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Joel Harper, associate professor, UM Department of Geosciences, 406-243-5867, joel@mso.umt.edu .

UM Professor Finds Greenland Absorbs Large Amounts Of Melt

Nov. 07, 2012

MISSOULA

University of Montana glaciologist Joel Harper will publish research findings in an article titled “Greenland Ice Sheet Contribution to Sea Level Rise Buffered by Meltwater Storage in Firn” in the Nov. 8 issue of the journal Nature.

According to the research, a large storage sink for meltwater generated on the surface of Greenland exists in a layer of old compacted snow called firn, which is up to 100 meters thick. Because some of the melt is absorbed by the firn, not all of the melt from the huge ice sheet contributes to rising sea levels, yet.

Scientists know that sea level currently rises about 3.2 mm per year, with about half of that rise comes from melting ice around the world. Researchers estimate that 20 to 40 percent of that new water comes from Greenland.

Harper led a team of researchers from the University of Wyoming, University of Colorado-Boulder and the Universite de Liege in Belgium to study the meltwater path on the Greenland ice sheet from 2007 to 2009. Funded by a National Science Foundation grant, the team set out to determine just how much of the meltwater generated in Greenland travels to the sea and what happens to the melt that doesn’t.

“The melt generated from low elevations easily escapes to the ocean, but there’s a large area at higher elevation where the fate of the melt is highly uncertain,” Harper said. “Quantifying how melt can infiltrate and refreeze in cold snow and firn is critical to understanding sea-level changes.”

The researchers spent three months spread over three summers traversing the ice sheet on snowmobiles and skis while taking measurements. They used a radar system to track water deep in the firn, drilled more than 30 ice cores and installed sensors within the ice to track the migration of water.

They found that some of Greenland’s melt can penetrate deeply into the firn, bypassing thick lenses of ice where previous water has refrozen. Since completing the field research, the team has run computer models based on their data to investigate the problem further. Their calculations quantify how much of the firn layer already has been filled by meltwater and how much additional storage remains.

Despite the fact that Greenland absorbs some of its massive melt, the firn is filling at a rate faster than winter snowfall can create more. Harper said the firn will hold an additional 322 to 1,289 gigatons of water, where 1 gigaton is equivalent to a cubic kilometer of water. Based on measurements of the melt already absorbed, the researchers were able to determine a rough estimate of how much longer the firn will continue to take melt.

“Under a range of plausible climate scenarios, filling this firn will take on the order of decades,” Harper said. “It’s not a year or two of heavy melt that’s going to do it, but it’s also not centuries. The rate that the storage sink is filled will dictate changes to the area of the ice sheet that contributes melt to the oceans.

“You can only route water into this storage sink for so long before it fills up and meltwater from higher elevations is rerouted to the oceans.”

The research team’s findings come at a crucial time for Greenland. Satellites have monitored the amount of melt on the ice sheet daily since 1980. These measurements show that the area experiencing melt and the duration of melt each summer have increased on average. This year was the first time since monitoring began that the entire ice sheet experienced melt, though ice cores show that this has happened on a few occasions in the past.

Despite the existence of the water storage sink in the firn layer, multiple lines of independent measurements show that Greenland has lost mass and the rate of mass loss has increased.

“Since the firn can absorb some of the surface melt, this means that understanding other mechanisms for mass loss, such as iceberg calving and melt of the nonfirn-covered areas are critically important,” Harper said.

The next step is to study meltwater at the bottom of the ice sheet by drilling 1,000-meter long boreholes to take measurements beneath the ice. In August, Harper and a partner at the University of Wyoming were awarded a $1.3 million NSF grant to investigate how the melt generated at lower elevations finds its way to the base of the ice and influences the movement of the massive ice sheet.

For more information call Harper at 406-243-5867 or email joel@mso.umt.edu.

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