A submarine fiber-optic cable has revealed how glacier calving stirs fjords long after the splash. The measurements capture surface tsunamis and towering internal waves triggered by glaciers that mix warm and cold layers.
The result is faster underwater melting and a powerful feedback loop that helps glaciers shed ice.
Researchers from the University of Washington and partner institutions recorded calving across South Greenland’s Eqalorutsit Kangilliit Sermiat by laying a sensing cable along the fjord floor. The work shows how to quantify a process that is accelerating ice loss and affecting ocean circulation and ecosystems.
Remote view of glacier edge
The front of a glacier is no place to linger. Ice blocks the size of buildings break free and hurl into the sea. Traditional instruments cannot safely sit in the impact zone.
“We took the fiber to a glacier and we measured this calving multiplier effect that we never could have seen with simpler technology,” said co-author Brad Lipovsky, a geophysicist at the University of Washington. “It’s the kind of thing we’ve just never been able to quantify before.”
Light tracks waves from glaciers
The team used Distributed Acoustic Sensing (DAS). A 10-kilometer cable was unspooled from a boat near the glacier’s mouth. A compact receiver pulsed light and read tiny changes as the cable strained.
Those optical backscatter patterns recorded ground motion and temperature at thousands of points along the line, for three weeks.
The setup turned the fjord into an instrument. It registered the instant of impact when ice hit the water and followed the wake as icebergs sped past – some as big as stadiums and moving 15 to 20 miles per hour. It also captured what the eye cannot see.
Glaciers, waves, and warm water
Calving first launches a surface surge. These calving-induced tsunamis rake the fjord’s top layers. Then the surface calms. The cable, however, kept shaking. Beneath the still water, internal gravity waves rolled between density layers.
“When icebergs break off, they excite all sorts of waves,” said lead author Dominik Gräff, a postdoctoral fellow at the University of Washington.
The internal waves were as tall as skyscrapers and lasted longer than the surface swell. They mixed the water column, lifting warmth upward and pushing cold water downward. That sustained stirring renewed contact between the glacier face and warmer water at depth.
Feedback loop accelerates melt
Glaciers are top-heavy where they meet the ocean, while most of their mass hides below the surface. Warm water erodes the submerged ice and hollows the base.
Afterward, calving sheds the overhanging top. The splash does more than make noise. It stirs the fjord like a spoon in a drink.
Gräff used a simple image. Around an ice cube, still water turns cool and forms a thin insulating layer. Stirring strips that layer away, making the cube melt faster.
In the fjord, calving does the stirring, and the boundary layer at the ice front is disrupted again and again. The researchers observed a large event every few hours.
Why it matters well beyond one fjord
The Greenland ice sheet is shrinking. It is a frozen cap roughly three times the size of Texas. If it melted, sea levels would rise by about 25 feet, inundating coasts and displacing millions.
Scientists also worry about the Atlantic Meridional Overturning Circulation (AMOC), which moves heat and nutrients around the globe since ice loss and freshwater inputs can weaken that system.
“Our whole Earth system depends, at least in part, on these ice sheets,” Gräff said. “It’s a fragile system, and if you disturb it even just a little bit, it could collapse.”
“We need to understand the turning points, and this requires deep, process-based knowledge of glacial mass loss.”
Improving models and warning systems
Before this expedition, no one had attempted to capture calving with a submarine fiber-optic array.
“We didn’t know if this was going to work,” Lipovsky said. “But now we have data to support something that was only an idea before.”
The cable recorded internal waves not only from the splash but also from glaciers as they cruised down the fjord. Earlier studies relied on isolated bottom sensors and strings of thermometers, which offered only snapshots.
The fiber delivered a movie, with both space and time resolved along the entire line. That level of detail can improve models and inform warning systems for calving-induced tsunamis in narrow fjords.
Fiber optics in glacial science
“There is a fiber-sensing revolution going on right now,” Lipovsky said. “It’s become much more accessible in the past decade, and we can use this technology in these amazing settings.”
The approach is scalable. Cables can be redeployed, linked, and left in place. They can watch a fjord through seasons, storms, and heatwaves and can record the subtle shifts that push a glacier toward a tipping point.
The lesson is clear: watch the waves you cannot see, and you will learn how glaciers vanish. With that knowledge, scientists can better forecast sea-level rise and its cascading effects – from coastlines to currents to communities.
The study is published in the journal Nature.
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