For years, scientists have theorized that the surface lakes formed during the summer months on the Greenland ice sheet could carve through the half-mile thick ice and reach bedrock. Now they know the answer is yes.
For years, scientists have theorized that the surface lakes formed during the summer months on the Greenland ice sheet could carve through the half-mile thick ice and reach bedrock.Now they know the answer is yes.
But, according to research conducted by glaciologists Sara Das of the Woods Hole Oceanographic Institution and Ian Joughin of the University of Washington’s Applied Physics Laboratory, that meltwater is only a minor reason why Greenland’s outlet glaciers — the sections of glacier which meet the ocean and break off as icebergs — accelerated their race to the sea by 50 to 100 percent in the last two decades.
“We found clear evidence that supraglacial lakes — the pools of meltwater that form on the surface in the summer — can actually drive a crack through the ice sheet in a process called hydrofracture,” said Das, an assistant scientist in the WHOI Department of Geology and Geophysics. “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.”
Das and Joughin, whose findings appear in the online journal Science Express and will be published next month by Science magazine, worked with colleagues in the summers of 2006 and 2007. They used seismic instruments, water-level monitors and Global Positioning System sensors to closely monitor two lakes and the surrounding ice sheet. They also used helicopter and airplane surveys and satellite imagery to monitor the lakes and track the progress of glaciers moving to the coast.
The result was the first documentation of the sudden and complete drainage of a lake of meltwater from the top of the Greenland ice sheet to its base.
In July of 2006, the science team’s instruments recorded the draining of a lake that had once covered 2.2 square miles of the surface and held 11.6 billion gallons of water. The entire lake emptied in a 24-hour span and the majority of water drained out in a 90-minute span. A WHOI statement on the research likened the process to “the draining of a bathtub” and stated that, at maximum, the drainage rate was faster than the average flow rate over Niagara Falls.
“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 debate in the scientific community as to whether such processes could exist, even though some theoretical work has hypothesized this for decades.”
With the ice sheet moving ever faster, the scientific community has also discussed whether that meltwater, once it reaches bedrock, could be acting as a lubricant for the ice sheet itself. The team’s findings indicate that it is one part of the equation but not the primary factor.
Over an entire year, surface meltwater was responsible for only a few percent of the movement of the six outlet glaciers monitored, says Joughin. Even in the summer, it appears to contribute, at most, 15 percent to the total annual movement of these fast-moving outlet glaciers. Often, the percentage is far less.
According to the numbers, that’s not the case for the broad, slow-moving sections of the ice sheet, where summer meltwater is accelerating ice flow by 50 to 100 percent.
That difference is a hard concept to grasp but Das explained that the outlet glaciers are moving so much faster than the broader sections — one to two orders of magnitude, she said — so that the same “boost” in speed is a much smaller percentage of the outlet glacier’s overall speed.
While most of the ice sheet moves at less than 1/10 of a mile per year, some outlet glaciers move 7.5 miles per year, making outlet glaciers a concern because of global warming and potential rises in sea level.
“Considered together,” said Joughin, “the new findings indicated that, while surface melt plays a substantial role in ice sheet dynamics, it may not produce large instabilities leading to sea-level rise. There are still other mechanisms that are contributing to the current ice loss and likely will increase this loss as the climate warms.”
In a statement released by the University of Washington, Richard Alley, a professor of geosciences at Penn State University and who is not connected with the papers, said: “What Joughin, Das and their co-authors confirm is that iceflow speed up with meltwater is a widespread occurrence, not restricted to the one site where previously observed. But, they also show that the really fast-moving ice doesn’t speed up very much with this. So we can expect the ice sheet in a warming world to shrink somewhat faster than previously expected, but this mechanism will not cause greatly faster shrinkage.”