Last month, an unusually warm weather system swept across Greenland, triggering a widespread melting of surface ice and snow that typically occurs only once in a lifetime. Such extensive melting is exceedingly rare, but it could exacerbate a trend noted in a new study: Since 2000, increasing amounts of surface melting have been darkening the lower-elevation portions of Greenland’s ice sheet, setting the stage for more frequent melting in the future.
On Greenland, as on most ice sheets, the majority of snow accumulates at colder, higher elevations. There, the increasing pressure of ever-deepening layers compresses snow into ice, which then begins to flow toward the sea. Snow that accumulates at lower elevations around the fringe of the ice sheet often melts before it gets pressed into ice. But previous analyses of ice cores drilled from Greenland's ice sheet reveal that on rare occasions—about once every 153 years, on average—even snow at the highest elevations melts, probably due to a brief warm spell. The last such melting event chronicled in ice cores occurred in 1889.
Using data gleaned from satellites, researchers have discovered that much of Greenland’s ice sheet has been gradually getting darker—and therefore absorbing more solar radiation, which in turn stimulates more melting—since 2000. The darkening trend is apparent across about 97% of the ice sheet, says Jason Box, a glaciologist at Ohio State University, Columbus. In July 2000, the ice sheet, on average, absorbed a little less than 25% of the sunlight that fell upon it; the rest was reflected back into space. And even though increased snowfall across nearly two-thirds of the accumulation area is helping to curb the darkening, it isn’t strong enough to completely stifle the overall trend. In 2012, the ice sheet absorbed slightly more than 30% of the incoming sunlight, Box and his colleagues report online in The Cryosphere.
Recently, a number of factors have conspired to render Greenland's ice darker, the researchers explain. One of the largest contributors is a stubborn weather pattern set up by persistent low pressure over Iceland during summers since 2007. That low-pressure system has steered warm air northward along the western coast of Greenland, which in turn warmed snow at the surface, making its grains larger and decreasing its ability to scatter light. That decreased reflectivity, along with the decreased cloudiness in recent years, has dramatically boosted summertime melting. Reduced snowfall, the final ingredient in the recipe, has failed to cover and therefore brighten the darker snow, Box notes.
At the upper elevations of Greenland's ice sheet, where snow typically accumulates year-round, reflectivity is dropping to the point where the snow almost absorbs more energy than it reflects—a sign that the surface of the ice sheet may routinely suffer widespread melting in the near future, possibly within another decade, if weather patterns hold. The amount of energy absorbed by the Greenland ice sheet this July is enough to melt 136 million metric tons of ice, if that ice is already at its melting point of 0°C. That's almost 55% of the ice lost during summer months between 2003 and 2009, the researchers say. Altogether, the amount of ice melted across all of Greenland during July 2012 due to changes in reflectivity alone—about 100 cubic kilometers—is about double the average seen in recent years and is enough to cover Manhattan with ice to a depth of more than 1 kilometer.
As the reflectivity of snow decreases, it’s easier to warm the snow and produce melting, says Jan van Angelen, a polar meteorologist at Utrecht University in the Netherlands. Although the July melting event was brought about by an unusual warm spell, it’s reasonable to suggest that such events will become more common in coming years, he notes.
The exceptional melting event Greenland experienced in July was brought about by "a couple of days where the temperature was a couple of degrees above normal," says Stef Lhermitte, a remote sensing analyst at the Royal Netherlands Meteorological Institute in De Bilt. Nevertheless, he notes, Box and his colleagues "make a strong point that the whole of Greenland is getting darker." That trend, plus changes in climate that boost average summertime temperatures, increase the probability of widespread melting in the future. "I don’t think it will take another 150 years to have another such event," he says.