When the remnants of the second European heat wave migrated across Greenland at the end of July, more than half of the ice cover began to melt for the first time since 2012. A study published on Wednesday in Nature shows that such megacities, exacerbated by climate change, are not only allowing Greenland to shed billions of pounds of ice. They cause the remaining ice to become denser.
"Ice sheets" – solid ice sheets that span hundreds of square miles and can become 50 feet thick – spread out over the porous, air-filled surface of the Greenland ice sheet melts and freezes more frequently. From 2001
As the ice sheets continue to expand, the study's authors predict that Greenland's surface will increasingly become a "run-off" zone that reinforces ice Sheet's contribution to global sea-level rise and possibly unexpected changes.
"We see an ice sheet changing its condition quickly before our eyes, which is frightening," says Mike MacFerrin, senior study author, a glaciologist at the University of Colorado, Boulder.
A "turtle shell" for ice
Greenland can easily be thought of as a solid, impenetrable piece of ice. In fact, 80 percent of the ice cover resembles a snow cone: a dust of fresh snow covers a thick layer of old snow, called firn, which is slowly compressed into glacial ice, but still contains many air pockets. When the top of this snow cone melts in the summer, liquid water trickles into the firn and absorbs it like a thirty-meter-thick sponge.
MacFerrin and his colleagues got the first hint that the firn could lose its absorbency in the spring of 2012 as they drilled holes through the firn in southwest Greenland. They began to find dense, dense layers of ice in one core after another just below the seasonal snow layer. It was, says MacFerrin, as if a "turtle shell" had formed over the firn.
MacFerrin and his colleagues immediately wondered if this shell could prevent the ingress of meltwater in the firn.
"That was in May 2012," says MacFerrin, "and July was this record-breaking year of melting, and we've gotten our answer very quickly."
This summer, meltwater from that part of Greenland began to drain for the first time
When the researchers realized that they were witnessing a significant event, they drilled more nuclei across a larger area to determine how large the ice tray was, and found it to be 40 kilometers long
These results, published in Nature Climate Change in 2016, were the springboard for the new study, with MacFerrin and his colleagues now using radar data from the IceBridge air campaign NASA and ground-based surveys created a unique map of ice sheets on the entire surface of Greenland.  Based on modeling results Researchers believe that the shell was widely used in the early 2000s. According to the new analysis, in 2014 it was about 4 percent of the Greenland area. With each summer, when it comes to a strong melting, it thickens and spreads inland to colder, higher soils.
"Every few years, these big melting seasons make a number on the fire," says MacFerrin. "As a result, the whole process grows pretty fast inland."
Sea level rise and unexpected consequences
According to the new study, ice sheets have already increased Greenland's runoff by about 26 percent. So far, the additional outflow has only raised the global sea level by about a millimeter. Greenland now contributes a little less than a millimeter per year to sea-level rise in ice due to a combination of icebergs that break off glaciers and melt that occurs on the surface and on the ground.
However, if the surface of Greenland hardens stronger, the runoff can rise dramatically. In a worst-case scenario where carbon emissions continue to rise until the end of the century, researchers calculated that ice plate proliferation by 2100 would result in a 3-inch sea-level rise and almost 3-fold overall sea ice rise contribution to the ice sheet Inch could increase a third. In a middle-of-the-road scenario where emissions peak in the middle of the century and high emissions peak, the amount of outflow from the interior of the country nearly doubles by the end of the century.
However, a major runoff is just one possible consequence of the transformation in Greenland ice. Kristin Poinar, a glaciologist at the University of Buffalo who was not involved in the study, pointed out that solid ice sheets are not nearly as reflective as bright white snowfall.
"And when we start getting this ice When plates form near the ice, it can cause the ice to absorb more solar radiation and heat up," she says. "And that would produce more ice sheets."
And the runoff of ice sheets does not have to flow into the ocean, said Indrani Das, a glaciologist at Columbia University who was not involved in the study. She worries about how it might invade the great crevices that exist at lower levels on the ice sheet. From there, the runoff could possibly drain down to the bedrock and lubricate the area where the ice is in contact with it.
"This could make the ice sheet flow faster," says What could lead to glaciers Spill their contents faster into the ocean, like ice slipping from a piece of cake.
For Poinar, the most important contribution of the new study is that scientists can improve their predictions about future sea-level rise coastal communities the information they need to prepare. At the same time, the study highlights the fact that the more carbon we emit into the atmosphere, the more likely we are to turn the Earth's northern ice shield into insidious and unexpected ways. And that could have hard-to-predict consequences.
"We have never seen an ice sheet that behaves like this," says Poinar. "It's unprecedented in human history of science."