The snow atop Greenland’s ice sheet has warmed markedly over the last 60 years, a trend driven by percolating meltwater that will accelerate in coming years, according to research by Dartmouth scientists and their colleagues.
The rapid melting of most of the world’s glaciers is well established, but the new findings shed light on the mechanisms that control how the interior of the ice warms and how melt leaves the ice sheet to contribute to sea level rise. The study appeared in the journal Geophysical Research Letters.
The researchers re-measured temperatures in shallow boreholes in the firn at locations first observed from 1952 through 1955 across the northern Greenland ice sheet. Firn is layers of old snow that compacts over time and eventually becomes glacier ice. The results show a pattern of substantial firn warming (up to 10 degrees Fahrenheit) over the past 60 years at mid-level elevations of the ice sheet, but little change at higher elevations.
To explain the pattern of temperature increases they observed, the researchers modeled the process of meltwater percolating into the firn. The modeling showed that when melt occurs and water percolates down into the firn, it carries latent heat with it several meters downward. There, under highly insulating layers of snow, the heat from consecutive melt events accumulates and is partly stored for several years. The amount of firn warming is highly sensitive to the amount of meltwater produced at the surface, but the impact endures for an extended time, requiring more than four years for the firn temperature rise to drop below 50 percent of the peak impact from a percolation event.
“The important implication of this is that the energy delivered into the firn by percolating meltwater and released as heat by refreezing the meltwater can accumulate over multiple years,” says lead author Chris Polashenski ’07, Thayer ’07, ’11, an adjunct assistant professor at Thayer School of Engineering and a research geophysicist at the U.S. Army Corps of Engineers Cold Regions Research and Engineering Laboratory.
“Such an amplified warming within the layers of the firn is important to understanding sea level rise,” Polashenski says. “A lot of the melt currently produced on the Greenland ice sheet never makes it to ocean because the firn is still cold enough to refreeze the meltwater that enters it. The accumulation of heat year after year within the firn, however, ‘ripens’ the ice sheet and makes it less capable of recapturing future melt events—a process that these observations show is well under way."