Just yesterday, Nature World News reported on Greenland's mysteriously vanishing lakes, which can drain entirely in just a matter of a few hours. But now, a subsequent study is saying that while warming temperatures have created more of these supraglacial lakes, they are not likely to worsen Greenland's contribution to sea level rise.
Each summer, Greenland's ice sheet - measuring three times the size of Texas - begins to melt. Pockets of melting ice form hundreds of large, supraglacial lakes on the surface of the ice. Many of these lakes drain through cracks and crevasses in the ice sheet, called moulins, creating a liquid layer over which massive chunks of ice can slide. This natural conveyor belt can speed ice toward the coast, where it eventually falls off into the sea.
"It's essentially a check on the inner ice starting to move along this fast conveyor belt," Laura Stevens, a graduate student in MIT's Department of Earth, Atmospheric and Planetary Sciences, said in a statement. "One of the big questions about the Greenland ice sheet is how much of the ice sheet [travels towards the coast] during the summer, and how much is entering into the ocean. Our hypothesis that inland lakes are less likely to drain locally suggests the ice sheet in that region won't speed up. That's good news, at least for the time being."
During the study, researchers at MIT, Woods Hole Oceanographic Institution (WHOI), and elsewhere observed the drainage of North Lake, a 10-meter-deep, 2-kilometer-wide lake on the western side of Greenland. They found that each summer, the lake, like many others, drained quickly, completely emptying in just a couple of hours.
Based on GPS data, they figured out that just six to 12 hours before the lake drained, some water from the lake trickled to the bottom of the ice sheet through moulins. Water would subsequently collect at the bottom of the ice sheet, pushing up on the surface of the ice, triggering cracks that would cause the lake to disappear.
"That water will cause the ice above it to be jacked up like a dome, and then you've created tension at the surface that allows the ice sheet to start to fracture," Stevens explained. "Once a fracture gets beneath the lake, then water just starts to pour into that fracture, and the whole thing goes."
Like the previous study, these findings are worrisome and suggest that all this water being dumped into the ocean is likely going to raise sea levels. However, North Lake is located within the coastal region of Greenland, where the ice sheet is thinner. That means along Greenland's coast there are more moulins that can move water to the base of the ice sheet.
Lakes further inland, on the other hand, are situated at higher elevations and form over thicker ice. According to the new research, it's unlikely that inland lakes would drain, as there are fewer moulins near inland lakes. Without these channels, large cracks wouldn't form in the lake basin, and lakes would stay intact, simply refreezing in the winter or overflowing into a surface stream.
"It is critical to understand how and why these lakes drain in order to predict how much mass the ice sheet will contribute to sea-level rise in our warming climate," Stevens concluded. "We find that while lakes are forming inland, they probably won't drain by this...mechanism. The inland lakes will more likely drain their water via surface stream runoff, which transfers the water to the bed in more coastal areas of the ice sheet. So, while we see inland ice beginning to speed up as more melt happens inland, the draining of inland lakes likely won't exacerbate the situation."
These latest findings were published in the journal Nature.
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