All over Greenland, meltwater collects beneath the ice, gradually carving out an intricate network of passageways called moulins. The moulins form an ever-changing plumbing system that regulate where water collects between the ice and bedrock at different times of the year. According to Howat, meltwater increases as ice melts in the summer, and decreases as water re-freezes in winter.
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, to the tune of 100 meters per year, he said. The water passageways quickly expand, however, and reduce the water pressure so that by mid-summer the glaciers are flowing 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 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. “Some of these outlet glaciers accelerated in the wintertime, and some of accelerated over long periods of time. The changes didn’t correlate with any time that you would expect there to be more melt,” he added.
So if meltwater is not responsible for rapidly moving outlet glaciers, then what is responsible? 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.
Howat did point out two cases in which 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.
At one point, he and his colleagues witnessed the latter effect first hand. They detected a sudden decrease of sub-glacial meltwater inland, only to see a giant plume of dirty water burst out from under the ice at the nearby water’s edge.
The dirty water was freshwater -- glacial meltwater. It sprayed out from between the glacier and the bedrock “like a fire hose,” Howat said. Since saltwater is more dense than freshwater, the freshwater bubbled straight up to the surface. “This was the equivalent of the pipes bursting on all that plumbing beneath the ice, releasing the pressure.”
That kind of turbulence stirs up the warm ocean water, and can cause more ice to melt, he said.
“So you can’t just say, ‘if you increase melting, you increase glacial speed.’ The relationship is much more complex than that, and since the plumbing system evolves over time, it’s especially hard to pin down.” ---