The disintegration of Greenland’s ice sheet is currently the biggest contributor to global sea level rise, and the breakneck pace of its climate change-driven dismemberment doesn’t look set to slow down. It shed 3.8 trillion metric tons of ice between 1992 and 2018, a staggering amount in such a short timeframe. If the entire ice sheet completely melted, sea levels would rise by about 20 feet.
It is, however, a complicated mass of ice, providing scientists with so much information that it has been difficult for them to process and interpret it all.
A new research endeavor takes advantage of cutting-edge technology to generate an unprecedentedly detailed dataset for the glacial titan. Twenty years of shots taken by NASA’s Terra satellite—18,000 images, covering the entire ice sheet, documenting every single day of every single melt season from 2000 to 2019—has provided a time-travelling atlas of the proliferation of ominous, yet eerily beautiful, lakes of meltwater atop it.
The images would normally take a scientist over a year to process. But scientists took advantage of Google Earth Engine, whose cloud computing platform allowed them to handle all that data in just five days.
The chronicle revealed that the number of lakes on the ice sheet has increased on average by 27 percent over the past two decades. The last nine years were full of extremes: 2019 had the fifth highest count of lakes during that time, with 2011, 2012, 2015, and 2016 containing even more. Presenting their yet-to-be-published work at the annual gathering of the American Geophysical Union in San Francisco, California this week, the scientists said that 2019’s hot Arctic summer, which peppered the ice with meltwater lakes, could be Greenland’s “new normal.”
It wasn’t just a numbers game; the locations of some of the lakes also caught their eye. The ice sheet is a bit like a dome, rising up to colder heights as you head inland. The images found that lakes were appearing at 6,600 feet in the ice sheet’s elevated interior—well away from the ice mass’s fingery edges that jut out into the ocean—a place they weren’t expected to appear until 2050. At such elevations, the lakes can drain through cracks down to the ice sheet’s underbelly, providing a lubricating layer and potentially causing it to flow faster, hastening its instability and ultimate demise.
There are many complex processes at work here and far more research is required to determine how the appearance of these high-altitude lakes will affect the ice there. But if lakes are appearing inland at higher elevations, it is a clear sign that the ice sheet as a whole is getting warmer. (See the routes opening up as the Arctic melts.)
“It’s very much a canary in the coal mine for climate change,” says Tom Chudley, a doctoral student studying the Greenland ice sheet at the University of Cambridge. “Another one to add to the pile.”
Sinister pools of cerulean
Meltwater lakes appear when temperatures heat up enough that the ice thaws and pools, which makes them “quite a nice visual indicator of how much melt is actually going on in the ice sheet,” says James Lea, a glaciologist and UKRI Future Leader Fellow at the University of Liverpool who presented the research at AGU.
Although water from the lakes doesn’t always drain through the ice, they are capable of doing so. Water is denser than ice, so if it hits a narrow crack and there is enough water there, the pressure can force the crack open. That process, known as hydrofracturing, often doesn’t stop once it gets going, so meltwater lakes can generate giant waterfalls that go right through to the bedrock under the ice sheet. And, says Chudley, these watery highways can stay open for years.
What effects the meltwater has on the ice sheet’s movements is still somewhat unclear. In some cases, at lower elevations closer to the coast, the complex hydrological motions may slow the flow of the ice sheet.
At higher elevations, it’s thought that when meltwater pushes through cracks onto the bedrock it can lubricate the ice above it, a bit like the air jet at the base of a hovercraft. That can potentially give the ice sheet’s shoreward migration a short-term speed boost.
Many processes are operating between the ice sheet and the bedrock, creating some level of uncertainty as to whether or not this drainage is helping various sections of the ice sheet to flow or not. But it is still a worrying sign whenever lakes appear at higher elevations in the ice sheet’s interior because they are supplying meltwater to areas that don’t normally see it. If those waterfalls do boost the flow of ice, parts of the ice sheet will be delivered to lower elevations. The heart of the ice sheet subsequently thins, and lower-altitude, warmer air drifts over it, cooking the ice further, setting up a dangerous feedback cycle.
Knowing how many lakes are appearing high on the sheet, then, is of paramount importance.
New data dawns
Research has been pointing toward a proliferation in the number of meltwater lakes, and a recent study by Chudley suggests that scientists have been underestimating how many of them are draining into the ice sheet. This new imagery—taken by the Terra satellite’s Moderate Resolution Imaging Spectroradiometer, or MODIS instrument—has allowed scientists to track the development of those lakes with more detail than ever before.
The idea of processing 18,000 images so rapidly only a decade ago would have been seen as madness. As the processing power of cloud-based Google Earth Engine and similar platforms are now demonstrating, “we’re well and truly in the age of big data,” says Chudley.
The 27 percent increase in the lake population is worrying, but not surprising, because over the years, scientists have themselves seen these lakes getting larger, appearing earlier, and higher up. Past work has sometimes required a bit of extrapolation to determine long-term trends, but this new data show that “none of this is hypothetical anymore,” says Mike MacFerrin, a glaciologist at the University of Colorado, Boulder, who was not involved with the work.
“It’s right there,” he says. “It’s happening and it’s not slowing down.”
The increasing number of lakes, including those higher up, aren’t just appearing because the atmosphere is warming. Meltwater normally filters down into a sponge-like snowy layer within the ice sheet. But a recent study by MacFerrin found that the cavity-laden snow band is being overridden by a far less permeable layer of freezing meltwater, preventing surface melt from percolating down and giving it a chance to pool into sizeable lakes.
An awakening of ice giants
More research is required to understand all of these observations, says Lea. But already, it is raising important questions: What happens when lakes keep appearing in increasingly higher places? What does that mean for the stability of the ice sheet?
Punching a lot of warm water into Greenland’s interior ice sheet is an understudied phenomenon, but it’s probably not a good thing. “It is a little terrifying to see,” says MacFerrin.
There is also not a simple one-to-one relationship of more meltwater runoff matching up with more lakes. But it is clear that there are more lakes there than there used to be, a trend that will continue in a warming world. And what we are seeing in Greenland is also transferrable to places like the West Antarctic ice sheet, which Chudley describes as “a sleeping giant of sea level rise.”
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December 11, 2019 at 10:44PM
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Newly spotted lakes on Greenland ice sheet are speeding up its demise - National Geographic
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