Greenland's Glacial Lakes are Mysteriously Vanishing, and Here's Why
Greenland's many supraglacial lakes have been seen suddenly and mysteriously draining as climate change causes this region to warm, and now scientists have finally explained the mechanism behind this phenomenon.
The findings were published in the journal Nature.
The Greenland Ice Sheet, the second largest ice sheet in the world, has long been of concern to scientists as rising temperatures continue to melt its glaciers. Consequently, this meltwater forms thousands of lakes atop the ice sheet each spring and summer. And yet, these vast bodies of water can empty in a matter of just a few hours.
When they drain, they send torrents of water to the base of the ice sheet thousands of feet below, lubricating the interface between rock and ice. That allows the ice sheet to flow faster to the ocean and discharge ice into ocean, which causes sea levels to rise faster.
So what exactly is causing this sudden drainage?
In 2008, scientists from Woods Hole Oceanographic Institution (WHOI) and the University of Washington documented for the first time how the icy bottoms of lakes atop the Greenland Ice Sheet can crack open suddenly. Initially, they thought the sheer weight of water was putting too much pressure on the ice to the point that it cracked. However, this theory did not explain why some lake bottoms cracked while others did not.
"Our discovery will help us predict more accurately how supraglacial lakes will affect ice sheet flow and sea level rise as the region warms in the future," lead author Laura Stevens from WHOI said in a press release.
Stevens and her colleagues deployed a network of 16 GPS units around North Lake, a 1.5-mile-long supraglacial lake in southwest Greenland. This lake was among those seen suddenly cracking and draining. They recorded ice movement before, during and after three rapid lake drainages in summer 2011-2013.
It turns out that tension from below the ice sheet, and not above it from the lakes, was causing cracks in the ice. (Scroll to read on...)
According to the data, in the 6 to 12 hours before the lake cracked and drained, the ice around the lake moved upward and slipped horizontally. The scientists say that meltwater had begun to drain through a nearby system of moulins - vertical channels through the ice - which connected the surface to the base of the ice sheet 3,215 feet below. The accumulating water creates a bulge that floats the entire ice sheet, creating tension at the surface underneath the lake. The stress builds up until it is relieved by a sudden large crack in the ice below the lake.
"In some ways, ice behaves like Silly Putty - if you push up on it slowly, it will stretch; if you do it with enough force, it will crack," Stevens explained. "Ordinarily, pressure at the ice sheet surface is directed into the lake basin, compressing the ice together. But, essentially, if you push up on the ice sheet and create a dome instead of a bowl, you get tension that stretches the ice surface apart. You change the stress state of the surface ice from compressional to tensional, which promotes crack formation."
Though once this tension initiates the crack, the volume of water in the lake does play a role. Lakewater pours into the crack and widens it even more, keeping it filled with water all the way to base of the thick ice sheet and forming what are called hydrofractures.
"You need both conditions - tension to initiate the crack and the large volume of water to amplify it - for hydrofractures to form," Stevens said. Meaning, the weight from meltwater alone is not enough to cause supraglacial lakes to suddenly drain.
This entire underlying mechanism is so impactful, it can reportedly drain more than 11 billion gallons of water out of North Lake in about 90 minutes. That's even faster than the water goes over Niagara Falls.
By better understanding these sudden drainages scientists hope to determine how supraglacial lakes will affect sea level rise as climate conditions shift in the future.
For more great nature science stories and general news, please visit our sister site, Headlines and Global News (HNGN).