Remember the age-old adage, “If a tree falls in the forest and no one is around, does it make a sound?” For centuries philosophers have tested our minds with such questions, and certainly the answer depends on how the individual chooses to define the word sound. Scientists would say that if by sound, we mean the physical phenomenon of wave disturbance caused by the crash, we would undoubtedly concur. Indeed, in recognizing the uniqueness of audio frequencies, the scientific practice of studying environmental soundscapes has proven effective at providing information across a varied range of phenomena. But glaciers represent a relatively new soundscape frontier.
“Glaciologists just opened their eyes to studying glaciers about 150 years ago. We started to look at glaciers from different angles, perspectives, satellites — but we forgot to open our ears,” said Dr. Evgeny Podolskiy, an assistant professor at the Arctic Research Center at Hokkaido University in Sapporo, Japan. “I’ve been studying glacier geophysics for quite some time and I found that there is this kind of natural zoo, or a universe, of sounds which we kind of totally ignored until recently.”
His research then became directed toward the glacial soundscape, and last month he published an article in Geophysical Research Letters about the sounds he recorded, not with expensive geophysical sensors, but with a smartphone from Bowdoin Glacier (Kangerluarsuup Sermia), located in northwestern Greenland. His recordings captured a unique sound which he used to describe a specific drainage process within the glacier — one that is impossible to observe from the surface: Meltwater drainage through a crevasse.
Ponds of meltwater that pool on top of the glacial surface drain through the crevasses, entering into the drainage system of the glacier. As the water travels to subglacial environments, it warms up the ice, makes it softer, and increases the subglacial water pressure that causes the glacier to slide faster into the ocean. In his paper, Podolskiy presented the first evidence of unexplained acoustic phenomena being generated by water drainage through a crevasse.
This acoustic signal is distinct from other drainage processes due to the “two-phase” interaction between air and water. “The main point I want to make is that we totally forgot that there’s air,” he said. The air produces vibrations on water in the near surface environment where they mix. “By listening to these sounds, we can actually determine the type of flow regime — the way fluid flows in these systems — just by looking at the analysis of the signals,” he said.
After many years in the field as a glaciologist, Podolskiy found that different types of glacial environments produce their own unique soundscapes. For instance, during the daytime at a Himalayan debris-covered glacier, exposed ice cliffs slowly melt and the rocks on top tumble down the slope, producing noisy avalanches. Podolskiy noticed that during the afternoon, there is a lot of this particular sound. At night, if a glacier is not shielded by insulating debris cover, the ice begins to contract as it gets extremely cold, and the tensile contraction of the ice produces cracking sounds.
Podolskiy’s most recent research concerns the soundscape of Bowdoin, a tidewater glacier. These fast-flowing valley glaciers begin in mountains or on more distant ice sheets and reach their terminus at the ocean where their icy cliff edges occasionally break off, or calve, into the sea. Glaciers recede when the rate of calving and/or englacial melt exceeds the rate of new snow accumulation at higher elevations.
Bowdoin was initially being monitored by Podolskiy and his colleagues because melt and glacier retreat recently began accelerating in the area. Amazingly, the scientists were able to walk right up to the calving front where the icebergs detach, something that is quite uncommon in these environments, making Bowdoin a great study site for all types of glacial research.
The idea of using sensors to passively study the ocean has been around for awhile. In the 1950s, Navy surveillance systems discovered unknown repetitive pulses of traveling through the sea, and they were later attributed to finback whale courting displays. This actually provided much of the stimulus for the early design of ocean acoustic equipment and techniques for observation. According to Acoustics Today, the proposal that “these powerful [acoustic] tools could be applied to a pressing and difficult measurement problem in polar regions: the monitoring of tidewater glaciers with hydroacoustics,” came about in 2008 at a workshop in Bremen, Germany.
Though his paper only references sounds recorded from his smartphone, Podolskiy pointed to a drawing he made behind him on his whiteboard and explained: “We also have seismic and GPS stations to observe tide-modulated motion of the ice and its fracturing. We have hydroacoustic sensors under water so we can hear processes like bursting or pressurized air bubbles within the melting ice, calving, and even whales. On a mountain nearby we have infrasound sensors, which are basically sensors used to measure air pressure because when icebergs fall, they displace air and produce air pressure waves that can tell us where calving occurred,” he said.
Podolskiy held up handfuls of hard drives and explained that instead of going through terabytes of complex geophysical data, he realized a simple fact: “Audible sounds recorded with my smartphone over various drainage systems contain a lot of unique acoustic information. Every place you look has a very different signature. We can fingerprint different ways of water flowing into the ice by sound and the fingerprinting of different flow regimes is useful for understanding the glacial hydrology”
“But when I walk on that glacier I just close my eyes and I realize there are so many sounds, audible sounds — not these fancy seismic, infrasound, hydroacoustic recorded sounds we have been collecting there for years — just sounds audible to our ears,” he said.
Podolskiy explained that, after the many summers at Bowdoin, one of the things that directed him to studying acoustics was the sounds of seabirds at the calving front. Birds, like the black-legged kittiwake, are attracted to tidewater glacier discharge plumes which form when meltwater exits from underneath the glacier and, due to its low density, rises in the seawater toward the surface, bringing with it nutrients and zooplankton on which arctic seabirds feed.
Seabird Sounds at Calving Front. Source: Evgeny Podolskiy
“On the surface I listen to the birds and then I listen to the crevasses,” Podolskiy said. Crevasses are deep, open fractures on the glacier surface that form as a result of changing stresses as the ice moves and flows toward the ocean. Crevasses can open up overnight. “It is the most intense process on Bowdoin. We can hear it as shooting sounds, like gunshots,” he said. This ice splitting process should not be confused with the description of meltwater drainage through the crevasse which was articulated at the beginning of the article.
Calving, Podolskiy explained, does not happen as frequently, just several events per day. But calving is very distinct and very loud and can last ten minutes when the ice is collapsing. It produces an array of strong seismo-acoustic signals.
Moulins are circular-like shafts within a glacier through which water enters from the surface. They are normally found in areas that are heavily crevassed and they too produce their own unique sounds.
As the climate warms, understanding the various flow regimes in the englacial conduits is valuable because of their influence on glacial mass flux. In addition to contributing to global sea level rise, the influx of fresh glacial water to the ocean affects global scale heat transport by weakening circulation patterns. Fresh surface water does not sink like dense, salty water, so it slows the overturning movement of the ocean, a powerful regulator of global climate.
“What is clear is that the Greenland Ice Sheet, the Antarctic Ice Sheet, and all the glaciers around the world are getting wet because they’re melting over increasingly larger areas, and all this produced meltwater is bringing our cryosphere into a new state” Podolskiy said. The meltwater flows through the englacial system and affects glaciers from the inside, and he presumed part of this story could be studied with microphones. Certainly, near-source acoustic methods offer advantages over more conventional remote sensing methods because satellites are unable to see how the meltwater enters and flows through the crevasses.
Polar explorers and mountaineers were sensitive to glacial sounds for centuries, but now with acoustic instruments we have the ability to learn the things we missed without them. “I hope it will inspire people,” he said, “to pay attention and to just try to see the world like whales or dolphins do because these guys, they don’t see much — they hear the configuration. They are living in soundscapes.”
Read More on GlacierHub: