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University of Wyoming Geologist Neil Humphrey and colleagues have taken a significant step to better understand glacial movement in Greenland, which is fundamental to projecting future sea-level rise worldwide. The key may lie in the effect of the basal or sub-glacial drainage system on the Greenland ice sheet.
Sub-glacial drainage systems are formed from the hydraulic pathways that contain and transfer water located close to the contact between an ice mass and its substrate.
But rather than the melting ice draining out from under the ice sheet within a matter of days -- as a decades-old theory of a glacial river or conduit system purports -- the melting ice moves much more slowly, as ice melts from everywhere on the ice sheet and meltwater moves underneath in what Humphrey terms a “distribution system.”
“It’s socially relevant science. It’s directly related to sea-level rise,” says Humphrey, a UW professor in the Department of Geology and Geophysics. “From a human perspective, 20 percent of the world’s population lives within 50 feet of sea level. If Greenland is to melt, it’s beyond catastrophic.”
Humphrey is co-author of a paper, titled “Basal Drainage System Response to Increasing Surface Melt on the Greenland Ice Sheet,” that will be published in this month’s issue of Science. This marks the fifth time he has contributed a paper that has been published in Science, Humphrey says.
Toby Meierbachtol, a doctoral student at the University of Montana, is the paper’s lead writer. Joel Harper, an associate professor in the Department of Geosciences at the University of Montana, was the other contributing writer. Humphrey and Harper have worked as colleagues in the field for 20 years.
During the summers of 2010-12, the research group used hot water methods to drill 23 vertically straight bore holes into the ice sheet bed -- in ice up to 1 kilometer thick -- at sites along an east-west transect in western Greenland.
Humphrey designed and constructed a hot water drill at UW. The drill pumps cold surface water through a series of heaters and high-pressure pumps to melt a hole -- at 120 meters per hour -- through the ice sheet. Drilling allowed for the installation of sensors for monitoring water flow and ice motion.
Measurements beneath the Greenland ice sheet indicate limited growth of the basal hydrologic network due to basal water pressures unfavorable to water-draining conduit development extending inland beneath deep ice. Slow melt-back of ice walls limits conduit growth, inhibiting their capacity to transport increased discharge.
“We’re the first people to actually go out and observe, and drill to the bed,” Humphrey says.
The logistical difficulty of accessing the bed of the Greenland ice sheet has limited researchers’ ability to fully understand this drainage system’s response to surface ice melt, Humphrey says. As a result, geologists have been forced to interpret Greenland’s velocity changes based on theory developed from 40 years of observations on smaller, more accessible mountain glaciers.
Mountain glacier geometry generally promotes rapid development of water-draining basal conduits through melt-back of the overlying ice roof. In contrast, conditions beneath much of Greenland do not support such fast growth of the basal drainage system through ice melting, Humphrey says.
“Our research suggests that, because of geometric differences between the Greenland ice sheet and mountain glaciers, key aspects of the (previous) theory are not directly transferrable,” Humphrey says. “It requires a total rethinking of how the mechanism works.”
Still, questions persist.
Ice comes off of Greenland in two ways, Humphrey says. The ice either deforms or slides off.
“The sliding of the ice is all related to the basal drainage,” Humphrey says. “It’s a very complicated system we don’t understand. It’s like the ice is floating.”
The National Science Foundation (NSF) and the Greenland Analogue Project (GAP), a consortium of Canadian, Finnish and Swedish companies (with contributions from Britain and Denmark), provided $1.3 million and $1.6 million, respectively, to fund the research. The NSF is providing another $1.2 million to continue the research, Humphrey says.