Watching flow sharpens picture of moving glaciers
It took almost a month for meltwater to accumulate atop
At its height, the torrent exceeded that of
Some scientists have suggested that an increased number of
similar events could spur a collapse of much of
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
In regions of
“For huge ice streams, the effect isn’t terribly
significant,” says Waleed Abdalati, a glaciologist at NASA’s Goddard Space
Flight Center in
Das, S.B., I. Joughin, et al. In press. Fracture propoagation to the base of the Greenland ice sheet during supraglacial lake drainage. Science.
Joughin, I., S.B. Das, et al. In press. Seasonal speedup along the Western flank of the Greenland ice sheet. Science.