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The pitches are caused by wind striking snow dunes, and it's an eerie sort of song. But, the researchers argue, it's also an early warning sign for one of the nightmare scenarios in climate change: the disintegration of Antarctica's largest ice shelf, and consequent slide of glaciers into the ocean.
The song slows down when snow begins in the ice shelf's top layers. That's already happened.
The ice warbled to itself for centuries: a discordant song whose verses told the stories of cold winds and shifting snow dunes vibrating across Antarctica.
It was not music as we think of it. Days or months might pass between each tonal shift, composed of notes so low and slow they were inaudible to human ears. But if you could lie for 1,000 years on the ice Ross Shelf Shelf and feel every minute shiver that passed through it – if you were snow itself – then you would know the chorus.
In January 2016, the song went flat.
Sped up thousands of times in the frequency range of human hearing, it sounds like it's way to the end of the world. A song that warned of melting snow.
If the worst fears of climate scientists come true – if in some particularly warm month this century, the 500-mile-long Ross Ice Shelf collapses like a ruined border wall, allowing Antarctica's interior glaciers to flow past it into swelling oceans – we might see little of the calamity's beginning.
When a smaller ice shelf collapsed on the other side of West Antarctica in January 2002, we were blind.
"Scientists monitoring daily satellite images of the Antarctic Peninsula watched in amazement as the whole of Larsen B Ice Shelf splintered and collapsed in just over one month," NASA wrote in its memorial to that 10,000-year-old platform of ice.
"It collapsed between pictures of a satellite," Julien Chaput, a geophysicist at Colorado State University, told The Washington Post. "One picture, it was there, The next, it was not."
But the ice shelf was sick long before its spectacular death. As Chaput explained, the early stages of disintegration are insidious and largely invisible to satellites.
Repeat heat waves cause the carpet of snow atop the ice shelf to melt and refreeze. With each refreeze, the snow gets harder. Eventually, it gets so hard that the pools of water form on the snow and trickle downward, carving tunnels in the snow to reach the ice beneath.
The ice weakens like a rotting boat hull under the meltwater's assault. It cracks. Only one of the following is the extent of the satellite, when the entire shelf – ice, snow and all – breaks apart and dissolves into the ocean within days.
This is, to put it mildly, a lousy warning system for the end of the world we know it.
But as Chaput and his team demonstrated in a paper published by the American Geophysical Union last week, it will be long before it shows them to us.
The discovery was "a complete accident," Chaput said. No one expected ice to sing.
Several years ago, a different team of researchers installed dozens of seismic stations across the Ross Ice Shelf. Like many climate scientists, they were interested in the idea of having the world go by in 2002, titanic glaciers behind it would be free to escape the mainland of Antarctica, eventually raising ocean levels by several feet.
"For now, the Ross Ice Shelf seems to be stable," Chaput said. "But that could change very rapidly and without warning."
The seismic stations are designed to measure the Earth's crust and mantle are doing beneath the ice – massive vibrations on the scale of earthquakes.
But as a result, you can see it in the sine waves: a subtle song, vibrating through the top layers of snow.
"You had these pitches, these incredibly defined tones, persist and defined at each station," he said. "They'd change all the time, with changes in the air and weather events and wind events."
Even the movement of a snow, Chaput said. It was an old record of phonograph records, humming with the rustle of the atmosphere above.
The notes are hovering around 5 hertz, about four times lower than human ears can detect. But Chaput could easily speed up to hear – long-running rhythms into minutes or seconds.
That's how it was able to hear what happened in early 2016 – when an especially warm summer came to Antarctica and the phonograph skipped.
Chaput did not discover the great melt event of January 2016. As Chris Mooney wrote in The Washington Post
The two-week melt left nothing so obvious on the surface of the Ross Ice Shelf. Rather, it's a patch of snow the size of Texas wet and slushy as the air temperature rose to above freezing. Scientists detected it at first through the presence of vapor, Mooney wrote, then used microwave satellites to confirm the damage.
But when in the music of the snow, the melting was impossible to miss.
At seismic stations across the ice shelf, the warbling vibrations grew quiet. Notes stretched out into a long drone at some locations, like a tornado siren going off. To Chaput, it sounds like a two-week-long groan.
"It does not sound super happy to me," he said.
The music of the ice, it was explained, is made by the wind and snow by the snow – called a firn. "Snow is 80 percent air, with flaky bonds between crystals," Chaput said. "As they get weaker, the velocity of the travels gets lower, so the tones go down.It both lowers and gets quieter."
All this might simply mean Chaput found a depressing soundtrack for the melting of an ice cap. But as described in his paper, the music also contains a measurement tool – something like a sonogram for the health of snow and ice in future warming evens, of which he expects many.
That does not mean we'll like what we hear.
The ancient warble of the Ross Ice Shelf turned back to the heat wave in late January, as the watery snow refroze and crystals reforged their bonds. But at many of the listening stations, it no longer sounds the same. The warble now has something like a rasp.
"You can see the physical impact," Chaput said. "When it gets cooled again, the hunt and rebounds in some ways, but not entirely."
It does not know whether the Ross Ice Shelf will regain its original structure and voice, or whether it has been permanently erased.
For now, however imperfectly, it continues to sing.
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This article was written by Avi Selk, a reporter for The Washington Post.
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