Antarctica's vast Ross Ice Shelf has been captured "singing" in remarkable new observations – something that could help scientists to monitor its changes from afar.
Winds blowing across snow dunes on the shelf cause the Texas-sized ice slab's surface to vibrate, producing a near-constant set of seismic "tones" described in a just-published study.
The ice shelf – which aircraft flying to New Zealand's Scott Base land upon in the summer season – is Antarctica's largest and is fed from the icy continent's interior that floats atop the Southern Ocean.
Most of it lay in the Ross Dependency, claimed by New Zealand.
It buttressed adjacent ice sheets on Antarctica's mainland, impeding ice flow from land into water, like a cork in a bottle.
When ice shelves collapsed, ice flowed faster from land into the sea, raising sea levels.
Ice shelves all over Antarctica have been thinning, and in some cases breaking up or retreating, because of rising ocean and air temperatures.
Prior observations have shown that Antarctic ice shelves can collapse suddenly and without obvious warning signs, which happened when the Larsen B ice shelf on the Antarctic Peninsula abruptly collapsed in 2002.
To better understand the physical properties of the shelf, researchers buried 34 sensitive seismic sensors under its snowy surface.
The sensors allowed the researchers to monitor the ice shelf's vibrations and study its structure and movements for over two years, from late 2014 to early 2017.
Ice shelves are covered in thick blankets of snow, often several metres deep, that are topped with massive snow dunes, like sand dunes in a desert.
This snow layer acts like a fur coat for the underlying ice, insulating the ice below from heating and even melting when temperatures rise.
When the researchers started analysing seismic data on the shelf, they noticed something odd: its fur coat was almost constantly vibrating.
When they looked closer at the data, they discovered winds whipping across the massive snow dunes caused the ice sheet's snow covering to rumble, like the pounding of a colossal drum.
They also noticed the pitch of this seismic hum changed when weather conditions altered the snow layer's surface.
They found the ice vibrated at different frequencies when strong storms rearranged the snow dunes or when the air temperatures at the surface went up or down, which changed how fast seismic waves travelled through the snow.
"It's kind of like you're blowing a flute, constantly, on the ice shelf," said Julien Chaput, a geophysicist and mathematician at Colorado State University, and lead author of the new study.
Just like musicians could change the pitch of a note on a flute by altering which holes air flowed through or how fast it flowed, weather conditions on the ice shelf could change the frequency of its vibration by altering its dune-like topography, Chaput explained.
"Either you change the velocity of the snow by heating or cooling it, or you change where you blow on the flute, by adding or destroying dunes," he said.
"And that's essentially the two forcing effects we can observe."
The hum was too low in frequency to be audible to human ears, but the new findings suggest scientists could use seismic stations to continuously monitor the conditions on ice shelves in near real-time.
Studying the vibrations of an ice shelf's insulating snow jacket could give scientists a sense of how it was responding to changing climate conditions, said Douglas MacAyeal, a glaciologist at the University of Chicago.
Changes to the ice shelf's seismic hum could indicate whether melt ponds or cracks in the ice were forming that might indicate whether the ice shelf is susceptible to breaking up.
"The response of the ice shelf tells us that we can track extremely sensitive details about it," Chaput said.
"Basically, what we have on our hands is a tool to monitor the environment, really. And its impact on the ice shelf."
If the Ross Ice Shelf collapsed, scientists say it would have major implications for global sea level rise.
The wider West Antarctic Ice Sheet stores an equivalent 3.2m of sea level rise, while the much larger East Antarctic Ice Sheet is estimated to contain 58m of equivalent sea level rise.
Before 2012, Antarctica lost ice at a steady rate of 76 billion tonnes per year – a 0.2 mm per year contribution to sea level rise.
But since then there has been a sharp, threefold increase.
Between 2012 and 2017 the continent lost 219 billion tonnes of ice per year – a 0.6 mm per year sea-level contribution.
And between 1992 and 2017, the Antarctic Ice Sheet lost about three trillion tonnes of ice - equivalent to around 8mm of mean sea-level rise.
During this 26-year period, ocean-driven melting led to a tripling of ice-loss rates from West Antarctica, from 53 billion to 159 billion tonnes per year.
The rate of ice loss from the Antarctic Peninsula increased from about 7 billion to 33 billion tonnes per year as a result of ice-shelf collapse.
In West Antarctica, ice shelves were being eaten away by warm ocean water, and those in the Amundsen and Bellingshausen seas are up to 18 per cent thinner than in the early 1990s.
At the Antarctic Peninsula, where air temperatures had risen sharply, ice shelves have collapsed as their surfaces have melted.
Altogether, 34,000 sq km of ice shelf area had been lost since the 1950s.