Scientists “listen” to glaciers to discover ocean secrets; hear the sounds

Pop, pop, pop: the sound of a glacier. Large, densely packed bodies of ice may look like immobile masses, but they flow and fracture and grow and shrink, and these processes are anything but silent.

In fact, glacial ice is notoriously effervescent. Cubes of it have long been used on cruise ships in Alaska, added to a whiskey or gin and tonic, as the ice emits a unique hiss as it slowly releases the highly pressurized air that has been trapped there for hundreds and sometimes thousands. of years.

But the sounds made by glaciers can be used for more than just novelty ice cubes. With many glaciers around the world shrinking because of the climate crisis, scientists are analyzing these noises to predict exactly how fast the ice is melting and what that could mean for sea level rise.

“Glaciers are undergoing rapid retreat as the atmosphere and ocean warms,” says Grant Deane, a research oceanographer at the Scripps Institution of Oceanography in San Diego, California. “If we want to (predict) sea level rise […] We need a way to monitor these glacial systems, and underwater sound could be an important and interesting way to do that.”

The sound of glacier ice melting in Norway

Deane, who has worked in the field of underwater sound for more than two decades, explains that there are two main processes by which glaciers retreat, and both produce a distinct noise.

There’s the “bright, energetic sound of bubbles exploding in the water as the ice melts,” he says, which he likens to fireworks or sizzling bacon. And then there’s the “deep, ominous rumble” of a detachment event, when a block of ice breaks off the edge of a glacier, which he says sounds like prolonged thunder.

Both events take place on the border where the ice meets the ocean, normally a very dangerous area for humans. This is one reason why acoustics, which can be monitored from afar, can be so valuable.

counting bubbles

Using underwater sound to predict ice melt is still a relatively new field. In 2008, renowned oceanographer Wolfgang Berger co-authored a paper in the science journal Nature Geoscience that proposed using hydroacoustics (sound in water) to monitor the Greenland ice sheets. This inspired Deane — who was already listening to ocean waves crashing to understand how gases transfer from the sea to the air — to turn his ears to the glaciers.

“As the ocean rises, it will affect a lot of our civilization. We need to be able to predict the stability of these ice sheets so that we can plan well and live well as our environment changes,” he says.

Piece of ice breaks off from Hans Glacier terminal in Svalbard

Using underwater microphones to record the sound of calving events on the Hans Glacier in Svalbard, northern Norway, along with time-lapse photography, Deane and Oskar Glowacki of the Polish Academy of Sciences demonstrated that the amount of ice loss can be estimated from the noise produced when an iceberg falls into the ocean. Their findings were published in the journal Cryosphere in 2020.

Air bubbles can also reveal vital information. “If we can count how many bubbles are coming out of the ice in any given unit of time, we can figure out how much ice has melted,” says Deane.

This could be the key to understanding how much ice will melt in the future.

It is simple as an idea, but far from simple in practice. The volume of air bubbles changes depending on how they are released, says Deane, and there is a possibility that noise levels vary between glaciers because of geology and local conditions.

But Deane’s research, predominantly focused on Svalbard, has shown that the intensity of sound generated by air bubbles increases as water temperature rises, showing that volume can be an indicator of ice melting.

“With each expedition, we get closer to the real answer, where we can turn those signals into the numbers we need,” he says.

Several different methods, and some much more developed, already exist for studying glaciers, including seismology, satellite photography, underwater sonar, and ice-penetrating radar. But Deane insists that acoustics can complement these methods and offer some advantages.

Hydrophones (underwater microphones) can be deployed in glacier fjords and monitored remotely over long timescales, he says, and unlike satellite observations, which don’t work in the six months of the year when it’s dark at the north and south poles, the Acoustic technology operates year round and is cheaper than other methods.

ripple effects

Listening to glaciers not only shows us how they are melting, it can also teach us more about the marine ecosystem. Glaciologist Erin Pettit used acoustic technology to determine that glacier fjords are some of the noisiest places on the ocean, thanks to the constant hiss of air bubbles released as the ice melts, and that noise can provide refuge for marine mammals.

Pettit and his team of researchers watched as seals swam into glacier bays in Alaska and Antarctica, possibly to protect themselves from predatory whales that don’t like loud noises.

“The ecosystem changes as the soundscape changes,” she says, adding that if the volume goes up or down, there will be a ripple effect.

“If the glacier leaves the fjord and there is less ice in the water, the sound will slowly subside…then it will no longer be noisy and it will no longer be a safe place for seals.” In this way, acoustic measurements can provide information on the decline of seal populations in these areas.

Pettit notes that the field of acoustics is still in its early days, and to measure long-term changes in glaciers, scientists will need to collect more sound data. But she believes the technology holds great promise.

“Sound doesn’t give us all the answers — but it does provide a relatively inexpensive and easy-to-deploy means of capturing the entire environment of fjords and glaciers,” she says. If the hydrophones were deployed over a long period of time, they could help scientists understand “normal” noise levels from a glacier and detect abnormal sounds that could indicate instability, she adds.

Deane’s goal is to follow in the footsteps of the late Wolfgang Berger and establish long-term acoustic monitoring stations in Greenland to help track the stability of its ice sheet, which could raise sea levels by seven meters if it melted completely.

“I want recording systems that work from south to north around the Greenland glaciers,” he says. “The first job is to make sure we can understand the sounds. If we can prove that we can do this, then we can argue that we should continually listen to these glaciers.”

“The future of the oceans depends on us (humans)”, he adds. “We need to start listening to what they are telling us.”