Bursting Air Bubbles Accelerate Glacier Melting
Large glaciers that flow into the ocean, known as tidewater glaciers, produce a constant sizzling and hissing sound as their icy undersides melt in contact with seawater. These underwater noises, resembling the sound of cooking food, occur due to the release of air bubbles that were previously trapped within the ice.
However, these small bubbles under pressure are not only producing sound. Recent studies have demonstrated that the energy released from the bursting bubbles can accelerate the melting of these glaciers underwater. Laboratory trials have revealed that glacier ice with bubbles melts twice as quickly as ice without bubbles.
According to Erin Pettit, a coauthor of the recently published study in Nature Geoscience, the bursting of air bubbles measuring in millimeters has a significant impact on the melting rates of tidewater glaciers.
The finding may clarify the reason behind the accelerated melting of certain tidewater glaciers, including Xeitl Sít’i (LeConte) Glacier in Alaska, which is occurring more rapidly below the water’s surface than what was previously anticipated based on theoretical models.
When compressed air is forcefully released from melted ice, bubbles are created which produce a crackling sound as they grow and ascend in the ocean. Source: Erin Pettit
Gauging Melting
One of the main ways that tidewater glaciers contribute to the rise of sea levels is by releasing icebergs from their steep fronts, where the ice meets the ocean. This process is known as calving. They also release meltwater through streams that flow along their bases and by melting directly in warm ocean water.
Researchers are interested in comprehending the process of underwater melting due to its potential impact on glacier stability and iceberg calving. However, direct measurement of this phenomenon is difficult. As a result, they rely on theoretical models to approximate the amount of ice melting using data on ocean temperatures and currents. These models also provide insight into how glaciers may be affected by rising ocean temperatures as a result of climate change.
Pettit mentioned that the issue with ice melt models is that they only consider bubble-free sea ice. However, up to 10% of glacier ice is composed of air, which becomes trapped between ice crystals as snow compresses over time. The air within these bubbles can be at 20 times the regular atmospheric pressure at sea level.
A decade ago, Pettit began researching glacier air bubbles by using hydrophones to listen to the sounds near a glacier in an Alaskan fjord. During a conversation with Meagan Wengrove, a coastal engineer from Oregon State University, they came up with the idea for a lab experiment on bursting bubbles. Wengrove is the main author of the recent study.
Testing a Hunch
According to Wengrove, the impact of high-pressure bubbles on ice melt had not been studied before, despite the fact that “air bubbles are well-known for causing turbulent flow in liquids.”
Wengrove and her team suspected that air bubbles could disrupt the thin layer of cold water that typically covers the undersides of glaciers. This disruption could potentially expose the glacier ice to warmer water, leading to increased melting.
The scientists conducted an experiment to validate their hypothesis by filming glacier ice from Oregon State University’s lab as it melted in a tank of saltwater. Additionally, they utilized laser light and tracer particles to monitor currents in the water surrounding the ice. The same procedure was repeated with bubble-free ice provided by an ice sculptor.
The credit for the image of glacier ice floating in LeConte Bay near Petersburg, Alaska goes to Oregon State University.
Wengrove and his team discovered that the presence of air bubbles caused ice to melt faster by bringing in warm, fast-moving water to the surface. Through their observations, they saw that the air bubbles would burst out of the thawing ice with force, creating low-pressure areas in the protective layer. This allowed warm seawater to rush in and fill the voids. The researchers also noted that the rising air bubbles brought warm water up with them, causing currents that further melted the ice.
Wengrove and Jonathan Nash, a fellow researcher at Oregon State University, created a computer simulation to study the impact of air bubbles on glaciers. Their results showed that the bubbles led to the most significant melting of the glacier in water depths less than 60 meters. This was due to lower water pressure, which caused the bubbles to expand quickly and stay in the water for a longer period of time.
This study is a significant advancement in enhancing the precision of our ice melt predictions.
According to Nash, this suggests that the findings are most applicable to tidewater glaciers located in the shallow waters near the Arctic. He also mentioned that since Antarctic glaciers melt at deeper water depths, the impact may not be as significant in those areas.
Twila Moon, a glaciologist from the University of Colorado Boulder’s National Snow and Ice Data Center, stated that air bubbles have a significant effect on melting that has been previously ignored. She acknowledged the study’s efforts to measure this impact as a crucial step in enhancing the precision of ice melt models.
At present, predictions about ice melt are primarily based on ocean temperature and the intensity of freshwater plumes originating from the base of a glacier. However, scientists are aware that there are additional physical factors, such as the recently discovered bubble effect, that could enhance the accuracy of these predictions.
More Field Data
Keith Nicholls, an oceanographer from the British Antarctic Survey who was not part of the research, commented that the experiments were interesting. He added that while air bubbles may contribute to the difference between predicted and actual glacier melt, they likely do not fully account for it.
According to Nicholls, the reason for this is the intricate nature of the environment where tidewater glaciers meet meltwater and seawater. These processes are challenging to simulate in theoretical models or laboratory settings.
Wengrove and team stated that further field data is necessary in order to determine the proportion of melting caused by air bubbles. In addition to the water conditions surrounding a glacier, other characteristics of the ice, such as its surface texture, can impact the rate of melting.
The group has just come back from their expedition to Xeitl Sít’i Glacier, where they utilized remotely operated vehicles to gather vital data. According to Nash, their lab experiments have given a new meaning to the common sight of bubbles. “Whenever we approach an iceberg now, all we can hear are these bubbles loudly disappearing as they melt.”
—Erin Martin-Jones (@Erin_M_J), Science Writer
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Citation: Martin-Jones, E. (2023), Popping bubbles make glaciers melt faster, Eos, 104, https://doi.org/10.1029/2023EO230402. Published on 25 October 2023.
Text © 2023. The authors. CC BY-NC-ND 3.0
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