Jan. 29, 2015
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 College 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. A PDF is available on request.
The researchers re-measured temperatures in shallow boreholes in the firn at locations first observed from 1952–1955 across the northern Greenland ice sheet. Firn is old layers of snow that compacts over time into granular, porous ice and eventually glacier ice. The results show a pattern of substantial firn warming (up to 10 degrees Farenheit) over the past 60 years at mid-level elevations of the ice sheet but little change at higher elevations.
The researchers’ modeled the process of meltwater percolating into the firn to explain the pattern of temperature increases they observed. 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, an adjunct assistant professor at Dartmouth’s 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. 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 underway.“
The researchers also found that the firn warming in recent years was part of a long-term increase in meltwater percolation and was not predominantly driven by the Greenland ice sheet’s unusually widespread surface melting in 2012.
This process means that full-year average temperatures on the ice sheet are much less important than summer temperatures. “The large impact that percolation intensity has on firn temperatures and runoff production highlights a potentially obvious but important system dynamic,” Polashenski says. “Firn warming and runoff potential are much more dependent on the amount of melt produced during the warmest several days of the year than the overall annual average temperature at the site. The location of the runoff limit, and likely the amount of runoff produced from the ice sheet, is being controlled by the conditions that occur during the critical several weeks of summer when melt is possible. Conditions that favor melt during this window, such as increased episodic atmospheric transport of warm air onto the ice sheet, formation of optically thin clouds and low surface albedo (all of which are cited as contributing to the 2012 melt) will enhance percolation and cause rapid firn warming. Consecutive years with particularly active melt seasons could warm the firn and move the runoff production farther up the ice sheet than the rise in the mean annual temperature would imply and therefore have outsized impacts on sea level rise.”
Chris Polashenski is available to comment at email@example.com.