UW Researchers Measure Meltwater Drainage on Southwest Greenland Ice Sheet

The southwest Greenland ice sheet is not one giant, solid mass but, rather, a tapestry of gushing rivers, lakes and streams that efficiently drains meltwater off its surface -- and stores about 25 percent of this runoff deep below, either in the body of the ice itself or at the base of the ice sheet between the ice and underlying rock.

This recent discovery contradicts previous modeling efforts that assumed that all meltwater produced on the ice sheet’s surface was rapidly discharged to the ocean, says Carl Legleiter, a University of Wyoming assistant professor in the Department of Geography.

“One of the take-home messages from this study is that if you only rely on climate model studies, you will overestimate how much water is going to be discharged,” Legleiter says. “That model does not take into account storage of meltwater within or beneath the ice.”

In that sense, it’s good news. Sea-level rise may not be as great as feared because of this buffering mechanism, he adds. “Everything that melts on the surface of the ice sheet does not necessarily end up in the ocean, at least not in the next week -- the response is not immediate.”

Legleiter is co-author of a paper, titled “Efficient Meltwater Drainage Through Supraglacial Streams and Rivers on the Southwest Greenland Ice Sheet.” The paper was published online earlier this month in the Proceedings of the National Academy of Sciences (PNAS). The paper is scheduled to appear in the print edition Jan. 27. The journal is one of the world's most prestigious multidisciplinary scientific serials, with coverage spanning the biological, physical and social sciences.

Research was conducted during July 2012 in southwestern Greenland during the summer melt season. Meltwater runoff from the Greenland ice sheet accounts for half or more of its total mass loss to the global ocean, but remains one of the least-studied hydrologic processes on Earth. The runoff is a key contributor to global sea-level rise.

Each summer, a complex system of supraglacial meltwater ponds, lakes, streams, rivers and moulins (essentially sinkholes) develops across large sections of the southwestern Greenland ice sheet surface. For a look at how these water bodies behave on the Greenland ice sheet, go to the video in this L.A. Times story at http://www.latimes.com/science/sciencenow/la-sci-sn-catastrophic-greenland-melt-20150112-story.html.

The study concludes that the ice sheet surface is efficiently drained under optimal conditions; that digital elevation models alone cannot fully describe supraglacial drainage and its connection to subglacial systems; and that predicting outflow from the climate models alone may overestimate true meltwater release from the ice sheet.

“The hydrology is a little more complicated than previously thought. There are potential storage reservoirs where some of the meltwater can be detained before it goes into oceans,” Legleiter says. “If there is significant storage, that would delay runoff. The meltwater will make it out there (the ocean) eventually, but at a slower rate.”

Legleiter says this theory has been speculated before but, to his knowledge, this is the first time that meltwater was actually measured on the ice sheet and in pro-glacial rivers discharging from beneath the ice sheet and emptying into the ocean. This data set allowed the researchers to quantify the rate of meltwater production and discharge to the ocean and, thus, infer storage.

The paper has extra meaning for Legleiter. Alberto Behar, a professor in the School of Earth and Space Exploration at Arizona State University and a colleague during the research, died in a plane crash Jan. 9.

“PNAS allowed us to dedicate this paper to Alberto. We got to be pretty close during the brief time we were together,” Legleiter says of the time he and Behar spent working together on the Greenland ice sheet in 2012.

In addition to Behar, other contributing writers to the paper were Brandon Overstreet, a UW doctoral student; researchers from the University of Utah, Rutgers University and the City College of New York; the NASA Jet Propulsion Laboratory in Pasadena, Calif.; U.S. Army Cold Regions Research and Engineering Laboratory in Hanover, N.H.; and Earth Vision Trust in Boulder, Colo. Laurence Smith, professor and head of the Department of Geography at UCLA, was the paper’s lead writer.