UW Professors' Work Offers New Picture of Glacier Hydrology
For years now, Neil Humphrey and Joel T. Harper have made a living atop mountain glaciers, decked out in cold weather or rain gear, drilling and instrumenting boreholes in sheer ice and working to make sense of the data.
One of their latest experiments, on Bench Glacier in coastal Alaska, produced an unexpected result that could change scientists' view of basal water drainage and glacial sliding. Their findings were published today (Thursday) in Nature, the world's foremost weekly scientific journal and the flagship journal for Nature Publishing Group (NPG).
While researchers have long known that the manner in which waters moves inside a glacier will control how the ice slides down its valley, Humphrey says the Nature report illustrates a differing picture of glacier hydrology.
"In the past, the mental picture that people have of glaciers is that you had ice flowing over bedrock and that there was a thin sheet of water and dirt between the ice and the bedrock. The glacier slid over this layer of dirt and water and this layer was thought of as a simple 2-dimensional surface," says Humphrey, a professor in the University of Wyoming Department of Geology and Geophysics. "But what we discovered when we started looking in great detail was that the bed of the glacier is not just a simple 2D surface. The ice actually has enough cracks in it that the water is not flowing through just a little gap, it's actually flowing through a complex 3-dimensional zone.
"Our observations show that considerable water is stored englacially, in other words, away from the bed, in a 3D flow system. That changes the picture completely."
Through their work, Humphrey and Harper, a geoscientist at the University of Montana in Missoula, hope to make progress on one of the fundamental problems of glacier motion, namely the poorly understood link between glacial water drainage and bulk sliding of the ice.
"We can currently predict how fast glaciers will melt but not how fast they can slide, and the sliding of ice from the big ice sheets of the world will be what controls sea-level rise resulting from global warming," Humphrey says.
In their report, titled "Vertical extension of the subglacial drainage system into basal crevasses," Humphrey and Harper detail findings from field experiments in 2003 and 2006 in which they first drilled and instrumented 28 boreholes in the temperate valley glacier near Valdez and then studied detailed video inspections and water pressure records from the holes.
Each hole, about 600 feet deep, was drilled with a piece of equipment designed and built at UW, which boasts the "fastest and deepest portable drill in the world," Humphrey says. Researchers were able to drill about 300 feet per hour, sometimes making up to five holes in a single day.
"The equipment was totally a key to the project," Harper says. "Without the drill, we would have had no way of making measurements at the bottom of the glacier and this discovery simply would not have been made."
Their findings in Alaska served as motivation for the researchers' current project in Greenland, a country whose surface is covered primarily by the appropriately-named Greenland ice sheet, the second largest ice body in the world.
The researchers' work in Greenland focuses primarily on how the ice
sheet "might react to global warming," says Humphrey.
A 2004 study by the University of Reading in England showed that if the Greenland ice sheet were to completely melt away, the world's sea level would rise by more than 23 feet and threaten to swamp low-lying cities such as Los Angeles and London.
"We found out (in Alaska) that water not only flows along the contact between bedrock and the overlying ice, but that water can sometimes also move way up into the ice. We found basal crevasses at Bench Glacier, and there is reason to believe that they can sometimes exist on other glaciers," says Harper. "We need figure out how common they are in other places and, this is, in part, our motivation for drilling boreholes in Greenland."
For their work in Greenland, where temperatures remain below freezing for nine months of the year, Humphrey has designed a new drill, also built in the UW machine shop, capable of penetrating up to 2,000 meters, or nearly 2,200 yards, of ice.
Humphrey and Harper have worked together, oftentimes atop mountain glaciers, since the early 1990s. They have spent considerable time in Alaska, as well as the Rocky Mountains, the Himalayas in Asia, Greenland and Antarctica.
John Bradford, of the Center for Geophysical Investigation of the Shallow Subsurface at Boise State University in Idaho, and Toby W. Meierbachtol, a senior graduate student from the University of Montana, were co-authors on the Nature paper.
University of Montana researchers Joel T. Harper and Toby W. Meierbachtol work on the Bench Glacier in Alaska. (Neil Humphrey photo)