What does a melting glacier sound like? 'Gunshots.'

Tiny, pressurized air bubbles, trapped in ice, are accelerating the decline of some of the world’s glaciers, scientists have recently discovered.

In tidewater glaciers—large rivers of ice that pour into the sea—the steady melting of ice underwater causes billions of these bubbles to burst into the water “like little tiny gunshots,” says Erin Pettit, a glaciologist at Oregon State University in Corvallis, who is part of the team that made the discovery.

That violent pop of air stirs up the cold layer of water that hugs the surface of the glacial ice—bringing warmer water, several inches away, into contact with it. The bubbles then rise buoyantly through the water—driving turbulent currents that also bring warm water into contact with the ice.

The glaciers of Alaska, where Pettit’s team conducted the study, are currently losing over 70 billion tons of ice per year, a loss that causes seas to rise around the globe.

This new discovery stems from a 15-year effort by Pettit to understand an important but dangerous environment. In addition to improving our understanding of climate change, her efforts could also explain sharp reductions in the abundance of seals in some Alaskan fjords.

Unlocking the anatomy of a melting glacier

Few people realize it, but glaciers are full of tiny air bubbles. They form as snow, accumulating over thousands of years, slowly compresses into ice under its own weight—squishing the air that was trapped between snowflakes into microscopic pores. A cubic foot of glacial ice can hold over five million bubbles. Those bubbles are compressed to as much as 20 times the pressure of the Earth’s atmosphere.

Pettit and her colleagues had suspected for several years that these bubbles might cause the ice in tidewater glaciers to melt more quickly. To test this idea, they undertook a series of laboratory experiments.

They harvested blocks of bubble-rich ice from a tidewater glacier in Alaska called Xeitl Sít’ in Tlingit (also known as LeConte Glacier) and watched as the ice melted in a fish tank filled with sea water. As a comparison, they also melted blocks of bubble-free ice, which they purchased from a local ice sculpting artist.

As the ice melted, the resulting fresh water rose, because it was less dense than the surrounding sea water. This created a rising current along the vertical face of the ice—a microcosm of what occurs at the front of a real tidewater glacier. When the glacial ice melted, the current that it produced was six times faster than what was seen with the bubble-free ice because the rising bubbles pulled the water up more quickly. The glacial ice melted 2.25 times more quickly than the bubble-free ice.

“That’s a very powerful effect,” says Keith Nicholls, a polar oceanographer with the British Antarctic Survey in Cambridge, who was not part of the team. “If that’s the reality in nature, then it’s quite serious.”

The discovery was published today in the journal Nature Geoscience.

Discovering the “snap, crackle, and pop” of glaciers

Pettit’s first inklings that bubbles could be important came from monitoring glacier changes from afar.

Although huge amounts of melting happen at the calving fronts of tidewater glaciers, scientists are reticent to approach them too closely, due to the risks involved. The fronts of these glaciers tower up to 200 feet above the water, forming sheer walls that can send 50-ton ice blocks plunging downward at any moment, triggering waves that can crush or capsize small boats.

In 2009, Pettit tried to monitor the ice front from a safe distance in Icy Bay, Alaska, using hydrophones to record sounds underwater. She expected to hear icebergs calving off the glacier—perhaps even the low, garden-hose gurgle of a subglacial river gushing out from underneath the glacier.

But the main noise captured by these recordings was something more continuous, “like a sizzling pan of cooking food—a bit of snap, crackle, and pop,” says Pettit.

At 120 decibels, “the sounds were off the charts,” she says, louder than a car horn or a kitchen blender. The sounds were so loud that Jeffrey Nystuen, an oceanographer from the University of Washington who loaned her the hydrophones, believed the equipment was malfunctioning.

(Chile's glaciers are dying, and you can actually hear it—read more.)

Only after several more years of capturing recordings in other fjords did Nystuen finally embrace Pettit’s interpretation: that the sounds came from air bubbles popping out of the ice as it melted.

How bubbles expose glaciers to warming oceans

When Pettit published those observations back in 2015, she hoped to use the underwater sounds to monitor the rate of melting, and how it changed over the seasons. The full importance of the bubbles didn’t emerge until 2018, when she happened to discuss them with a newly hired professor at Oregon State.

As Pettit chatted over wine with Meagan Wengrove, an engineer and the study’s lead author, who studies the turbulence of rivers, they realized that those bubbles might actually mix up the thin ‘boundary layer’ of cold water that often insulates glacial ice from warm water. That very afternoon they rushed to out to a pet store and bought an aquarium that they would use in their newly published experiments.

Jonathan Nash, an oceanographer at Oregon State who is part of the team (and also Pettit’s husband), thinks that the bubbles probably exert their strongest melting effects in glaciers that thin substantially as they flow into the ocean, in places like Alaska, Canada, and Greenland—bringing ice that had been deeply buried up close to the surface.

These conditions will bring highly pressurized bubbles into the shallow part of the ocean (say the upper 300 feet) where the pressure inside the bubble is much higher than the surrounding water pressure—allowing the bubble to expand explosively and rise quickly.

Nash doesn’t expect these bubbles to have as much widespread impact in Antarctica, where most of the melting currently happens at far deeper depths, where the water pressure is higher—blunting the bubbles’ explosive effects.

What do bursting bubbles mean for rising seas?

These new results don’t mean that tidewater glaciers will melt and retreat twice as quickly as scientists had expected. But the new findings could help solve a long-standing mystery: in some tidewater glaciers in places like Alaska, Canada, and Greenland, the ice front is melting 10 times more quickly than scientists think it should, based on the water temperature.

The newly discovered bubble effect could explain some of that extra melting, says Mathieu Morlighem, a glaciologist at Dartmouth College in Hanover, New Hampshire. “It’s improving our understanding, but it’s not painting a darker picture of what’s happening today,” he says.

The new discovery will help scientists like Morlighem improve their models to better predict the future shrinkage of glaciers as oceans warm in the coming century. “It’s really, really critical,” he says. “We need a lot more work like this, to better understand the interaction between ocean water and the ice, and what drives that melt rate.”

New theories about bubbles in icy ecosystems

Pettit speculates that these glacial bubbles may have yet other, unseen effects in places like Alaska—perhaps even shaping aquatic ecosystems.

She notes that in Alaska, many fjords with tidewater glaciers have large populations of harbor seals. The animals shelter there while molting and raising pups. But in Glacier Bay, where the tidewater glaciers retreated many miles inland, the seal populations have declined. ­­­­

Pettit now suspects that the roaring patter of exploding bubbles provides a hiding place where seals can avoid detection by hungry orcas, which often find their prey by listening. The bubbles may mask the seals’ sounds—at least until the ice retreats out of earshot.

This may turn out to be yet another way in which these tiny bubbles have surprisingly large-scale effects—the proverbial ‘butterfly effect’—whereby tiny flapping wings spawn storms in distant places.

“Can these sub-millimeter bubbles actually affect the global ocean circulation” and global sea level? asks Nash: “Maybe they can.”