What Lies Beneath

[youtube=http://www.youtube.com/watch?v=EP4ONFKZFAs] For years, scientists thought that melted water beneath Greenland's coastal glaciers such as the Jakobshavn and Helheim lubricated the giant sheets of ice above, accelerating their plunge into the ocean and contributing to loss of sea ice. Turns out, that was an over-simplified explanation, said Ian Howat, assistant professor of earth sciences at Ohio State University.

Speaking in a press conference Wednesday at the annual meeting of the American Geophysical Union (AGU), the NASA-funded, CPS-supported scientist explained that the subsurface dynamics beneath glaciers is significantly more complex than previously thought.

"In the science community it's been accepted that basal lubrication due to increased melting and warming is responsible for accelerating glacial advance and breaking off," said Howat. "We're finding out that's not true."

Specifically, a complex, subglacial "plumbing" system involving the ocean, meltwater, and ice evolves, which drives the glacial calving. In fact, early evidence from Howat's research suggests that ocean changes have a greater impact on the rate at which outlet glaciers spill into the sea than does meltwater.

Much of the melt water comes from early summer hot temperatures, which melt the glacier's surface. The water flows through cracks in the ice to the ground surface.

In the early summer, the sudden influx of water overwhelms the subglacial drainage system, causing the water pressure to increase and the ice to lift off its bed and flow faster—up to 100 meters per year, he said. The water passageways quickly expand, however, and reduce the water pressure so that by mid-summer the glaciers flow slowly again.

Inland, this summertime boost in speed is very noticeable, since the glaciers are moving so slowly in general. But outlet glaciers along the coast, such as the Jakobshavn, are already flowing out to sea at rates as high as 10 kilometers per year -- a rate too high to be caused by the meltwater.

"So you have this inland ice moving slowly, and you have these outlet glaciers moving 100 times faster. Those outlet glaciers are feeling a small acceleration from the meltwater, but overall the contribution is negligible," Howat said.

His team looked for correlations between times of peak meltwater in the summer and times of sudden acceleration in outlet glaciers, and found none. So if meltwater is not responsible for rapidly moving outlet glaciers, what is? Howat suspects that the ocean is the cause.

Through computer modeling, he and his colleagues have determined that friction between the glacial walls and the fjords that surround them is probably what holds outlet glaciers in place, and sudden increases in ocean water temperature cause the outlet glaciers to speed up.

However, Howat said meltwater can have a dramatic effect on ice loss along the coast. It can expand within cracks to form stress fractures, or it can bubble out from under the base of the ice sheet and stir up the warmer ocean water. Both circumstances can cause large pieces of the glacier to break off, and the subsequent turbulence stirs up the warm ocean water, and can cause more ice to melt.