November 28, 2013
Warm Water Causes Arctic Glaciers To Sizzle As They Melt
[ Watch the Video: Melting Glacial Ice Has Its Own Sound ]April Flowers for redOrbit.com - Your Universe Online
One of the most prominent sounds of a warming planet has been recorded and identified by scientists at the University of Alaska: the sizzle of glacial ice as it melts into the sea.
Trapped air bubbles squirting out of the disappearing ice causes a noise that might provide clues to the rate of glacier melt and help researchers better monitor the fast-changing polar environments.
While kayaking in the frigid northern waters, geophysicist Erin Pettit had often heard the popping and crackling noises. She also picked up these sounds with underwater microphones set up off the Alaskan coast. These underwater sounds were at a much louder volume than above 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," she said. "In glacial fjords we record that level of sound almost continually."
Despite a suspicion that the noise was caused by melting ice, Pettit couldn't confirm her hypothesis without a more controlled experiment. To conduct this experiment, she enlisted the aid of Kevin Lee and Preston Wilson, acoustics experts from the University of Texas. After Pettit sent chunks of glacier to the two researchers, they mounted the chunks in a tank of chilled water and recorded video and audio of the ice as it melted. They were able to match sounds on the recording to the escape of bubbles from the ice.
"Most of the sound comes from the bubbles oscillating when they're ejected," Lee said. "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," he said, meaning the smaller the bubble, the higher the pitch. The sounds were recorded in the middle of the frequency range audible to humans, around 1 – 3 kilohertz.
It has been known for decades that bubbles form in glaciers when snow crystals trap pockets of air, which slowly get squashed down under the weight of more snow. The compacting snow turns into ice, pressurizing the air pockets. Such bubbles are evenly distributed throughout the ice because of the regular way bubbles form. This is an important characteristic for researchers trying to use the sound intensity of bubble squirts to measure ice melt rate.
Though images carry a stronger emotional impact than the symphony of melting ice, sound still has its own story to tell. The research team says they envision using hydrophone recordings in glacial fjords to monitor relative changes in glacier melting in response to one-time weather events, seasonal changes, and long-term climate trends. Recording microphones can be placed a safe distance from unstable ice sheets because sound travels long distances underwater. Along with time-lapse photography and salinity readings, the sound recordings would complement other measurements of ice melt.
The study findings will be presented at the 166th Meeting of the Acoustical Society of America (ASA) held in San Francisco in December.