If a 10-billion-ton hunk of glacial ice falls into the Arctic Ocean and no one is around to hear it, does it make a sound?
Erin Pettit and researchers at Oregon State University were set to find out.
They’ve been studying melting glaciers by dropping hydrophones — specialized underwater microphones — into the water near these massive ice formations.
It turns out, glacial ice has a surprising song: Hissing, popping, and sizzling of pressurized bubbles bursting, and low rumbling and crackling of glacial calving events.
Some of these distinct noises can tell researchers how the formations have changed over time.
Pettit joins us to discuss her research and show us some of the sounds of glaciers she’s collected.
Note: The following transcript was transcribed digitally and validated for accuracy, readability and formatting by an OPB volunteer.
Dave Miller: From the Gert Boyle Studio at OPB, this is Think Out Loud. I’m Dave Miller. We turn now to the sizzling, crackling, crashing sounds of loss. For nearly 20 years, Oregon State University glaciologist Erin Pettit has been using underwater microphones to record the sounds of melting and calving glaciers. She joins us now to talk about her research and what these sounds can tell us about the ice that we are losing. Erin Pettit, welcome back to Think Out Loud.
Erin Pettit: Hi, thanks for having me.
Miller: I want to start with a clip that you shared with us that you recorded in Antarctica.
[Recording plays of water droplets, cracking ice]
Miller: There are a lot of different sounds all mixed together there. Can you break them down for us? What were we listening to?
Pettit: This is a sound that comes from actually fairly deep in the ocean, underneath an ice shelf. An ice shelf is where the big Antarctic ice sheet, a glacier there, flows off into the ocean; it doesn’t fall apart the same way that Alaskan glaciers do, because the water is cold. So these big glaciers flow out onto the ocean. And we were able to drill through almost 1,000 feet of ice, to put some microphones underneath that ice. There are a lot of sounds, and breaking it down and trying to figure out what we’re listening to is part of our goal. We do hear there the little tiny bits of snap, crackle and pop are most likely bubbles popping out of the ice. And then in the distance we hear some cracking – that’s the ice cracking. And then sometimes we hear seals off in the distance that will go through and just make these shrieks, or other animals.
All of these give us clues as to what physical or biological processes are going on. Acoustics has been used for a long time to look at the biological world in the ocean. But it wasn’t until a little less than 20 years ago that we started listening to that physical process, the melt of glaciers. In Alaska we started, and then more recently took it down to Antarctica to try to understand what’s happening.
Miller: If I use one of your microphones, and I got some ice out of my freezer and listened to that ice melting – minus the seals, I suppose – would it sound anything like what we just heard?
Pettit: The ice in your freezer is not that different from glacier ice. But the special thing that glacier ice has is these thousands of tiny pressurized bubbles inside it. Glacier ice comes from compacting snow, in the process way up in the mountains or the center of the Antarctic ice sheet, it compresses that snow and that snow turns into ice that’s filled with billions of tiny pressurized bubbles. So that’s the main difference. If you had a freezer that made an ice chunk big enough to crack like a glacier calving into the ocean, then you might hear a similar cracking sound. But the special thing about glacier ice is that sizzling background sound that is the bubbles popping out as the ice melts.
Miller: Let’s have a listen one more time, actually to that same clip, now informed with a little bit more knowledge, and especially to listen for that snap, crackle and pop of those air bubbles escaping.
[The same recording plays of water droplets and cracking ice]
Miller: What prompted you to start putting these hydrophones – underwater microphones – under or near glaciers to begin with, nearly 20 years ago?
Pettit: It’s actually a bit of a funny story, a lovely story from my perspective. I’ve been a glaciologist for almost 30 years now. I was up in Glacier Bay National Park, looking at some of the glaciers there and interacting with them. We were on the water in some kayaks and there was this lovely humpback whale that was swimming around. It was a wonderful experience just being in that place. And I suddenly asked myself the question, what does the whale think of the big, crazy calving sounds, that big activity that comes from tidewater glaciers in a place like Alaska? The glaciers that tumble into the ocean and that a lot of cruise ships visit, they’re beautiful, fascinating examples of glaciers on our planet. And the whales are so sensitive to hearing and they communicate through sound. So I suddenly asked myself, what do the whales think of glaciers? What does a glacier sound like to a whale? So that was my first thought of putting it down, and putting a hydrophone into the water and just listening.
Miller: Is it fair to say that when you started, you were assuming you’d be paying most attention to those massive sounds as opposed to the minute, second by second crackling of the bubbles?
Pettit: Absolutely, yeah. The first project, we intended to try to use it to estimate the size of these calving events or to get some idea of how much ice might be lost to those big calving events that produce the icebergs. I totally was focused on that as being the sound that was going to be the most important sound that we were interested in.
So discovering this additional sound was interesting because our first thought was, is our instrument not working right? Is there some kind of noise in the hydrophone electronics that is generating this background hiss? And that’s something that we do all the time. If we see something in our data that doesn’t quite seem real, we always ask ourselves, is this an instrumental issue, are we doing something wrong, before we will put it out there and say “yes, this is a real sound that has real meaning for our analysis.” And that was definitely the case. It was actually several years before we were able to fully convince ourselves that this indeed was what ice melting sounded like.
Miller: Now that you’ve done that and now that you have this data that goes back nearly 20 years, what kinds of comparisons are you able to make with all of this sort of longitudinal data at this point?
Pettit: That’s a really good question, because sure, we can listen to this and it sounds funky and all this stuff. But what can we do with it and why would that matter is always a question going on in the back of our head. What we’ve noticed casually was that warmer fjords, like in Alaska, are louder because normally the ice there does melt faster just naturally, compared to, say, Greenland or some of the places in Antarctica. So we’ve noticed that there are variations. There’s some commonalities among all of these environments, but there’s also some very specific differences. For example, a fjord in Alaska that used to have a glacier in it but doesn’t anymore sounds very different than a fjord that has an actively calving glacier in it.
So what that triggered us to really want to be able to think about is, listening to this long term, it helps us understand what changes are going on. All of the glaciers around the world are undergoing change right now. Most of them are retreating. And being able to understand not just if they are retreating, but what is triggering them, what is making them behave in different ways than they used to. And the way we do that is using some forms of machine learning. It takes a lot of time to listen to a glacier and we’d have to have a lot of help to have people listening to everything going on. So if we can get a computer to help listen for us and translate that into different sounds and changes in those sounds, then we can start to piece together a time series of different behaviors.
For example, if you have a coffee shop that you go to regularly, you use those sounds around you to kind of help navigate the world. We all do this, whether it’s vibrations or actually audible sounds. And you would notice one day if you went into the coffee shop and there was a different sound. For example, maybe they got a new espresso machine, or maybe the espresso machine is broken, or maybe COVID has hit and suddenly there’s nobody in the coffee shop. If you just measure that ambient sound in a certain environment and you get to know its normal behavior, maybe there’s a weekly cycle of a coffee shop or a daily cycle of a coffee shop. And then when suddenly something is different, you can start to ask that question of what just happened, why did it happen now, what does that mean for the particular environment we’re in, what has changed that has triggered this difference in sound that we hear?
Miller: And obviously, lurking underneath all of this for us as humans is the terror that a lot of us feel about melting sea ice and rising sea levels, and what climate change and warming oceans could mean for us all.
To go back to the broadest correlation that you described, that in general, when water is warmer, more air comes out of glaciers, and the snap and crackle is louder, have you seen an increase in volume over time in various places that you’ve been studying?
Pettit: We do see the changes seasonally, because the waters in the fjords will get warmer or cooler over the course of a season, or maybe when a storm comes in and moves some of the water around, or the winds blow. We don’t yet have enough information to be able to say that a particular fjord is getting louder just due to warming water. Water temperature is something we also often measure. Because we can often see the water getting warmer, we want to take it one step further and try to understand, is that warming water, how is that getting translated into the glacier? Is that causing it to really just melt faster? Or is it feeding back into the dynamics and such that it’s actually calving more and still melting? And the change in the speed of the glacier, there’s a lot of variables that go into it. So we can start to ask a lot of questions when we get those signals that do show change. I wish it were a simple 1-to-1, it’s getting louder, so therefore it’s warmer.
But it does give us a really rich dataset, which is what the soundscape to us is, a dataset. It gives us a really rich dataset to work with, to start to ask more questions and to then probe some other data more specifically, or look at the changes. Because there are diurnal changes, there’s daily changes we see. We have to get rid of data where a tour boat comes and goes over our hydrophone, because we can actually hear those changes. So we can start to piece together a lot of the different changes that we see. But it’s not just a simple 1-to-1 relationship, but it gives us a rich soundscape in which to try to interpret and bring together multiple data sets.
Miller: Let’s listen to another piece of audio data that you and your team have recorded. This is from a place called Lake Bonney. We’ll have a listen and then you can tell us more about it.
[Recording plays of dripping water, cracking ice and deep booming sounds]
Miller: So what is Lake Bonney?
Pettit: Such a bizarre sound, isn’t it? Lake Bonney is a perennially frozen lake in Antarctica. It is near McMurdo Station. For anyone who is familiar with the Antarctic geography, it’s not too far from there. But it’s a lake, not the ocean. And in this case, there’s a glacier that flows into that lake. And if anyone is curious about that particular glacier, if you look up a feature called Blood Falls, which was a site that I’ve studied before, it shows you the pictures of the glacier that flows into this lake.
So the glacier flows into this lake, but the lake is frozen. So we don’t have as many of those popping out bubbles because this is lake ice, not as much of the glacier ice. There’s a little bit of glacier ice there, but not as much. But what we’re hearing is lake ice that’s about 10 feet thick being compressed by the glacier, because it’s got a rock wall on the far side of the lake and then the glacier is flowing into one side of the lake. It’s confined, it’s not even a very big lake. [It] has like 100 feet of water and then 10 feet of ice on top of it. So it’s this very contained cavity and the sounds are then also able to echo across the lake.
It’s an absolutely fascinating, fascinating place. But what we’re hearing there is the lake ice getting squashed by the glacier and that’s what’s causing the big cracks that you hear. And it feels echoey because it’s this enclosed basin, because it’s a lake that’s maybe a couple of miles across.
Miller: Let’s listen to one more piece of audio. This is from a dramatic calving event. As you said, you really are paying a lot of attention to the snapping and crackling from the melting. But you do also capture some of those glaciers breaking off and smashing into the water. Let’s have a listen.
[Recording plays of crackling ice and large ice chunks smashing into the water]
Miller: That is from the LeConte Glacier in Alaska. I’m curious broadly, how or if doing this research has changed your relationship to sound more broadly, even when you’re not listening to underwater sounds?
Pettit: Absolutely. I notice it helps me pay attention more to what’s going on around me. I notice those changes beyond just what’s going on in the glacier. This idea of using ambient sound to understand our environment is fascinating, both in terms of my research, but I do notice that I use it on a day-to-day basis. And I think we all do deep down, whether it’s just from the vibrations, whether or not you are a hearing person.
But those of us that can hear do use sounds a lot to navigate and to decide when and where, what’s going on around us. And if we all pay attention a little bit more, I think you’ll find just some really cool sounds out there, whether it’s the leaves rustling in the wind or the way certain pathways sound different when there’s lots of people walking down them, versus on a quiet afternoon. There’s a lot of variations in our environment that we can use to help understand what’s going on and how things are changing.
Miller: Erin, thanks very much.
Pettit: Yeah, thanks for having me.
Miller: Erin Pettit is a glaciologist at Oregon State University.
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