Scientists deployed an innovative fiber optics in Greenland

As glaciers melt, huge chunks of ice break free and splash into the sea, generating tsunami-size waves and leaving behind a powerful wake as they drift away. This process, called calving, is important for researchers to understand. But the front of a glacier is a dangerous place for data collection.

To solve this problem, a team of researchers from the University of Washington and collaborating institutions used a fiber-optic cable to capture calving dynamics across the fjord of the Eqalorutsit Kangilliit Sermiat glacier in South Greenland. This allowed them to document — without getting too close — one of the key processes that is accelerating the rate of glacial mass loss and in turn, threatening the stability of ice sheets, with consequences for global ocean currents and local ecosystems.

“We took the fiber to a glacier, and we measured this crazy calving multiplier effect that we never could have seen with simpler technology,” said co-author Brad Lipovsky, a UW assistant professor of Earth and space sciences. “It’s the kind of thing we’ve just never been able to quantify before.”

Their findings have just been published in the print edition of the journal Nature. The research was conducted as part of the GreenFjord project, led at EPFL by Julia Schmale, assistant professor and director of the Extreme Environment Research Laboratory (EERL) in Sion.

• About the GreenFjord project

The Greenland ice sheet — a frozen cap about 40 times bigger than Switzerland ­­­— is shrinking. Scientists have documented its retreat for the past 27 years as they scramble to understand the consequences of continued mass loss. If the Greenland ice sheet were to melt, it would release enough water to raise global sea levels by about 25 feet, inundating coastlines and displacing millions of people.

Researchers also speculate that ice loss is weakening the Atlantic meridional overturning circulation, a global current system that controls the climate and nutrient distribution by circulating water between northern and southern regions.

Understanding turning points

“Our whole Earth system depends, at least in part, on these ice sheets,” said lead author Dominik Gräff, a postdoctoral researcher of Earth and space sciences. “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.”

For the researchers, that meant taking a field trip to South Greenland — where the Greenland ice sheet meets the Atlantic Ocean — to deploy a fiber-optic cable. In the past decade, researchers have been exploring how these cables can be used for remote data collection through technology called Distributed Acoustic Sensing, or DAS, that records ground motion based on cable strain. Before this study, no one had attempted to record glacial calving with a submarine DAS cable.

“We didn’t know if this was going to work,” said Lipovsky. “But now we have data to support something that was just an idea before.”

Researchers dropped a 10-kilometer cable from the back of their boat near the mouth of the glacier. They connected it to a small receiver and collected ground motion data and temperature readings along the length of the cable for three weeks.

Underwater observation

The backscatter pattern from photons passing through the cable gave researchers a window beneath the surface. They were able to make nuanced observations about the enormous chunks of ice speeding past their boat. Some of which, said Lipovsky, were the size of a stadium and moving faster than a car on the freeway.

Glaciers are huge, and most of their mass sits below the surface of the water. Mass loss proceeds faster underwater, eating away at the base and creating an unstable overhang. During a calving event, the overhanging portion breaks off and splashes into the sea. Gradual calving chips away at the glacier, but every so often, a large event occurs. During the experiment, the researchers witnessed a large event every few hours.“Icebergs are breaking off and exciting all sorts of waves,” said Gräff.

Following the initial impact, surface waves — called calving-induced tsunamis — surged through the fjord. This stirs the upper water column, which is stratified. Seawater is warmer and heavier than glacial melt and thus settles at the bottom. But long after the splash, when the surface had stilled, researchers observed other waves, called internal gravity waves, propagating between density layers.

As tall as skyscrapers

Although they were not visible from the surface, the researchers recorded internal waves as tall as skyscrapers rocking the fjord. The slower, more sustained motion created by these waves prolonged water mixing, bringing a steady supply of warmer water to the surface while driving cold water down to the fjord bottom.

Gräff compared this process to ice cubes melting in a warm drink. If you don’t stir the drink, a cool layer of water forms around the ice cube, insulating it from the warmer liquid. But if you stir, that layer is disrupted, and the ice melts much faster. In the fjord, researchers hypothesized that waves, from calving, were disrupting the boundary layer and speeding up underwater melt.

Internal gravity waves

Researchers also observed disruptive internal gravity waves emanating from the icebergs as they moved across the fjord. This type of wave is not new, but documenting them at this scale is. Previous work relied on site specific measurements from ocean bottom sensors, which capture just a snapshot of the fjord, and temperature readings from vertical thermometers. The data could help improve forecasting models and support early warning systems for calving-induced tsunamis.

“There is a fiber-sensing revolution going on right now,” said Lipovsky. “It’s become much more accessible in the past decade, and we can use this technology in these amazing settings.”