The eerie song of Antarctica - melting
Research published last week by the American Geophysical Union documents a chaotic, low-frequency hum across the Ross Ice Shelf - a platform the size of France that floats off the coast of West Antarctica.
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 science: the disintegration of Antarctica's largest ice shelf, and consequent slide of glaciers into the ocean.
The song slows down when snow begins to melt 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 wasn't 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 great Ross Ice 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 into the frequency range of human hearing, it sounded as though the ice's warble faded to something like a dial tone - a moaning dirge that lasted for two of the warmest weeks on record for the polar continent. 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 almost the entire 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 wasn't."
But the ice shelf was sick long before its spectacular death. As Chaput explained it, 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 pools of water form on the snow's surface 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 near the end is the extent of the damage obvious to satellites, 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 as we know it.
But as Chaput and his team demonstrated in a paper published by the American Geophysical Union last week, a wounded ice shelf will sing about its troubles 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 concerned that if the France-size floating ice platform ever collapsed like Larsen B did 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 extremely rapidly and without warning."
The seismic stations were designed to measure what the Earth's crust and mantle are doing beneath the ice - massive vibrations on the scale of earthquakes.
But as Chaput reviewed the data set from late 2014 to 2017, he noticed something in the sine waves: a subtle song, vibrating through the top layers of snow.
"You had these pitches, these incredibly defined tones, persistent and defined at each station," he said. "They'd change all the time, with changes in air temperature and storm events and wind events."
Even the movement of a snow dune could alter the frequencies, Chaput said. It was as if the entire snow bed were grooved out like an old phonograph record, humming with the rustle of the atmosphere above.
The notes hovered around 5 hertz, about four times lower than human ears can detect. But Chaput could easily speed them up enough to hear - compressing days-long rhythms into minutes or seconds.
That's how he was able to hear what happened in early 2016 - when an especially warm summer came to Antarctica and the phonograph skipped.
Chaput didn't discover the great melt event of January 2016. As Chris Mooney wrote in The Washington Post, it disturbed scientists who learned of it at the time.
The two-week melt left nothing so obvious as a lake on the surface of the Ross Ice Shelf. Rather, it turned 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 clouds above the ice shelf, 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 sounded like a two-week-long groan.
"It doesn't sound super happy to me," he said.
The music of the ice, he explained, is made by wind passing over snow dunes and sending vibrations through trillions of compressed ice crystals in the snow bed - called a firn. "Snow is 80 percent air, with flaky bonds between crystals," Chaput said. "As they get weaker, the velocity a wave travels gets lower, so the tones go down. It both lowers and gets quieter."
All this might simply mean that Chaput found a depressing soundtrack for the melting of an ice cap. But as described in his paper, the music also holds potential as a measurement tool - something like a sonogram for the health of snow and ice in future warming evens, of which he expects many.
That doesn't mean we'll like what we hear.
The ancient warble of the Ross Ice Shelf returned shortly after the heat wave ended in late January, as 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 firn partially heals and rebounds in some ways, but not entirely."
He doesn't know whether the Ross Ice Shelf will regain its original structure and voice, or whether it's been permanently damaged, as the Larsen B Ice Shelf must have been long before it broke apart.
For now, however imperfectly, it continues to sing.