier observations.
But they found that the surface melt has a more subdued influence – up to 15% – on the fast-moving outlet glaciers that discharge ice to the ocean by calving icebergs. Most of the Greenland Ice Sheet moves less than one-tenth of a mile a year but some outlet glaciers flow at 7.5 miles a year.
“Many of Greenland's glaciers have doubled their speed and the rate at which they discharge ice to the ocean over the last decade,” Ian Joughin of the University of Washington told environmentalresearchweb. “Our results show that while surface melt is important on the slower moving parts of the ice sheet, in relative terms, its influence is small for the fast-moving outlet glaciers. Instead our results show that, at least on Jakobshavn – the largest glacier on Greenland's west coast – the loss of its floating ice shelf in the late-1990s was probably the dominant cause for its doubling in speed.”
According to Joughin, surface melt-enhanced lubrication is not likely to have a catastrophic influence on ice sheet stability as it is not having a large effect on fast-moving outlet glacies. “Instead, other processes seem to be having a greater effect,” he said. “While we are beginning to understand these processes, there is still much to be done to understand their link to climate and how significant they will be in a warming climate.”
Now the researchers plan to continue their satellite and ground-based observations to determine how the ice sheet is changing. They will incorporate this data into improved ice sheet models to make better estimates of future ice sheet sea level.
The researchers reported their work in two papers in Sciencexpress.