Intensely blue, supraglacial lakes like those dotting Petermann glacier in northwestern Greenland are beautiful but can endanger the floating tongues of ice on which they form. As such lakes fill and then drain rapidly through cracks, the resulting stresses caused by shifting loads can weaken and even fracture ice. That’s what happened to Antarctica’s Larsen B ice shelf in 2002, and it’s bad news: Ice shelves as well as the floating tongues of tidewater glaciers act like buttresses, holding glaciers back from flowing into the ocean and contributing to sea level rise.
Now the first study of the effects of supraglacial lakes on the periphery of the Greenland ice sheet has found that some of Petermann glacier’s lakes are probably draining in a gradual and innocuous way that spares the floating tongue of ice from a similar fate. An analysis of satellite images indicates that the meltwater empties into a river on the glacier’s surface that funnels the runoff into Petermann Fjord. Such glacial rivers might act as a harmless export mechanism for supraglacial lake water, the scientists suggest. However, these rivers are rare.
Half as Much Water
Scientists working in Antarctica had previously shown that supraglacial lakes can destabilize ice shelves. However, such effects of these lakes in Greenland, whose ice sheet is melting much faster than Antarctica’s, have not drawn similar attention.
To study Petermann glacier, Alison Banwell, a glaciologist at the University of Colorado Boulder, and her colleagues turned to observations collected by the Landsat 8 satellite from 2014 to 2016. The team focused on the glacier’s floating terminus in Petermann Fjord.
Scrutinizing the high-resolution imagery, Banwell and her collaborators tabulated lakes, measured their areas, and estimated their depths across three melt seasons. Banwell was at the University of Cambridge in the United Kingdom at the time this study was conducted.
The researchers found that Petermann’s supraglacial lakes tended to form in June and peak in number, area, and volume around the end of the month before dissipating by August. Even when the lakes were most numerous, however, they covered less than 3% of the glacier’s tongue, the team reported this month in Annals of Glaciology. By contrast, more than 5% of Antarctica’s Larsen B ice shelf was covered in supraglacial lakes when it collapsed 16 years ago.
Banwell and her colleagues were relieved to find that supraglacial lakes covered less of Petermann glacier. “The fact that there’s half as much water on Petermann is a good sign,” said Banwell. “We’d be a bit worried if Petermann had as much water on it as Larsen B.”
Supraglacial lakes can destabilize ice shelves in several ways, studies in Antarctica have shown. Water is less effective at reflecting sunlight than ice is, so supraglacial lakes absorb more heat and cause the surrounding ice to melt more quickly. Furthermore, water can percolate into fractures in the ice, which can then expand. “The force of the water helps to propagate that crack downward,” explained Banwell. When supraglacial lakes drain rapidly—through a fracture, for example—millions of kilograms of water are quickly removed, which flexes and weakens the ice shelves.
A Glacial Relief Valve
Banwell and her colleagues found that Petermann glacier’s lakes sometimes coalesced with neighboring lakes, forming larger bodies. They also occasionally drained away in just a few hours or days. Sometimes, however, the lakes changed in volume only gradually, leading Banwell and her collaborators to presume that the water was draining away not in a gush but in a trickle. The researchers had to look only at the center of the glacier’s tongue to find the likely outlet.
Petermann glacier’s “Blue River” is a several-kilometer-long turquoise waterway bisecting the glacier’s tongue and plunging into Petermann Fjord. Lake water is likely trickling into the Blue River, slowly releasing pressure on the ice shelf and minimizing its flexure, the team concluded.
“The runoff is important,” said Richard Alley, a geoscientist at Pennsylvania State University in University Park who was not involved in the research. “We need to understand the occurrence of lakes and how they behave,” he added.
Fortunately, the Blue River appears to be a recurring feature of Petermann glacier. It was visible every melt season in the team’s satellite images, evidently reforming each summer after the winter snowfall. Thanks to this glacial relief valve, said Banwell, “we don’t think Petermann is currently prone to collapse.”
Nonetheless, other glaciers in the Arctic may be more vulnerable, Banwell said. “This is the only example [of a glacial river] in Greenland.”