Cracks in Greenland's Ice Sheet May Be Making It Even More Unstable

The meltwater lake before and after the draining
Photo: Thomas R. Chudley

Greenland’s melting ice sheet is the world’s single biggest contributor to sea level rise. In a new study, scientists used drones to show how water flows through cracks in the ice, which creates dramatic waterfalls—and could be making it more unstable.

The report, published in the Proceedings of the National Academy of Sciences on Monday, was the first one to ever use drones to observe how cracks form under meltwater lakes on Greenland’s ice shelf.

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“To date, most observations are provided by satellites,” said lead researcher Dr Poul Christoffersen. “These allow us to see what’s happening over the whole ice sheet, but drone-based observations give a lot more nuance to our understanding of these lake drainages.”

The team found that the cracks create caverns, or moulins, which the meltwater moves through. Since the ice sheet is about a kilometer (0.62 miles) thick, the flow of water into the moulins can create the world’s largest waterfalls.

In the summer of 2018, the UK researchers captured footage of a meltwater lake on the ice sheet’s draining five million cubic meters of water from the ice sheet’s surface in just five hours. That’s enough water to fill 2,000 Olympic swimming pools.

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The water flowed through a moulin to the base of the ice sheet, reducing the volume of the lake by two thirds. The quick flow of water from the lake also helped raise the surface of the ice by a little more than half a meter (1.8 feet). That could mean that the water flowed into hydrofractures, or spaces between the ice sheet and its base.

When water flows to an ice sheet’s base, it “increases the tensile stresses near other lakes, triggering further hydrofractures,” the scientists wrote. In other words, it can trigger a chain reaction, creating more cracks, loosening the bottom of the sheet and making it even more unstable. This can cause the glaciers to break off the ice sheet—especially when it’s draining fast. And the researchers say that the kind of fast drainage they observed may be happening much more often than scientists previously thought.

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Photo: Thomas R. Chudley

“It’s possible we’ve under-estimated the effects of these glaciers on the overall instability of the Greenland Ice Sheet,” the study’s co-lead author and drone pilot Tom Chudley said in a statement.

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Earlier this year, scientists found that Greenland’s ice sheet it may have reached a tipping point where the cool periods of weather just pause the ice melting, instead of refreezing the ice like it used to. In 2012, the ice sheet lost more than 400 billion tons of ice—nearly four times what it lost in 2003. Other recent findings show ice is melting six times faster than it was in 1980. And just this past summer, a widespread heat wave caused ice sheet to melt at an unprecedented rate, shedding 12.5 billion tons ice in one day.

As the Earth continues to warm, it will be increasingly important to see how the Greenland ice sheet will respond. In an attempt to understand its response, the researchers are now using drilling equipment to explore how climate change might affect the ice sheet’s drainage systems. But last summer’s disappear lake serves as quite a warning.

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“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,” said Chudley.

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