If the entire glacier in the northeast of Greenland melted, then global sea levels would rise by an astonishing 46 centimeters (18 inches). Unfortunately, it seems the rate at which this glacier, known as Zachariæ Isstrøm, is sliding into the ocean and breaking apart has dramatically increased since 2012. At its current rate, it is losing a worrying 4.5 billion tonnes (5 billion tons) of ice per year. The researchers think that the glacier is being dealt a double blow, with a warming ocean melting it from underneath while a warming atmosphere is thawing it from the top.
“North Greenland glaciers are changing rapidly,” says Jeremie Mouginot, who led the study published in Science. “The shape and dynamics of Zachariæ Isstrøm have changed dramatically over the last few years. The glacier is now breaking up and calving high volumes of icebergs into the ocean, which will result in rising sea levels for decades to come.”
The glacier used to have a large ice shelf which jutted out from the bottom of the main body of ice, floating on top of a deep channel of ocean and anchoring to a projection of rock around 40 kilometers (24 miles) out to sea. During the last eight years, however, global warming-induced melting has shrunk this floating ice shelf by around 95 percent. This is important, because the rate at which a glacier loses mass is strongly linked to changes in its ice shelf. In effect, the shelf acts like a cork in a bottle, holding the rest of the glacier back and stopping it from sliding into the sea.
Researchers prodding icebergs away from the delicate equipment on the boat, used to calculate how the glacier has changed over time. Maria Stenzel/UC Irvine
Where the base of the glacier at the ground meets the ocean is known as the “grounding line.” The researchers, using data from aerial surveys as well as satellite- and boat-based observations, were able to determine and track the thickness of the glacier at different points and where exactly this grounding point lies, over 40 years.
Worryingly, they’ve found that the grounding line has been shifting backward at an ever-increasing rate. In fact, they estimate that it is now moving four times faster – from 230 meters to 875 meters per year (754 to 2,870 feet) – than from before 2011. They’ve also found that the glacier is now calving icebergs at the grounding line, meaning that the glacier has become “unmoored.”
The scientists also discovered the speed at which the glacier is moving towards the sea has tripled, while its rate of thinning has doubled. This is clearly bad news, as the glacier is slowly being broken up and disintegrating.
It also neighbors another large glacier, called Nioghalvfjerdsfjorden, and while the researchers found that the rate of retreat for this second one is less than that for Zachariæ Isstrøm, it’s also melting. Considering that together the two masses of ice account for 12 percent of Greenland’s ice sheet, if they were both to melt entirely, global sea levels would rise by 99 centimeters (39 inches).
“Not long ago, we wondered about the effect on sea levels if Earth's major glaciers were to start retreating,” says Eric Rignot, another of the co-author’s from the University of California, Irvine. “We no longer need to wonder; for a couple of decades now, we've been able to directly observe the results of climate warming on polar glaciers. The changes are staggering and are now affecting the four corners of Greenland.”