Melt pond falls through ice in Greenland

It took almost a month for meltwater to accumulate atop Greenland’s ice sheet in the summer of 2006. It took only 90 minutes for all that water — a lake so large it could fill New Orleans’ Superdome more than 12 times over — to pour through a crack in the kilometer-thick ice below it and drain the lake dry.

At its height, the torrent exceeded that of Niagara Falls, and its rumbling triggered seismic instruments nearby. GPS equipment indicated that the westward flow of ice in the region briefly surged, a sign that the water drained down to the bedrock and temporarily lubricated the boundary between ice and rock.

Some scientists have suggested that an increased number of similar events could spur a collapse of much of Greenland’s islandwide ice sheet, leading to sudden rises in sea level. But new analyses hint that the overall effects of an increase in such subglacial lubrication, while possibly substantial, would not be catastrophic. All ice on Greenland eventually flows to the sea, with that in glaciers and fast-moving ice streams outpacing the languid flow of most parts of the ice sheet.

The lake that suddenly disappeared in 2006, one of many such melt ponds that form atop Greenland’s ice sheet each summer, began accumulating in early July of that year, says Sarah B. Das, a glaciologist at the Woods Hole Oceanographic Institution in Massachusetts. By the morning of July 29, the lake covered 5.6 square kilometers and was in some places more than 12 meters deep.

At that time, instruments show, the lake level began to drop slowly but steadily, about 1.5 centimeters each hour for the next 16 hours. Then, literally, the bottom dropped out: Over about 84 minutes, the lake drained completely, losing on average about 8,700 cubic meters of water each second, she and her colleagues report online and in a paper to be published in Science.

That water quickly accumulated at the base of the underlying ice sheet, forming a subglacial lake that drained away during the following 24 hours. During that brief period, the seaward flow rate of the overlying ice sheet approximately tripled, then dropped back to its normal speed of 25 centimeters per day.

Analyses of space-based radar images of western Greenland suggest that the flow speed of the ice sheet increases, on average, between 50 and 100 percent during the summer — a phenomenon probably linked to increased amounts of meltwater reaching bedrock, says Ian Joughin, a glaciologist at the University of Washington in Seattle. He and Das collaborated on the new report and, along with another group of researchers, also analyzed satellite observations of the region that were gathered from September 2004 to August 2007. That report, too, will appear in an upcoming issue of Science.

In regions of Greenland where large glaciers dump ice into the sea, the effect of summer meltwater seems to be less pronounced, says Joughin, perhaps because the flow of subglacial water out of the glaciers is already brisk.

“For huge ice streams, the effect isn’t terribly significant,” says Waleed Abdalati, a glaciologist at NASA’s Goddard Space Flight Center in Greenbelt, Md. Nevertheless, he notes, the new findings have widespread implications for the Greenland ice sheet as a whole and could be important in the long term.