The image of a polar bear stranded on an ice floe. Herds of thousands of walruses hauled out on land, deprived of their sunning platforms of choice: sea ice. Videos from NASA simulating how carbon dioxide moves through the atmosphere and travels around the globe. The topic of climate change is usually presented to us in visuals and statistics.
What we are rarely exposed to, however, are the sounds of climate change. Some call the study of sound that comes from the landscape or the animals in a particular natural habitat “soundscape ecology,” and we’ve learned a lot about it in the last few years. Below, are just three examples.
Trees “cry for help” in drought situations
When a drought hits, trees suffer. And it turns out that trees make noises that can alert us to their distress.
In 2013, a team of French scientists in a lab setting captured the ultrasonic noise made by bubbles forming inside water-stressed trees. Trees draw groundwater up through specialized tubes in their trunks called xylem, which rely on the attractive forces between water molecules and water and plant cells to lift liquid to the highest leaves and branches.
However, because trees are so tall, the liquid in the xylem can be under intense pressure—many times that of the pressure of the atmosphere around us. Luckily, the attractive forces between neighboring water molecules keep the water column intact. But as groundwater dries up, the trees must pull harder on the remaining water. If the pressure is greater than the strength of the intermolecular forces, the column of water breaks and an air bubble forms, in a process called cavitation. Too many air bubbles can mean death for the tree.
To ensure that these air bubbles were causing the acoustic signature of drought-parched trees, the researchers made a mock-up of a tree in a lab. They placed a thin piece of pinewood into a capsule filled with a gel. They then evaporated the water out of the gel—causing a simulated drought—and simultaneously recorded video and sound of the cavitation in the xylem. The researchers discovered that about half of the sounds made by a tree are due to cavitation.
The findings could lead to the design of a microphone device that would attach to a tree and constantly listen for sounds of thirst. If needed, the instrument could then trigger an emergency-watering system before permanent damage to the tree sets in.
Glaciers “speak” of melting
In 2014, as part of the Glacier Acoustics Project, a pilot study headed up by geophysicists at the University of Alaska, researchers set out to record the sounds the Earth makes as it melts.
In one experiment, an anthropologist and a musician used hydrophonic technology to chart the underwater snap, crackle and pop of ice melting off glaciers and flowing into fjords.
When glaciers melt, freshwater shoots out in floods or rivers from underneath them. But finding a way to measure how big those floods or rivers are is extremely hard to do, since putting sensitive instruments beneath calving glaciers isn’t practical.
As the glaciers dissolve, though, air bubbles—trapped in the ice when it formed—then wiggle their way out into the water. Released from a nozzle or any orifice, a bubble will naturally oscillate at a frequency that’s inversely proportional to the radius of the bubble. Those frequencies can be measured by sound equipment.
In the anthropologist and musician’s research, a network of hydrophones was anchored to the seafloor off the coast of Alaska a mile or two away from a calving glacier. The two recorded the sounds of the trapped air bubbles that managed to escape from the glacier’s freshwater discharges, the large pulses of water that are a direct contributor to sea level rise. Such acoustic data will help to predict how much the seas will rise with increasing temperatures.
Whales are “singing” a new song
The mysterious new tune has such a low beat that it is scarcely audible. Humpback vocalizations typically have an audio frequency between 80 and 4,000 hertz (Hz). But the newly discovered melodies were found to have a significantly lower frequency of around 40 Hz.
Scientists can’t be absolutely sure that the humpbacks are making the noise. But I wouldn’t be surprised if they’ve composed and are singing a new song.
And it’s probably about climate change.
Here’s to finding your true places and natural habitats,