Greenland’s ice cap lakes can drain huge amounts of water, even in winter

Using satellite data to ‘see in the dark’, researchers have shown for the first time that Greenland’s ice cap lakes drain during winter, a finding that has implications for the speed at which the second largest ice cap in the world flows towards the ocean.

Researchers at the University of Cambridge used radar data from a European Space Agency satellite to show that even when heat from the Sun is absent, these lakes can release large amounts of water at the base of the ice cap. These “drainage events” are believed to play an important role in speeding up the movement of ice by lubricating it from below.

Previous studies of lake drainage have all been conducted during the summer months, using a combination of direct field observations and optical satellite data, which require daylight.

The approach developed by the Cambridge researchers uses radar “backscatter” – the reflection of waves back to the satellite from which they were emitted – to detect changes in lakes during the winter months, when Greenland is in. almost total darkness.

The results, reported in the newspaper The cryosphere, imply that the “ plumbing ” system under the Greenland ice cap is not only slowly leaking the water from the previous summer, but even in the depths of the arctic winter, it can be “ recharged ” , because large amounts of lake surface water cascade to the base of the ice cap.

Many previous studies have shown that the Greenland ice sheet is losing mass and the rate of loss is accelerating due to melt and runoff.

“One of the unknowns in terms of predicting the future of the ice cap is how fast the glaciers are moving – whether they will accelerate and, if so, by how much,” said the co -author, Dr Ian Willis, of the Scott Polar Research Institute, Cambridge. (SPRI). “The key control over how fast glaciers move is the amount of meltwater that reaches the bottom of the ice cap, where our work comes in.”

Lakes form on the surface of the Greenland ice sheet every summer as the weather warms. They exist for weeks or months, but can pass in a matter of hours due to hydrofracturing, transferring millions of cubic meters of water and heat to the base of the ice cap. The affected areas include sensitive regions of the interior of the ice sheet where the impact on ice flow is potentially significant.

“It was always thought that these lakes only drained in summer, simply because it was hotter and the sun was melting the ice,” said co-author Corinne Benedek, also from SPRI. “In winter, it gets dark and the surfaces freeze over. We thought that the filling of the lakes was what caused them to eventually drain, but it turns out that is not always the case.”

Benedek, who currently holds a doctorate. SPRI candidate, was first interested in what happens to surface lakes in winter when she was a master’s student studying thermal data by satellite.

“The thermal data has shown me that liquid water can survive in lakes throughout the winter,” she said. “Previous studies using airborne radar had also identified lakes buried a few meters below the surface of the ice cap in summer. These two elements led me to think about ways to observe the lakes throughout the year. The optical satellite imagery that we normally use to observe lakes is not available in winter, or even on cloudy days. “

Benedek and Willis developed a method using data from the Sentinel-1 satellite, which uses a type of radar called synthetic aperture radar (SAR). SAR operates at a wavelength that allows it to see through clouds and in the dark. Ice and water read differently using SAR, so they developed an algorithm that tracks sudden changes in SAR backscattering.

Over three winters, they identified six lakes that appeared to drain over the winter months. These lakes were buried lakes or frozen surface lakes. The algorithm was able to identify where the backscattering characteristics of the lake changed markedly between one image and the next recorded 12 days later.

The SAR data was supported by additional optical data from the previous fall and the following spring, which confirmed that lake area had significantly decreased for the six drained lakes. For three of the lakes, optical data along with data from other satellites were used to show that the snow and ice covered lakes collapsed, falling several meters, again confirming that the water had drained. .

“The first lake I found was surprising,” said Benedek. “It took me a while to make sure what I thought I was seeing was really what I was seeing. We used pre and post event surface elevation data to confirm what we thought. We now know that lake drainage during the winter is something that can happen, but we do not yet know how often this happens. “

“Glaciers slow down in winter, but they keep moving,” Willis said. “It must be this movement that causes fractures to develop in certain places allowing certain lakes to drain. We don’t yet know how widespread this phenomenon of winter lake drainage is, but it could have important implications for the Greenland ice sheet, as well as elsewhere in the Arctic and Antarctic. ”