For the first time ever, scientists have shown that lakes on Greenland’s ice sheet can drain during the winter months, in a phenomenon that could accelerate the rate of glacial melt.
The rate at which the second largest ice sheet in the world is draining into the northern Atlantic ocean may be occurring faster than we think, according to new research published in the Cryosphere on Wednesday.
As the new paper shows, water that collects on the surface of the Greenland ice sheet during the summer can remain in a liquid state during the winter and leak through cracks that appear along the surface, sending it down to the base below. The drained water then acts like a greasy lubricant, increasing the speed at which ice shelf can move.
That’s not an encouraging finding in this, the era of human-induced climate change where Greenland is already losing six times more ice than it was in the 1980s.
“One of the unknowns in terms of predicting the future of the ice sheet is how fast the glaciers move—whether they will speed up and if so, by how much,” Ian Willis, a co-author of the study and a research scientist from the Scott Polar Research Institute at the University of of Cambridge, explained in a statement.
“The key control on how fast the glaciers move is the amount of meltwater getting to the bottom of the ice sheet, which is where our work comes in.”
Scientists had previously shown that Greenland’s surface lakes form and drain during the summer, which they learned through field studies and optical satellite data. As the weather warms, water accumulates on top of the ice sheet, where it can linger for weeks and months.
This water will sometimes disappear in a matter of hours owing to a geological phenomenon known as hydrofracturing. When these cracks suddenly appear, millions of cubic meters of water—along with their associated warmth—are suddenly flushed to the bottom of the ice sheet.
Willis, along with his lone co-author, PhD student Corinne Benedek, also from Scott Polar Research Institute, sought to learn more about these lakes and what happens to them during the dark winter months—dark being the key word; Greenland is cast in near-total darkness in winter, making observations very difficult.
“The thermal data showed me that liquid water can survive in the lakes throughout the winter,” explained Benedek in the press release. “Previous studies using airborne radar had also identified lakes buried a few meters beneath the surface of the ice sheet in the summer.
Both of these things got me thinking about ways to observe lakes all year long. The optical satellite imagery we normally use to observe the lakes isn’t available in winter, or even when it’s cloudy.”
To overcome this hurdle, Willis and Benedek collected radar backscatter data, in which a satellite receives the bounce-back of its own radar signal. This allowed the team to measure changes in the surface lakes, even without the benefit of regular sunlight.
The duo developed an algorithm to “examine spatial and temporal variations in microwave backscatter from Sentinel-1 satellite synthetic aperture radar (SAR) imagery,” which they did to “document the location and timing of six separate lake drainage events over three different winters,” as the authors wrote in their study.
This technique proved useful, as the chosen microwave wavelength penetrated clouds and darkness, and because the SAR instrument was able to pick up water and ice as distinct signatures.
The researchers confirmed the winter lake drainage and offered estimates of how much water is getting lost by using optical data gathered by the Landsat 8 satellite during previous and subsequent melt seasons.
In total, the scientists surveyed 11,758 square miles (30,453 square kilometers) of the Greenland ice sheet from late 2014 through to early 2017. The results showed all six lakes studied—whether buried or covered in a layer of ice—were leaking during the winter months.
“The first lake I found was surprising,” Benedek said. “It took me a while to be sure that what I thought I was seeing was really what I was seeing. We used surface elevation data from before and after the events to confirm what we were thinking.”
To which she added: “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.”
The pace of glaciers’—which tumble down from the ice sheet to the sea—movements slow down during the winter months, “but they’re still moving,” said Willis.
These movements are apparently causing the leaky fractures to develop up top, but the scientists 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,” he added.
The new study, while interesting and illuminating, remains incomplete. As the authors themselves note in the paper, future research is needed to determine if the winter lake drainage is happening elsewhere in Greenland, and if it has happened during other years.
The scientists are also hoping to acquire a better understanding of the “triggering mechanisms,” and how the water cycle and chemistry (both geological and biological) are affected. And of course, they’d also like to know if “winter lake drainage will become more prevalent under future climate warming scenarios.”