Tidewater glaciers—colossal rivers of ice that flow into the sea—continually sizzle and hiss as their icy underbellies thaw in seawater. These underwater noises, sounding a bit like frying food, are caused by the release of once-trapped air bubbles.
But these tiny pressurized bubbles aren’t just making noise. New research has shown that energy liberated as the bubbles explode can enhance the underwater melting of these glaciers. Lab experiments showed that bubbly glacier ice melts twice as fast as bubble-free ice.
“These millimeter-sized air bubble explosions have an outsized influence on tidewater glacier melt rates,” said Erin Pettit, a glaciologist at Oregon State University and a coauthor of the new study, which was published in Nature Geoscience.
The discovery could explain, in part, why some tidewater glaciers, such as Xeitl Sít’i (LeConte) Glacier in Alaska, are melting faster underwater than predicted by theoretical models.
Gauging Melting
One of the most dramatic ways that tidewater glaciers contribute to sea level rise is through shedding icebergs from their steep-sided fronts, where ice meets ocean—a process called calving. They also eject meltwater via streams running along their bases and through direct melting of their submerged fronts in warm ocean water.
Scientists want to understand this underwater melting because it can influence glacier stability and iceberg calving, but measuring it directly is challenging. Instead, they use theoretical models to estimate ice melting based on ocean temperatures and currents. These models also give a picture of how glaciers might respond to ocean warming caused by climate change.
But the trouble with these ice melt models, explained Pettit, is that they are based on observations of bubble-free sea ice. Up to 10% of glacier ice is air, which gets trapped in between ice crystals as snow is compressed over time. The air inside these bubbles can be at 20 times normal atmospheric pressure at sea level.
Pettit started studying glacier air bubbles about a decade ago, when she used hydrophones to listen to the soundscape near an Alaskan glacier in a fjord. A chance conversation with Meagan Wengrove, a coastal engineer at Oregon State University, spurred an idea for their lab experiment on popping bubbles. Wengrove is the lead author of the new study.
Testing a Hunch
No one had previously investigated how high-pressure bubbles affect ice melt, Wengrove said, even though “air bubbles are widely known to create turbulent flow in liquids.”
Wengrove and her colleagues had a hunch that air bubbles might disturb the thin layer of cold water known to coat glaciers’ undersides, bringing warm water in direct contact with the glacier ice and enhancing melting.
To test their theory, the researchers used high-speed cameras to record glacier ice from Oregon State University’s ice core lab as it melted in a saltwater tank. They also shone laser light onto the ice surface and used tracer particles to track currents in the surrounding water. Then they repeated the experiment with bubble-free ice donated by an ice sculptor.
Wengrove and colleagues found that air bubbles increased melting by supplying fast moving warm water to the ice surface. In their recordings, they observed that air bubbles popped out of the thawing ice explosively, leaving behind low-pressure voids in the protective boundary layer. They think warm seawater then rushes toward the glacier to fill these voids. The team also found that these air bubbles pulled warm water upward with them as they rose, creating currents that melted the ice further.
Wengrove and coauthor Jonathan Nash, an oceanographer at Oregon State University, built a model simulation to explore the effect of air bubbles at the glacier scale. They saw that the bubbles caused the strongest submarine melting in water less than 60 meters deep, where water pressure is lower and therefore the bubbles released expand rapidly and remain in the water column longer.
“This work is an important step in improving the accuracy of our ice melt models.”
That means the results are likely most relevant to the tidewater glaciers in the shallow waters around the Arctic, Nash explained. Antarctic glaciers melt at a greater water depth, so this process may be less impactful there, he said.
“These air bubbles clearly have a significant, yet overlooked impact on melting,” said Twila Moon, a glaciologist at the National Snow and Ice Data Center at the University of Colorado Boulder, who was not involved in the study. “By quantifying that impact, this work is an important step in improving the accuracy of our ice melt models,” she added.
Currently, models rely mostly on ocean temperature and the strength of freshwater plumes coming from the glacier base to predict ice melt. Scientists know they are missing physics that improve their predictions, including the newly discovered bubble effect.
More Field Data
“These are really neat experiments,” said Keith Nicholls, an oceanographer at the British Antarctic Survey who was not involved in the study. He noted, however, that air bubbles probably don’t fully explain the discrepancy between model estimates and real observations of glacier melt.
That’s because the environment in front of tidewater glaciers, where meltwater and seawater mix, is particularly complex, Nicholls explained. “Those processes are difficult to replicate in theoretical models, or in the lab.”
Wengrove and colleagues said they need more field data before they can estimate the relative contribution of air bubbles to melting. Aside from the water conditions around a glacier, other ice properties, such as its surface roughness, can determine how quickly ice melts.
The team recently returned from fieldwork at Xeitl Sít’i Glacier, where they have been using remotely operated vehicles to collect these much needed observations. Following their lab experiments, the all-too-familiar bubbles have taken on a new significance, Nash said. “Now when we get close to an iceberg, all we can hear are these bubbles screaming out at us as they melt away.”
—Erin Martin-Jones (@Erin_M_J), Science Writer
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