In what some would perhaps consider an atypical form of scientific study, researchers have sought to measure the noise emanating from bubbles, squirting from disappearing ice, in the hope that these measurements might illustrate glacial melting. A recent press release describes this as the “sizzle” of a melting glacier, argued to be “… one of the most prominent sounds of a warming planet.”
What Do Melting Glaciers Sound Like?
Geophysicist and researcher Erin Pettit, working at the University of Alaska, claimed she heard popping and crackling sounds as she was kayaking in the icy northern waters. These sounds were also detected by a series of underwater microphones that Pettit had established along the Alaskan coast, registering sounds that far surpassed the volume of those heard on the surface.
“If you were underneath the water in a complete downpour, with the rain pounding the water, that’s one of the loudest natural ocean sounds out there… In glacial fjords we record that level of sound almost continually.”
Pettit dispatched large pieces of the glacier to a pair of acoustics specialists, based at the University of Texas, Kevin Lee and Preston Wilson. To test Pettit’s hypothesis, the Teas duo suspended the chunks of ice in a container of chilled water and produced video and audio recordings of the ice melting.
From this experimental setup, Lee and Wilson found that the recorded sounds belonged to the escaping bubbles, teeming from the melting ice. Lee explains that these sounds are caused by the bubbles oscillating, upon release from the ice:
“A bubble when it is released from a nozzle or any orifice will naturally oscillate at a frequency that’s inversely proportional to the radius of the bubble.”
On this basis, larger bubbles will oscillate at a lower frequency than when compared to smaller bubbles; small bubbles, therefore, emit a high pitch sound.
The researchers measured sounds between one and three kilohertz. Since human beings are capable of detecting sound frequencies of between 20 Hertz and 20,000 Hertz, the bubbles are discernible to the human ear.
The Trapping of Atmospheric Gas Bubbles
A number of prominent scientists have stated gas to become trapped within the crystalline structures of snow, as it settles across the surface of glaciers. An evident characteristic of glacier ice is the inclusion of cavities and gas bubbles, due to snow compaction. These bubbles merely consist of atmospheric air – including oxygen, nitrogen and carbon dioxide gases – destined to occupy the space between fused snow crystals.
Due to compaction, the snow eventually transitions into ice and the air bubbles become pressurized, and ordered into a regular distribution throughout the ice structure.
In contemplating the implications of the afore-mentioned findings, Pettit and Lee claim, using hydrophone recordings of glacial fjords, the rate of glacial melting could be inferred in response to alterations in weather and season, as well as long-standing trends in climate change.
Pettit believes, in listening to the underwater acoustic environment and collecting sample sounds from “different situations and environments,” they might find a novel means of measuring climate change:
“In order to be able to mitigate some of the potential future issues of the sea level rise, due to climate change, we need to be able to have some idea of how to predict how much sea level rise will happen… glaciers are one of the largest contributors to that sea level rise, and the fresh water discharge coming out from the glaciers is one of the largest components of that glacial contribution; if our acoustic method of listening to these glaciers can actually help us monitor that fresh water discharge, then we can have a much better dataset to predict future sea level rise.”
Additional research has shown that glacial fjords – narrow bodies of water, typically surrounded by U-shaped glacial valleys – may become subjected to warmer waters and, therefore, demonstrate a faster rate of melting.
For example, a recent study published by Fiammetta Straneo and colleagues, back in 2010, investigated the apparent increase in mass loss from Greenland ice sheets. They posit the accelerated loss of ice to be attributed to an elevation in the melting at the ice-ocean interface, provoked by “…synchronous warming of subtropical waters offshore of Greenland.”
Meanwhile, a research team, consisting of Eric Rignot, Michele Koppes and Isabella Velicogna, claim that rapid submarine melting was taking place because of contact with warm ocean waters, whilst warm air temperatures were responsible for surface melting.
The results of the latest study are due to be showcased during an upcoming presentation, entitled Underwater sound radiated by bubbles released by melting glacier ice, which will take place Thursday, Dec. 5, 2013.
By James Fenner