“We have never observed an ice sheet behaving this way before. It’s unprecedented in human scientific history.” – Kristin Poinar, Glaciologist, University of Buffalo
Greenland is, for the most part, a very moist environment (with the exception of northern Greenland which until recently had been a dry environment).
The ice sheet works like a sponge absorbing the meltwater of Summer. Now the slushie has turned into a popsicle. Massive ice slabs have formed that prevents percolation into the slushy layers of snow known as firn. This process is worsening runoff into the sea (as well as a lack of reflectivity of solar heat back to space).
When the remnants of Europe’s second summertime heatwave migrated over Greenland in late July, more than half of the ice sheet’s surface started melting for the first time since 2012. A study published Wednesday in Nature shows that mega-melts like that one, which is being amplified by climate change, aren’t just causing Greenland to shed billions of tonnes of ice. They’re causing the remaining ice to become denser.
“Ice slabs”—solid planks of ice that can span hundreds of square miles and grow to be 15 metres thick—are spreading across the porous, air pocket-filled surface of the Greenland ice sheet as it melts and refreezes more often. From 2001 to 2014, the slabs expanded in area by about 64,750 square kilometres, forming an impermeable barrier the size of West Virginia that prevents meltwater from trickling down through the ice. Instead, the meltwater becomes runoff that flows overland, eventually making its way out to sea.
As the ice slabs continue to spread, the study’s authors predict more and more of Greenland’s surface will become a “runoff zone,” boosting the ice sheet’s contribution to global sea level rise and, perhaps, causing unexpected changes.
“We're watching an ice sheet rapidly transform its state in front of our eyes, which is terrifying,” says lead study author Mike MacFerrin, a glaciologist at the University of Colorado, Boulder.
Stone continues noting that increased runoff is not the only consequence of the phenomenon.
“And so, if we start getting these ice slabs forming near the ice sheet’s surface, it could potentially…cause the ice sheet to absorb more solar radiation and warm up,” she says. “And that would create more ice slabs.”
And runoff from ice slabs doesn’t have to flow into the ocean, said Indrani Das, a glaciologist at Columbia University who wasn’t involved in the study. She worries about how it could seep into the large crevasses that exist at lower elevations on the ice sheet. From there, the runoff could, potentially, flow all the way down to bedrock, lubricating the zone where the ice makes contact with it.
“That could make the ice sheet flow faster,” Das says, which could cause glaciers to spill their contents into the ocean more quickly, like ice cream sliding off a piece of cake.
To understand how the growing ice slabs affect runoff, it's helpful to know that the Greenland Ice Sheet has three zones.
In the coastal melting zone, snow that falls in winter melts away completely, exposing darker ice that melts quickly during warm summer weather. High on the ice sheet is the permanent dry snow and ice zone, where there has been little melting, at least up to now.
Some of the ice slabs have thickened from 5 meters to 7 meters in just five years, and they are expanding inland.
The thick layers prevented water from percolating down, and so the scientists found slush fields and runoff channels on the surface that sent torrents of water rushing toward the ocean.
p class=”is-empty-p”>“Up until recently, nobody was paying attention to the cold slow moving ice in the interior, but now it's waking up, turning into warm wet ice very quickly,” MacFerrin said. “These interior regions of Greenland didn't use to run off, so you didn't have to worry about them in terms of sea level rise.”
The entire Arctic system, from its thawing permafrost to its diminished sea ice, is in a state of transition as global temperatures rise, said Mark Serreze, director of the National Snow and Ice Data Center.
“These ice slabs are not the sort of thing we were thinking about 20-30 years ago. Basically, the ice slabs are making the runoff process more efficient. Every time we look, we see changes that suggest more rapid melting of the Greenland ice sheet than we thought 20 years ago.”