A New Threat Lurks in Greenland's Melting Ice
Greenland’s melting ice sheets are contributing more water to the oceans than previously realized, and that’s going to lead to even greater amounts of sea-level rise around the world, according to new research.
The paper, published in the journal Nature Climate Change, reveals something scientists wouldn't have expected just five years ago. “It’s a very rapid change,” said one of the study’s authors, William Colgan of York University in Toronto. “The ice sheet is now losing about 8,000 tons every second, year-round, day in and day out.”
Colgan said the “lion’s share” of that loss—about 5,000 tons per second—comes in the form of meltwater. The ice sheet, like a sponge, used to be able to absorb most of what melted each year because the uppermost layers are composed of tightly packed but permeable snow, as opposed to the impermeable layers of ice much farther below. That porous surface, called “firn,” normally would allow meltwater to sink downward, where it would refreeze and stay within the glacier.
That started changing about a decade ago. The meltwater started staying on the surface, forming massive “rivers” that traveled 30 miles down the ice sheet to the sea.
These rivers have always existed to a degree, but they used to only go about half as far.
“That led us to speculate that the downward motion was being blocked somehow,” Colgan said.
Testing their assumption required spending several brutal weeks on top of the ice sheet before the melting season, where temperatures plunged as low as minus 40 degrees Celsius. “We don’t have heating for a month while we’re out there, and you’re working with ice all day,” he said. “It’s a recipe for getting really cold. We’re as comfortable as we can be, but it’s still closer to the hardships of a polar expedition than to a modern office.”
After three five-week expeditions in 2012, 2013, and 2015—all spent camping in unheated two-person tents—the research team obtained core samples that explained the meltwater rivers. They found that a layer of ice existed where it shouldn’t have.
It all stems from an extreme melt that took place in 2012. When that melted snow refroze, it formed a layer of ice several meters thick in the middle of the firn. Now melting water hits that thick ice layer and can sink no farther. Since it can’t go down, it goes sideways. “The rivers go downhill from the high interior of the ice sheet toward the coast,” Colgan said.
That means the Greenland ice sheet is losing its ability to absorb much of its own meltwater, something that had been assumed under previous climate change models, according to the study.
“All the projections we made assumed the water would keep percolating vertically until it filled up all of that firn space,” Colgan said. “Now we can say that’s probably not going to happen over large areas of the ice sheet.” That, he said, means those earlier projections now underestimate Greenland’s current and future contributions to sea-level rise.
Experts said this new research adds to scientists’ knowledge of the fragility of Greenland's ice sheet. "The ice in Greenland is a big, complicated beast, but every time we have a new result lately it turns out it's melting faster than we thought," said Josh Willis, a NASA climate scientist who was not involved with the study. "We're seeing now more and more ways in which the Greenland ice sheet is disappearing faster than we thought."
The situation might worsen. That new ice layer in the firn is darker than the snow that would normally be there. “That’s important because it absorbs more solar energy and makes the ice sheet melt faster,” Colgan said.
The same phenomenon, Colgan said, is occurring in the Canadian High Arctic. What it means there will need additional study.