An event in the life of glaciers that had never been observed before has just surprised glaciologists. A lake resting on the ice cap drained in the middle of winter, causing a cascade effect.
Greenland’s ice melts in the summer and is reconstructed in the winter. A cycle that repeats itself every year, at least that is what glaciologists thought until recent results. A French-Danish and American team discovered in a large dataset that 180 million m3 of water were released into the sea in the middle of winter by a coastal glacier in western Greenland. The source of this flood is a fifty year old glacial lake that has emptied.
This type of lake appears on the surface of an ice cap in summer when it is hot, the water runs off and accumulates. These are lakes that are several square kilometers in size,” explains Olivier Gagliardini, a glaciologist at Grenoble Alpes University in France. When winter arrives, the snow settles on the frozen surface of the lake, and isolates it from the cold. This leaves a large volume of liquid water trapped in the ice. In the summer it expands. Darker liquid water captures more solar energy and water that is heavier than ice takes up more space.
On March 9, 2018, upstream of Ilulissat 142 kilometers inland, and at an altitude of 1,600 meters on the Jakobshavn Isbræ cap, two glacial lakes disappear. “A fracture opened up and a conduit formed below the surface of the glacier,” says the glaciologist.
This event triggers an acceleration of the glacier’s downstream flow. “This is the first time that such a high flow velocity has been observed in winter. The passage of the flood has changed the physical constraints between the ground and the glacier, and other lakes have also drained downstream, a cascade effect,” he adds. The water was able to lift the hundreds of meters thick glacier by 20 cm.
Now, whether glacier flow will tend to accelerate due to subglacial flooding remains a matter of debate in the scientific community. “There are two schools of thought”, discusses the glaciologist. “One is that the acceleration of glacier flow will not occur because the water is cutting channels underneath them. The other thinks that crevasse fields may rise in elevation allowing meltwater to find the glacier bases further upstream, and would accelerate the flow of the upstream part of the glaciers.”
Eventually the two schools could come together. “They may agree,” he concludes, “that the glaciers will flow faster at higher elevations, and then downstream they would flow slower, but melt all the more.”
On the whole of the coastal glaciers of Greenland, the researchers located 4 other drainages of this type. “The one in 2018 represents a sea level rise of 0.0005 mm,” he calculates. That’s not much, but it shows that there is a lag of several decades for this water to reach the sea.” Information to consider when predicting sea level rise.
Camille Lin, PolarJournal
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