“Big calving event!” someone calls from the bridge of the R/V Adolf Jensen, our ice-strengthened research vessel. We quickly pull our oceanographic instruments from the water. The captain turns the vessel toward the towering front of the Eqalorutsit Kangilliit glacier in southern Greenland. A massive chunk of ice—roughly the size of a football stadium—has just broken off the glacier and crashed into the fjord. We watch as the iceberg rolls, splashes, and sends up a tsunami wave. It spreads outward like ripples from a stone dropped into a pond—only on a much larger scale. By the time it reaches us, just a few hundred meters away, the wave has lost most of its height—but it still rocks the boat.
This isn’t just a dramatic spectacle. It’s a dynamic and dangerous environment—one we planned to study in close detail by installing a subsea fiber-optic cable parallel to the glacier’s calving front, as wide as a city and as tall as a small skyscraper.
Ice-Ocean Interactions
Calving events like this, along with underwater melting, account for around half of the Greenland Ice Sheet’s total mass loss. If all of Greenland’s ice were to melt, global sea levels would rise by over seven meters. The outlet glaciers of the Greenland Ice Sheet act like conveyor belts, moving ice from the interior into Greenlandic fjords. Warm Atlantic waters creep into these deep fjords and melt the glacier from below. This undercutting destabilizes the ice front, often triggering more frequent and larger calving events.
The feedback loop between underwater melting and calving not only influences glacier stability but also regulates the input of freshwater into the North Atlantic—a critical part of the global climate system. Disruptions here may affect the Atlantic Meridional Overturning Circulation, a key driver of Earth’s climate.
A New Perspective from the Seafloor
To understand these ice-ocean interactions at the process scale, we needed to get dangerously close to the ice front. But instead of deploying sensors in the water column or on the ice, we used a method that allowed us to measure from a safe distance: distributed fiber-optic sensing. By laying a 10-kilometer fiber-optic cable onto the seafloor just 500 meters from the glacier front, we weren’t transmitting data—we were using the cable itself as thousands of sensors. Pulses of laser light sent down the fiber scatter back and are measured by devices called interrogators. Tiny changes in strain and temperature alter the backscattered light, allowing us to map the signatures of ice-ocean interactions in space and time.
The interrogators remained safely onshore, powered entirely by solar panels in Greenland’s long summer days. From there, we unspooled the fiber-optic cable off the back of the Adolf Jensen, navigating a maze of icebergs across the fjord. Cracking sounds echoed from the towering 80-meter-high ice front just 500 meters beside us. The cable settled on the fjord floor at depths reaching 300 meters, and once we reached the far side of the fjord, we brought the final section ashore. Back at the starting point, we powered up the interrogators, started the lasers, pressed record—and headed to our tents for the night.
The Wake-Up Call
At 5 a.m., I woke to what sounded like thunder—but it wasn’t a storm. Another massive calving event had occurred. A minute or two later, waves crashed against the shore near our field camp. My first thought: the fiber sensing interrogators must have recorded the entire calving sequence. I scrambled out of my sleeping bag, grabbed some breakfast, and walked down to the shore to check the instruments.
An Underwater Symphony
At first, everything looked fine. We had clearly recorded the acoustic and seismic signatures of iceberg calving: sharp ice-fracturing signals followed by vibrations as the iceberg detached. Seconds later, the passage of the calving-induced tsunami wave over the cable appeared in the data. Then came something more intriguing. Minutes after the main event, slow 20-minute oscillations appeared in the cable’s temperature readings from various depths—internal gravity waves, triggered by the calving event and traveling through the fjord’s layered waters, where cold freshwater from glacier melt floats above warm, salty Atlantic water. Near the center of the glacier front, strong harmonic vibrations emerged along the cable—coherent over tens of meters—accompanied by a sudden drop in temperature.
And then—silence. Or more precisely: instrument noise. From the midpoint of the cable onward, the signal was gone.
Later analysis revealed that as the stadium-sized iceberg drifted away from the glacier front, it pulled behind it a powerful internal wave wake—like the V-shaped trail behind a boat, but underwater and on a colossal scale. This internal wave wake heaved and depressed the fjord’s stratified water layers. As colder water was pushed to the seafloor, horizontal currents surged, tensioning the cable and causing it to vibrate like a guitar string. These strong currents likely tightened loose loops of slack and eventually kinked the cable, cutting the light signal—just like bending a garden hose blocks the water flow.
A New Lens on Glacier Dynamics
Our study offers a new view of the calving process chain through the lens of distributed fiber sensing. We show that calving-induced internal wave activity can enhance underwater melting—potentially accelerating ice loss in a positive feedback loop. What makes this work exciting is the perspective: instead of relying on a few point sensors, we used tens of thousands of sensing points stretched across the seafloor, directly in front of the glacier. It’s a leap forward in how we observe ice-ocean interactions—placing the sensor right where the future of Greenland’s ice is being decided.