‘Revolutionary’ seafloor fiber sensing reveals how falling ice drives glacial retreat in Greenland

(Press-News.org) 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. Data collected from the cable 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 in Earth and space sciences. “It’s the kind of thing we’ve just never been able to quantify before.”   

The data provides, for the first time, a deeper look at the relationship between ice and the water it collapses into, from surface waves to disturbances within the water column. 

Their findings were published in Nature on Aug. 13.  

The Greenland ice sheet — a frozen cap about three times bigger than Texas — 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 a global current system that controls the climate and nutrient distribution by circulating water between northern and southern regions, called the Atlantic meridional overturning circulation.   

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

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 football stadium and humming along at 15 to 20 miles per hour.     

Glaciers are huge, and most of their mass sits below the surface of the water, where ice melts faster. As warm water eats away at the base, the glacier becomes top-heavy. During a calving event, chunks of the overhanging portion break off, forming icebergs. Calving can be gradual, but every so often, the glacier heaves a colossal chunk of ice seaward. The researchers witnessed a large event every few hours while conducting their field work.

“When icebergs break off, they excite 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.  

Although these underwater waves 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 glacier’s boundary layer and speeding up underwater melt.   

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.”    

Other authors include Manuela Köpfli, a UW graduate student in Earth and space science; Ethan F. Williams a UW postdoctoral researcher in Earth and space science, Andreas Vieli, Armin Dachauer, Andrea Knieb-Walter, Diego Wasser, Ethan Welty of University of Zurich, Daniel Farinotti, Enrico van der Loo, Raphael Moser, Fabian Walter of ETH Zurich, Jean-Paul Ampuero, Daniel Mata Flores, Diego Mercerat and Anthony Sladen of the Université Côte d’Azur, Anke Dannowski and Heidrun Kopp of GEOMAR | Helmholtz Centre for Ocean Research Kiel, Rebecca Jackson of Tufts University, Julia Schmale, of École Polytechnique Fédérale de Lausanne, Eric Berg of Stanford University, and Selina Wetter of the Université Paris Cité 

This research was funded by the U.S. National Science Foundation, the University of Washington's FiberLab, the Murdock Charitable Trust, the Swiss Polar Institute, the University of Zurich, ETH Zurich, and the German Research Center for Geosciences GFZ. 

For more information, contact Dominik Gräff at graeffd@uw.edu.

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