NASA scientists detected a pulse of melting ice and water travelling through a major glacier in Greenland that was so big that it warped the solid Earth - a surge equivalent in mass to 18,000 Empire State Buildings.
The wave - which occurred during the 2012 record melt year - traveled nearly 15 miles through the Rink Glacier in western Greenland over four months before reaching the sea, the researchers said.
"It's a gigantic mass," said Eric Larour, one of the study's authors and a researcher at NASA's Jet Propulsion Laboratory. "It is able to bend the bedrock around it."
Such a "wave" has never before been detected in a Greenland or Antarctic glacier. The total amount of mass carried in the wave - in the form of either water, ice, or some combination of both - was 1.67 billion tons per month, or 6.68 billion tons overall over four months, the study, published in Geophysical Research Letters, found.
The study was led by the lab's Surendra Adhikari and was also co-authored by Erik Ivins.
"These solitary waves, they're fairly well known in rivers," said Ivins, also a researcher at the Jet Propulsion Laboratory. "Rivers can have inundations upstream where a lot of water is collected, and the water gets bunched up as it's going downstream, and doesn't ever really flatten out, it just remains as this wave and continues down a river."
However, the scientists don't know what the wave actually looked like or precisely what caused it - much of it was occurring below the surface of the glacier. They also don't know precisely what it was made of. "We are losing a combination of water and ice, we don't know what fraction," said Adhikari.
The researchers were only able to detect the wave because a GPS sensor, located in a rocky inland area a little over 12 miles, moved 15 millimeters as the wave went by, pushing down on the Earth's crust and causing a deep indentation.
"The GPS can sense that," explained Larour.
Richard Alley, a glaciologist at Penn State University who was not involved in the study, compared the effect something much more mundane and relatable in our everyday lives.
"Find a bed," Alley said by email. "Put a little piece of tape on the sheet. Put your fist right next to the tape and push down, while watching the tape. The tape will move down as you push down, and also will move horizontally toward your fist just a little. Put your fist farther away and the tape won't move as much. Push harder, and it will move more. While pushing down, slide your fist past the tape and you'll see a pattern of vertical and horizontal motions of the tape."
"A bed isn't exactly the elastic earth, but that's sort of what this team did," Alley continued. "They saw a 'fist' of mass sliding down the glacier past their GPS station, caused by extra meltwater."
The "wave" occurred in the wake of a 2012 summer melting event that saw most of the surface of Greenland become covered with liquid water, and that still has not been surpassed by subsequent warm years. The researchers suspect that some of that meltwater flooded beneath the ice sheet and then pulsed outward through Rink glacier.
"It's really related to the deep interior of Greenland that's full of melt and it's trying to get rid of that melt through gravitational processes," said Ivins.
The study also documented another, smaller "wave" at Rink Glacier in 2010, another major melt year.
Rink is far from the largest glacier in Greenland. It is about 3.4 miles wide at its front where it touches the ocean and a little over half a mile deep in the same location. Researchers have also shown that pulses of meltwater flow out from beneath the glacier in colorful silt-filled plumes, presumably through subterranean channels, which could be how some of this mass exited to the ocean in 2012.
The scale of the pulse, 6.68 billion tons, or gigatons, is still only a fraction of what Greenland contributes to the ocean every year in the form of water and ice. NASA has estimated that Greenland loses 287 billion tons annually at present (though it lost far more than that in the banner melt year of 2012).
Still, the research gives a sense of the tremendous magnitude of the changes now occurring on Greenland which is covered by enough ice to raise sea levels by over 20 feet if it were all to slide into the ocean.
And it pairs with other studies showing that the breaking off of large pieces from Greenland glaciers causes major earthquakes, and that enormous lakes atop the Greenland ice sheet can vanish within hours into its depths.
The study also raises questions about whether more huge ice and water pulses will be seen as the Arctic continues to warm, and Greenland to melt - and thus whether this is how a melting ice sheet exports its mass to the ocean.
But mostly, it's just staggering to contemplate.
If the analogy of 18,000 Empire State buildings isn't striking enough, the researchers offered another. The mass loss through Rink Glacier from the wave, they say, was equivalent to "150 million fully loaded 18 wheelers."
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