It is not difficult to envision how ice melts— just imagine a solid cube of water transforming into a liquid mess. Perhaps more surprising, this transition also produces sounds that are audible to human ears, if we listen carefully. The sounds occur because ice traps air bubbles as they are escaping from freezing water. The bigger the ice— glaciers or ice shelves, for example— the greater the number of air bubbles that it contains. Last month, a team of researchers published their work on the intensity, directionality and temporal statistics of underwater noise produced when icebergs melt. The study is a pioneer in the field of cryoacoustics research still in its nascence, since existing studies largely focus on larger forms of ice such as glaciers and ice shelves instead of icebergs.
In fact, different forms of ice produce different noise signals when melting. The key, in this case, according to Oskar Glowacki, the lead author of the paper, is the quantity of air bubbles trapped in ice. “Glaciers contain more bubbles than icebergs, which is obvious taking into account differences in size,” he explained to GlacierHub. “When the glacier is melting, millions of bubbles are released into the water at the same time. As a result, what we hear is a loud, constant noise described by a normal distribution (typical in nature). But when approaching melting icebergs, we can hear individual bursts of bubbles, and so the noise is much more impulsive.”
The study was conducted at Hornsund Fjord in Svalbard, Norway. The researchers gathered measurements for icebergs in four locations by deploying hydrophones at a depth of 1m from a boat during the spring and summer seasons. Hydrophones are devices that are used to record underwater sounds. Glowack said researchers can hear the sounds even while onboard the boat. But nothing beats diving in the cold waters of the Arctic fjords and listening to the noise of melting ice, an opportunity Glowacki recalls fondly as “the most amazing experience.”
Measures of underwater hissing produced during iceberg melt at the ice-ocean boundary pointed to the need for a remote method to gather quantitative data on the rate of subsurface melting. Iceberg melt has proven to be an important parameter in regional ocean models to estimate ocean circulation patterns and local hydrographic conditions such as in Greenland. However, it is still not easy to record underwater sounds in the harsh environments of the Arctic.
“The main difficulty is to really understand what we are listening to. When the goal is to accurately measure iceberg and glacier melting using underwater sound of bursting bubbles, we need to discover the exact relationship between the intensity of melt noise and exact ice loss,” Glowacki said.
In the study, the researchers noted that the cackle of icebergs changes based on its relative position to the hydrophone and speed of melting. Care must be taken to remove recordings that are made within 20m of an iceberg to avoid the effects of near-field noise interference, while calls from bearded seals also had to be excluded from analysis.
Moreover, this relationship can be different according to environmental conditions, as changing water temperature causes variation in the shape and size of air bubbles trapped in the ice, and hence the specific song that the ice sings under different conditions. Other complications include sound reflection from the sea surface or ocean bottom and changes in the direction of wave propagation driven by spatial and temporal differences in water temperature and salinity.
“Fortunately, we can take into account all of these factors using accurate mathematical models, which are available as computer programs,” Glowacki said. However, he reckons that transferring cryoacoustics into a real tool in glaciology may take a few years of intensive research, requiring laboratory experiments and studies in other ice-covered regions of Greenland, Alaska and Antarctica.
With more work, noises of melting glaciers might not only identify, but also accurately measure glacier retreat. Nevertheless, the sounds of melting ice are an obvious call from nature that climate change is real.