Researchers at Scripps Institution of Oceanography at the University of California San Diego are eavesdropping on an Arctic glacier in the name of science. In a new study, Scripps polar scientists Oskar Glowacki and Grant Deane describe a new method to measure glacier mass loss from iceberg calving by analyzing underwater acoustic recordings of icebergs as they fall into the ocean and make a splash.
Iceberg calving, a process during which ice breaks off from the edges of a glacier, contributes greatly to sea-level rise. As the planet warms, calving is expected to increase, but accurate estimates of ice loss at the ice-ocean boundary are difficult to obtain, say researchers. This is due to the remote locations of many glaciers, as well as the dangerous conditions that prevent researchers from making direct measurements at unstable ice cliffs.
In an effort to address these challenges, the Scripps team deployed in summer 2016 two underwater microphones (hydrophones) near the Hansbreen glacier in Hornsund fjord, Svalbard, an archipelago in the Arctic north of Norway. The recorders were placed by divers on the ocean floor more than 900 meters (3,000 feet) from the glacier cliff. Over the course of a month and a half, the hydrophones captured the sounds made by icebergs falling into the ocean. (Listen to a sample here.)
“An iceberg breaking off an ice cliff and falling into the water typically sounds like a cracking, rumbling splash,” said Deane, a research oceanographer at Scripps and study co-author. “It has a real bass feel to it.”
The hydrophones also recorded the sounds of air bubbles bursting from the glacier ice as it melted. Melting glacier ice is described as “more treble,” said Deane, noting that it sounds like popping or "bacon frying” if you're close to it and more like “hissing static” if it's far away.
“What we are actually recording is a sound cacophony, a mixture of noise made by all the chunks of ice floating around and the noise made by the glacier itself,” said Glowacki, a postdoctoral scholar at Scripps and lead author of the study.
Three time-lapse cameras placed near the glacier simultaneously photographed each calving event. Based on these images, the researchers estimated iceberg volumes and drop heights from 169 calving events. After retrieving the underwater microphones, the researchers analyzed the acoustic data and compared it with photographs from all 169 events, finding a strong relationship between the size of the iceberg breaking off from the glacier and the intensity of the resulting underwater sound.
This finding led the researchers to derive a mathematical formula that calculates the mass of the ice block from the noise it makes. This model can be used to measure ice loss due to calving from the underwater sound recordings of icebergs impacting the ocean, said the researchers.
“We demonstrated that glacier mass loss from iceberg calving can be measured safely by analyzing the underwater noise generated as icebergs impact the sea surface,” said Glowacki.
The researchers note other methods, such as satellite imagery, are effective ways to study large-scale, relatively slow changes in glaciers. But acoustic oceanography may offer some advantages for the study of interactions between land-based ice and the ocean. Low cost hydrophones are easily deployed, and acoustic data can be gathered continuously for several months or longer with a high sampling rate and low maintenance. Acoustic measurements are not hampered by conditions such as fog, cloud coverage, precipitation, or humidity.
“This study shows that we can measure the ice being lost from glaciers using sounds that they make as they break up, and we can do it reasonably accurately,” said Deane. “I think this is a significant addition to our list of methods for studying glaciers in the Arctic.”
Iceberg calving is a natural process of ice loss for any glacier terminating at the ocean, but the rate at which icebergs detach from glaciers increases with water temperature. As the planet warms and more heat is transported towards the glacier terminus, we can expect much higher calving rates and increased sea-level rise.
“We're losing ice from the Arctic at an accelerating rate. The melting ice is increasing sea level so it's important to monitor the glaciers and understand their behavior,” said Deane.
Glowacki and Deane said the next step will be to apply the acoustic technique to monitor ice loss due to calving for a long period of time and in different locations.
“We know very well that human-induced changes are responsible for ocean warming,” said Glowacki. “To fully understand the response of glaciers to climate shifts, we need to conduct long-term monitoring of the ice mass loss.”
They would like to build long-term monitoring stations around Greenland, “which holds enough ice to raise the global oceans by roughly twenty feet,” said Deane.
The researchers also plan to conduct further laboratory experiments to better understand the acoustic signal of iceberg calving, and to tease apart the sources of bursting bubbles—whether they’re coming from melting icebergs or from the melting glacier.
The study, “Quantifying iceberg calving fluxes with underwater noise,” was published in the March 17 edition of The Cryosphere.
This research was supported by the Ministry of Science and Higher Education of Poland, the National Science Foundation, the Polish National Science Centre, and the U.S. Office of Naval Research.
Attached file: Iceberg calving sample