Enormous lakes atop the Greenland ice sheet have a habit of suddenly vanishing: One moment they’re there, and the next they’re rapidly draining through holes in the ice.

Some scientists believe these events can increase the rate at which ice is being lost into the ocean. That’s a big concern for Greenland, where melt rates are currently at the fastest they’ve been in 12,000 years—and still speeding up.

Until recently, scientists believed these drainage events happen almost entirely during the summer, when the ice is melting and the lakes are growing. It seemed less likely during the winter, when much of the ice should be refreezing.

But new evidence suggests they can happen all year round.

new study, published Wednesday in the journal The Cryosphere, finds evidence for at least six winter drainage events between 2014 and 2017.

The study relies on a special kind of radar data known as “backscatter”—it’s essentially a measurement of the waves reflected back to the satellite emitting the radar signal. Backscatter tends to be higher with objects that are more reflective, such as snow and ice. That means a liquid lake should produce less backscatter than a frozen surface.

The researchers, Corinne Benedek and Ian Willis of the University of Cambridge, first combed the data to find lakes on top of the Greenland ice sheet. Then, they looked for instances in which backscatter over those lakes suddenly increased during the winter months. That would suggest that the liquid water in these lakes quickly drained away, leaving their frozen lake beds behind.

They found six examples, all taking place between November and February.

Additional data provided extra evidence for at least three of these events. The researchers used a special imaging method to estimate the way the lakes changed in volume after they drained, demonstrating that they really did shrink.

The study suggests that Greenland’s lakes don’t always refreeze in the winter. Some of them clearly retain some liquid water.

The next question is how widespread these winter drainage events really are.

“We know now that drainage of lakes during the winter is something that can happen, but we don’t yet know how often it happens,” said Willis in a statement.

They also aren’t totally sure what’s causing the winter drainage events.

In the summer, scientists believe that lakes may exert increasing pressure on the ice as they grow bigger. This added pressure can eventually cause cracks to open up in the ice. Other studies have suggested that the movement of the ice sheet itself, as it shifts and slides on top of the bedrock, can cause new cracks to form. More research is needed to figure out what happened to the winter lakes.

Scientists are still figuring out exactly how rapid drainage events affect the Greenland ice sheet, both in summer and winter.

When lakes drain, the liquid water often rushes straight to the bottom of the ice sheet. Some studies suggest that this can have a kind of lubricating effect, making it easier for the ice to slide over the bedrock beneath it. When that happens, some experts say, it can speed up the rate at which melting glaciers crumble into the ocean.

There’s still some debate about it. Some scientists believe that other processes may balance it out in the long run.

The U.N. Intergovernmental Panel on Climate Change’s last major report, published in 2014, suggested that drainage events may temporarily speed up the flow of ice where they occur but that the effect is short-lived. It also suggests that if these events become more common over time—a possibility, as melting increases on the surface of the ice sheet—than the slipping effect may subside.

In either case, figuring out just how often these events are happening is an important step. It can help scientists make better projections about the future of the Greenland ice sheet as the Arctic continues to warm.

“We don’t yet know how widespread this winter lake drainage phenomenon is, but it could have important implications for the Greenland ice sheet, as well as elsewhere in the Arctic and Antarctic,” Willis said.

Reprinted from E&E News with permission from POLITICO, LLC. Copyright 2021. E&E News provides essential news for energy and environment professionals.