Three have already lost their supporting ice shelf in the sea, the other five are at risk of entering accelerated melting, or are already doing so. This could have “dramatic consequences for sea level rise”, writes the French-Danish team that examines the state of the mapped glaciers in a professional journal Nature Communications. The glaciers together account for a sea level rise of approximately 2.1 meters.
Polar scientist Roderik van de Wal of Utrecht University qualifies that scenario: it is not very likely that it will come to that. “The situation in Greenland is completely different from Antarctica. What you see here is not an ice sheet about to collapse, but rather a kind of passive part that is falling off. I rather expect that a new equilibrium will emerge, with a slightly retreating ice sheet.”
So far, measurements have shown that Greenland is mainly losing mass in the southeast and the west. In the north there seemed to be little going on. “But that is a kind of early warning that something will happen here,” says Van de Wal. “Quite worrying, when you see it laid out like this.”
He is particularly concerned about the deeper cause of the weakening of the glaciers: apparently the sea water around the North Pole has warmed more than expected. That water gnaws at the parts of the northern glaciers that rest in the sea, causing them to melt, causing the glaciers to flow faster. Over the past 45 years, the floating ice shelves that hold back the glaciers have lost about 35 percent of their volume.
These are glaciers with names that are little known to laypeople, such as Petermann, Ryder, Steensby and Bistrup. Already between 2003 and 2010, the floating ends of the Zachariæ Isstrøm, Ostenfeld and Hagen Brae glaciers collapsed. But five other glacier ends also suddenly underwent ‘significant changes’ after 2000, according to the team, led by a glacier expert Romain Millan from the University of Grenoble.
The good news, says Van de Wal, is that most Greenland glacier points are wedged into narrow fjords. That actually slows down the glacier deflation. Furthermore, less of the glacier’s extensions lie below sea level than in Antarctica, where depth is the crucial weakness that makes many glaciers vulnerable to accelerated deflation. An exception in Northern Greenland is the Petermann Glacier, which accounts for a total of 38 centimeters of sea level rise.
Lower than the frost line
Sea levels are currently rising by about 3.7 millimeters per year. Greenland now contributes about 0.8 millimeters to this. Recently, an average of 300 billion tons of ice disappeared from the continent every year. About two-thirds of this is due to glaciers sliding into the sea. A third is due to melt on the surface of the ice-covered continent, which is large enough to include the Benelux, Great Britain, France, Spain and Portugal.
The fear is that global warming will cause more and more of the Greenland ice sheet to lie below the frost line, which will set in motion a self-reinforcing chain reaction of melting, without the disappeared ice being replenished with snow. “Then the ice cap is doomed to disappear,” Van de Wal predicts.