The Greenland Ice Sheet has reached a point of rapid retreat that it couldn't recover from even if global temperatures stopped rising instantly.
This Arctic ice sheet is the second-largest ice sheet in the world, after the one that blankets Antarctica. It covers the majority of Greenland and melts into the seas via outlet glaciers, which have been rapidly losing ice for decades. Now, new research finds that today's glacial ice loss is 14% greater than it was between 1985 and 1999. The ice sheet is losing approximately 500 gigatons of ice each year, more than is replenished by annual snowfall.
Projecting that trend forward, the ice sheet is set to lose mass 99 out of every 100 years, researchers reported Aug. 13 in the journal Nature Communications Earth & Environment.
"Glacier retreat has knocked the dynamics of the whole ice sheet into a constant state of loss," study co-author Ian Howat, an Earth scientist at The Ohio State University, said in a statement. "Even if the climate were to stay the same or even get a little colder, the ice sheet would still be losing mass."
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Increasing melt
Greenland's ice sheet has long been in peril. In January 2019, researchers reported in the journal Proceedings of the National Academy of Sciences that the ice sheet was losing ice at four times the rate it was in 2003. Other researchers have found that the ice sheet is sliding on its underlying bedrock faster than expected. This sliding can move ice from the cold interior of Greenland to the warmer edges, where it melts faster.
The new study combined three decades of satellite imagery with data from airplane surveys from Operation IceBridge,providing a detailed timeline of ice thickness, ice flow, ice elevation and the location of the calving front of the ice sheet — the line where the glaciers hit the sea and lose ice through the calving of icebergs. This method allowed the researchers to look at both seasonal changes in ice discharge as well as longer trends.
What they found was an increase of about 60 gigatons of ice loss a year between 2007 and 2018 compared with the period between 1985 and 1999. All told, the ice sheet has lost 4,200 gigatons of ice since 1985.
There are regional differences, with some of the accelerating ice retreat between 2001 and 2005 driven by glaciers in the southeastern part of Greenland moving from relatively shallow parts of the seafloor into deeper sections, where warm water can more easily circulate beneath them and melt them.
Ice-free future?
But regardless of the region of Greenland, the retreat of a glacier by 0.6 miles (1 kilometer) is consistently associated with a 4% to 5% increase in the rate of ice discharge from that glacier. Because these sheets are already retreating, further ice loss is baked in, even in a hypothetical situation in which temperatures stop rising.
"Glaciers have been sensitive to seasonal melt for as long as we've been able to observe it, with spikes in ice discharge in the summer," study lead author Michalea King, a researcher at The Ohio State University's Byrd Polar and Climate Research Center, said in the statement. "But starting in 2000, you start superimposing that seasonal melt on a higher baseline — so you're going to get even more losses."
With the ice sheet set to lose more ice than it gains each year, it might seem like the end of the Greenland Ice Sheet. But that future has yet to be written, Potsdam University physicist and climate scientist Stegan Rahmstorf noted on Twitter. Eventually, he said, the ice retreat will pull the ice sheet away from the ocean. At that point, warm ocean waters will stop melting the ice, and the glaciers will stop calving. The Greenland Ice Sheet will become an entirely land-based entity. When that occurs, a new dynamic will set in: It will be a race between the rate of the surface melt of the ice and the annual snowfall that decides when and whether the ice sheet disappears.
"Thus, Greenland ice loss is a real concern — already today it contributes a lot to sea-level rise," Rahmstorf wrote. "And Greenland does have a tipping point where the ice sheet becomes doomed to total loss. But the new paper by King et al. does not say anything about whether this has been crossed."
Originally published on Live Science.
