Scientists Watch Greenland Ice Sheet Lake Drain Five Million Cubic Meters of Water in Just Five Hours

Scientists working in the Arctic have watched a lake on the Greenland Ice Sheet drain five million cubic meters of water—equivalent to 2,000 Olympic-sized swimming pools—in just five hours. The rapid draining of the lake took place "without any precursory trigger" after it "expanded and reactivated a pre-existing fracture" on the ice, the team wrote in PNAS.

Researchers from the U.K. used custom built drones to monitor meltwater lakes on the Greenland Ice Sheet. This expanse of ice is the second largest in the world, after the Antarctic Ice Sheet. It is also the biggest contributor to sea level rise. It is estimated that if we continue on the current trajectory of global warming, the Greenland Ice Sheet will eventually disappear, raising sea levels by 23 feet.

Understanding how, when and why melting is taking place is important to forecasting how the ice sheet will respond to the climate in the future. Meltwater lakes form on the surface of the ice every year and they are known to drain away. With warming temperatures, these lakes are becoming more abundant, and what impact they are having on the glaciers is unclear, however.

"Previous fieldwork-based studies that have looked at rapidly draining lakes on the Greenland Ice Sheet have focussed on slow-moving, 'land-terminating' sectors of the ice sheet, as opposed to the fast-flowing, 'marine-terminating' area we studied here, where ice flows rapidly into the ocean in large-scale calving events, forming icebergs," lead author Thomas Chudley, from the University of Cambridge, told Newsweek.

"Meltwater draining to the bed of the ice sheet can impact its speed. This is important as, if the ice sheet speeds up, then there is a risk that there may be a higher contribution to sea level rise from processes such as iceberg calving at the ocean-terminating margins. However, this is a really complex story, because there's a whole web of positive and negative feedbacks relating to hydrology. Surface meltwater can accelerate the ice by lubricating the bed, but also slow it down in other cases by evacuating water that already exists beneath the ice."

Chudley said it had been thought these feedbacks would cancel each other out. However, research is now starting to suggest areas on the ice sheet—including the region he and his team studied—that the situation is more complicated. "We need to gain a more complete picture of these processes so that we can properly predict the impact of climate change on the Greenland Ice Sheet in the 21st century," he said.

With their drones, which were fitted with GPS monitors, the team was able to measure the volume and discharge of a meltwater lake. They were also able to assess ice flow, uplift and seismic activity that took place as the lake drained away.

Findings showed that over just five hours, five million cubic meters of water had gone. The lake did not fully drain—once it stopped around a third of the volume remained. Chudley said an important finding from their research was that the lake draining led to the formation of a moulin—a cavity that extends down to the base of the ice sheet. This opening means water can continue to get to the bed beneath long after the initial event.

"Previous research has shown that whilst only 3 percent percent of total surface melt is delivered to the bed via these drainage events, as much as 36 percent can continue to access the bed via moulins after the event," he explained. "Whilst satellite studies have previously estimated the number of lakes that rapidly drain to be between 10 to 30 percent, our lake drainage is actually small enough that it wouldn't be identified by these methods. So that suggests to us that there may be more of these rapid drainages—and the moulins that follow—than previously thought."

Researchers also found the drainage event caused the ice flow to speed up from 6.5 feet per day to over 16 feet per day. The ice rose by approximately 1.6 feet as the water entering below pushed it up. "We were also surprised to see that these impacts extended as far away as [2.4 miles] from the lake site itself, suggesting that these events can have quite widespread impacts in the short term," Chudley said.

The researchers are now continuing to work at the Greenland Ice Sheet to understand how networks beneath the surface impact the flow of the ice. "It's possible we've under-estimated the effects of these glaciers on the overall instability of the Greenland Ice Sheet," Chudley said in a statement. "It's a rare thing to actually observe these fast-draining lakes - we were lucky to be in the right place at the right time."