Some scientists have suggested that an increased number of similar events could spur a collapse of much of Greenlandâ€™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 Greenlandâ€™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 â€” 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.
â€œFor huge ice streams, the effect isnâ€™t terribly significant,â€ says Waleed Abdalati, a glaciologist at NASAâ€™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