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As glaciers recede, substantial ice masses detach and plunge into the ocean, creating tsunami-scale waves and leaving a substantial wake as they drift away. This phenomenon, known as calving, is crucial for researchers to comprehend. However, gathering data at a glacier’s front is a perilous endeavor.

To address this challenge, a group of scientists from the University of Washington and partner organizations employed a fiber-optic cable to record calving dynamics across the fjord of the Eqalorutsit Kangilliit Sermiat glacier in South Greenland. This method enabled them to document — without getting too proximate — one of the primary processes that is hastening the pace of glacial mass reduction and thereby endangering the integrity of ice sheets, which has implications for global ocean currents and local ecosystems.

“We transported the fiber to a glacier and recorded this astonishing calving multiplier effect that we previously could not have detected with simpler technology,” stated co-author Brad Lipovsky, a UW assistant professor in Earth and space sciences. “It’s something we’ve just never quantified before.”

Their results were released in Nature on August 13.

The Greenland ice sheet — a frozen expanse approximately three times the size of Texas — is diminishing. Scientists have tracked its retreat for the last 27 years as they rush to comprehend the ramifications of ongoing mass loss. Should the Greenland ice sheet fully melt, it would release enough water to elevate global sea levels by around 25 feet, submerging coastlines and displacing millions.

Researchers also theorize that ice loss is compromising the Atlantic meridional overturning circulation, a global current system that regulates climate and nutrient distribution by facilitating water flow between northern and southern areas.

“Our entire Earth system relies, at least partially, on these ice sheets,” remarked lead author Dominik Gräff, a postdoctoral researcher in Earth and space sciences. “It’s a delicate system, and if disrupted even slightly, it could collapse. We must grasp the turning points, which necessitates in-depth, process-oriented knowledge of glacial mass reduction.”

For the researchers, this called for a field expedition to South Greenland — where the Greenland ice sheet converges with the Atlantic Ocean — to deploy a fiber-optic cable. In the last decade, scientists have been investigating how these cables can enable remote data collection through a technology known as Distributed Acoustic Sensing, or DAS, which captures ground motion based on cable strain. Prior to this research, no one had attempted to record glacial calving using a submarine DAS cable.

“We were uncertain if this would succeed,” Lipovsky commented. “But now we possess data to substantiate something that was merely a concept before.”

The researchers deployed a 10-kilometer cable from the rear of their boat close to the glacier’s edge. They connected it to a compact receiver and gathered ground motion data and temperature measurements along the cable for three weeks.

The backscatter pattern from photons traveling through the cable provided researchers with a glimpse beneath the surface. They could make detailed observations about the enormous chunks of ice rushing past their vessel. Some of which, Lipovsky noted, were as large as a stadium and moving at 15 to 20 miles per hour.

Glaciers are massive, and most of their mass resides beneath the water’s surface. Mass loss occurs more rapidly underwater, eroding the base and creating an unstable overhang. During a calving incident, the overhanging section detaches and plunges into the ocean. Gradual calving wears away at the glacier, but occasionally, a significant event takes place. Throughout the experiment, the researchers observed a substantial event occurring every few hours.

“Icebergs are detaching and generating all sorts of waves,” said Gräff.

Following the initial impact, surface waves — referred to as calving-induced tsunamis — surged through the fjord. This disturbs the upper water column, which is layered. Seawater is warmer and denser than glacial melt, thus sinking to the bottom. However, long after the splash, when the surface calmed, researchers noted other waves, called internal gravity waves, traversing between density layers.

Though invisible from the surface, the researchers recorded internal waves towering as high as skyscrapers rocking the fjord. The slower, sustained motion engendered by these waves prolonged water mixing, delivering a continual influx of warmer water to the surface while pushing colder water down to the fjord bottom.

Gräff likened this process to ice cubes melting in a warm beverage. 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 significantly quicker. In the fjord, researchers theorized that waves from calving were disrupting the boundary layer and enhancing underwater melt.

Scientists also identified disruptive internal gravity waves arising from the icebergs as they traversed the fjord. This kind of wave is not unprecedented, but capturing them on this scale is. Earlier studies depended on location-specific readings from ocean bottom sensors, which only provide a brief view of the fjord, along with temperature measurements from vertical thermometers. The findings could enhance forecasting models and assist in establishing early warning systems for calving-induced tsunamis.

“There is currently a fiber-sensing transformation occurring,” remarked Lipovsky. “Over the last ten years, it has become significantly more accessible, enabling us to utilize this technology in these extraordinary environments.”

Additional contributors include Manuela Köpfli, a graduate student at UW 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 from the University of Zurich; Daniel Farinotti; Enrico van der Loo; Raphael Moser; Fabian Walter from ETH Zurich; Jean-Paul Ampuero; Daniel Mata Flores; Diego Mercerat; and Anthony Sladen from the Université Côte d’Azur; Anke Dannowski; and Heidrun Kopp of GEOMAR | Helmholtz Centre for Ocean Research Kiel; Rebecca Jackson from Tufts University; Julia Schmale, from École Polytechnique Fédérale de Lausanne; Eric Berg from Stanford University; and Selina Wetter of the Université Paris Cité.

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

For further information, reach out to Dominik Gräff at [email protected].

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