This known phenomenon is difficult to observe directly, but the glaciologists of the Scott Polar Research Institute at the University of Cambridge were fortunate when they arrived at Store Glacier in West Greenland in July 2018.
A few days after their arrival, in five hours, two-thirds of the lake, nearly 5 million liters, disappeared from the surface, engulfed in the ground. Aerial photos before / after taken by the small drone of the scientific team show a large dark blue oval shrink into a smaller, light blue circle, shallower.
"The interest of the drone is to allow us to make high quality surveys in areas that are not sure of access for scientists," says Tom Chudley, doctoral student and pilot of the drone.
The drone, by taking geolocated photos by GPS, has made it possible to reconstruct in three dimensions the evolution of the relief of the ice.
A glacier is a river of ice moving towards the ocean. When the ice reaches the water, it creates icebergs, which represent about 40% of Greenland's contribution to the rising waters (the rest is melting ice).
The Store Glacier advances 600 meters a year. What scientists have seen is that the sudden flow of the lake temporarily accelerated the speed from 2 to about 5 meters a day. The water passed under the ice lubricated the glacier.
More surprisingly, the water lifted the huge layer of ice by 55 centimeters for a few hours, say the researchers.
"One kilometer of ice raised half a meter, I let you imagine the level of pressure that implies," says Tom Chudley.
The interest of the study, published Monday in the journal PNAS, is to describe more precisely the formation of these huge faults, which then become "highways" to run water from the surface to the bed of the cap, which can accelerate the movement of glaciers.
"With climate change in Greenland, we see more lakes, bigger, and higher in the colder parts of the ice cap. And we see that some of these lakes are starting to empty, "said Tom Chudley.
"The volume of lakes draining will potentially increase in new places that we did not know before," he warns.