There is music in melting glaciers.

The earth is warming. And that causes glaciers on, for example, Greenland and Spitsbergen to melt. This is clearly visible: we see – for example on satellite images that are made year after year – that glaciers are getting shorter and thinner. But the melting of the glaciers can also be heard in the bays where these glaciers have been flowing for years. And the sounds made by the fading glaciers could potentially be used to estimate the impact climate change is having on large ice masses, researchers recently said at a meeting of the Acoustical Society of America.

The sound of a melting glacier

Before the meeting, the researchers looked at glaciers on Spitsbergen, which flow into the sea. Many of these glaciers are suffering from climate change, especially the warming of the water into which they flow. That warmer water affects the glacier tongues that rest on the water from below. This makes the glacier tongues thinner.

However, the glacier tongues are not only made of ice. The ice also contains air that has become trapped in the ice – often long ago. And when the ice melts due to the warming of the seawater, those air bubbles are released into the water under the glacier tongue. And you can hear that. “The release of each individual bubble trapped in the ice makes a popping sound,” said researcher Hari Vishnu. Scientias.nl. “And besides melting glacial ice, we also hear a lot of these random popping sounds that are a bit reminiscent of the sounds you hear when you fry bacon.”

The sound of a bubble

When air gets trapped in the ice, it forms bubbles in which the pressure can rise enormously over time: up to 20 atmospheres. “The gas pressure in the bubbles increases as the bubbles are compressed by the weight of the ice above,” explains researcher Grant Deane. And when the ice melts and such an air bubble – in which the pressure has increased enormously – is released, this results in detectable sound. “Bubbles in water generate musical tones as they form,” explains Deane. “All the musical qualities of running water – a babbling brook, the sound of a fountain and the crashing roar of a wave in the ocean – arise from sound pulses delivered by newly formed bubbles. Such a newly formed bubble has a neck or bridge of air that collapses, after which the bubble ‘relaxes’ and takes on that spherical shape. The collapse of that air bridge causes the bubble to ‘ringing’ as if it were a bell that is struck with a small hammer. A bubble released from melting glacial ice behaves similarly, except that the ‘ringing’ sound is now caused by the rapid expansion of the bubble as it escapes from the ice.”

Good audibility

And those escaping air bubbles are clearly audible, Vishnu emphasizes. “It is quite easy to perceive the sounds because they are very loud and clear and their frequency is within the range of human hearing. Moreover, during the melting season, these noises usually dominate.”

Monitor

The fact that melting glaciers make ‘music’ through air bubbles released is not just a remarkable fact. In theory, the ‘music’ also enables researchers to listen to glaciers in glacial bays and thus determine, while listening and counting air bubbles, to what extent glaciers are suffering from global warming. “If you know how many bubbles are released in a minute, for example, and you know how many bubbles there are on average in a certain volume of ice, you can calculate how much ice must have melted,” says Deane.

Spitsbergen

Deane and colleagues recently showed that it should in principle be possible to monitor glaciers in this way with an experiment in a glacier bay off the coast of Spitsbergen. “We observed that the intensity of the sound generated by melting glacial ice increased as the water temperature rose,” said Deane. “That makes sense, because you expect the ice in warmer water to melt faster and bubbles to be released more quickly in the ocean and therefore more noise will be generated.”

Surroundings

But the researchers also found that the sound isn’t just dictated by the rate at which the ice melts. Other factors also played a role. “Like where you listen, the distance to the glacier and the geometry of the glacier,” says Vishnu. “We removed the effect of the latter parameters by making an estimate of those effects using models. And that allowed us to determine to what extent the sound is related to the melting rate (and thus the water temperature) without being confused by other complicating factors. Once those disruptive effects disappeared, we were able to interpret the impact of the melting rate on the sounds more clearly and, based on that, also make an estimate of the melting rate itself.”

It means that researchers can’t simply listen to a glacier; in order to properly interpret the sounds, they must first have a good picture of the environment and be able to filter out all ‘noise’ – non-relevant sounds – in it. “It is crucial for us to understand the properties of the environment in order to interpret the sounds,” confirms Deane. “For example, we need to know how the properties of the bubbles that produce the sounds differ from glacier to glacier. But also how the water pressure – which increases with depth – influences the sound that the melting ice makes. And how waves and the seabed change the sound you hear at some distance from the tip of the glacier.”

Additional methodology

Although there are already several ways in which researchers can monitor glaciers – for example using the satellites mentioned above – an acoustic method is certainly not superfluous. “All measurement techniques have their strengths and weaknesses,” says Deane. “Cameras only work in light, so darkness, rain and fog are a problem. And satellite images are great, but you only get one every few days.” An additional limitation is that both cameras and satellites only record what happens above water. Hydrophones (suitable for recording sounds under water) therefore seem to be a nice addition. “They can give a better picture of the processes at play under water, such as the undersea glacial melt,” added Vishnu. And they can also continuously monitor the melt.

Challenges

“It’s very attractive,” concludes Deane about the new methodology. “If we can solve the problems.” Because there are still some hurdles to overcome before researchers get to the point where they can monitor glacier melt on a large scale and accurately using hydrophones. “We first need to take direct measurements of the melting ice and compare them with the acoustic measurements to make sure our estimate of the melting rate (based on the sound of the air bubbles, ed.) is correct,” says Deane. “And we need to examine the sounds made by Greenland glaciers — which are typically much larger than the glaciers we’ve studied to date — to make sure our ideas hold true for larger glaciers.”

So there is still some work to be done. But it is worth all the effort, Vishnu suspects. “Sounds have previously been used to estimate the amount of ice lost to calving, and now we’re focusing on another component: undersea melt.” And if it is up to the researchers, this invisible melt will soon be revealed by means of ‘ringing’ air bubbles. “In the long term, we hope to place hydrophones at glaciers in Greenland and Spitsbergen that will monitor the ice loss and stability of these glaciers for a long time.”