Jakobshavn is flowing ever faster. In the past decade it doubled its speed, to roughly 120 feet (37 meters) a day. By now it discharges 11 cubic miles (45 cubic kilometers) of ice each year, jamming the fjord with fresh icebergs.
The pace is picking up elsewhere around Greenland. Last year Eric Rignot reported satellite radar measurements showing that most glaciers draining the southern half of the Greenland ice sheet have accelerated, some even more dramatically than Jakobshavn. He calculated that Greenland lost a total of 54 cubic miles (225 cubic kilometers) of ice in 2005, more than twice as much as ten years ago—and more than some scientists were prepared to believe.
Two of the outlet glaciers have since slowed down. But other satellites detected a minuscule weakening of Greenland’s gravity, confirming that it is shedding ice at a rate of tens of cubic miles a year. Says Waleed Abdalati, a NASA scientist who oversees research on Greenland and Antarctica, “The ice sheet is starting to stir.”
Just as Jakobshavn accelerated, its tongue—the glacier’s seaward end, floating on the waters of the fjord—began to shatter and retreat. Since 2000, the tongue has receded by four miles (six kilometers), adding to the clutter of icebergs in the fjord. Many of the other Greenland glaciers racing to the sea have also lost part or all of their tongues, which may explain the speedup. “Floating ice acts as a buttress,” explains Abdalati. “It holds back the ice behind it, so that when it melts, it sort of uncorks the glacier.”
Greenland’s weather has warmed palpably. Winter temperatures at Steffen’s ice camp have risen about five degrees Celsius (9°F) since 1993. In the past, researchers riding snowmobiles to outlying instrument stations could still count on firm snow as late as May; last year they got stuck in slush. For the past two years Ilulissat—well above the Arctic Circle, a place where street signs mark dogsled crossings—has had long winter thaws. “It was supposed to be minus 20 (-29°C),” says Steffen, “and instead it was raining.”
Offshore, the middle depths of the Atlantic have warmed as well, by several tenths of a degree—enough to undermine an ice tongue that is also melting from above. Eventually all of Greenland’s floating ice could disintegrate. At that point the ice streams may stop accelerating. Then again, they may not, Steffen says. The weight of Greenland’s ice sheet has forced its bedrock down into a vast basin, much of it below sea level. As the glaciers retreat inland, the ocean may follow, prying them off their bed in a runaway process of collapse.