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Some Alaska cruises are skipping a popular excursion to Tracy Arm Fjord this year after a landslide-generated tsunami barreled through the narrow channel during peak season last August. A new analysis of the event from researchers at the University of Calgary and the University of Washington, published May 6 in Science, describes how glacial retreat caused by global warming primed the fjord for the colossal wave and what, if any, warning signs preceded it.

At 5:26 a.m. on Aug. 10, 2025, a piece of the mountainside one kilometer tall and 200 meters thick collapsed into the Tracy Arm Fjord, a scenic waterway south of Juneau. Rock crashed into the water, taking with it chunks of the South Sawyer glacier and producing a 481-meter high tsunami so powerful that it scraped surrounding hillsides bare.

The event would have been “unsurvivable for any ship of any size,” said co-author Gerard Roe, a UW professor of Earth and space sciences, but fortunately the tsunami occurred too early for tours and no one was harmed.

Later that day, as many as 20 boats, including large cruise ships, may have visited the fjord. Tourist vessels often draw near the fjord wall to get the best vantage point for photographs of towering glaciers and mountains. The slope that failed was only recently exposed to the water below it due to glacial retreat.

“It was only in the last few years that the glacier retreated back past the bottom of where the hillside failed,” Roe said.

Tracy Arm Fjord hosts two glaciers, the Sawyer and South Sawyer, which both stem from the Stikine Icecap, a frozen expanse spanning the Alaska-British Columbia border. The larger South Sawyer glacier terminates in the water, making it a tidewater glacier, while the Sawyer retreated onto land in 2023.

Satellite observations indicate that the ice has retreated nearly 10 kilometers since the beginning of the industrial era, with the pace accelerating after 2000.

Mapping the change in position and mass of a tidewater glacier can be difficult because they shrink in multiple directions. Exposed ice melts in the sun and chunks break off and fall into the water at the glacial front. Glaciers around the world have been retreating in response to global warming, but tidewater glaciers don’t always follow general trends.

To understand the link between global warming and the 2025 tsunami, researchers used a computational method developed by Roe and Mira Berdahl, a UW research scientist in Earth and space sciences. Their approach combines hundreds of simulations from various computer models to estimate how different certain climates would look without human influence.

“With these data, we can quantify how unusual the observations are compared to the expected natural variability in the climate had we not been burning fossil fuels,” Berdahl said.

In the study, they conclude that 100% of the industrial-era warming in this region of Alaska is human-caused. As it gets warmer, less snow accumulates and the ice retreats.

“Snowline elevations are rising, ice is thinning, and the ice cap is shrinking. Even though tidewater glaciers can be more complicated to study, we are fully confident that the retreat is primarily due to the changing environment, and we are the cause of the changing environment,” Roe said.

It is possible that glacial retreat destabilized the slope that failed, but specific landslide triggers are notoriously difficult to discern. Either way, if the surface beneath the slope had been glacial ice, the slide wouldn’t have produced such a massive tsunami.

Although no one was harmed by the wave, those nearby raised the alarm. Kayakers awoke early in the morning to water flowing past their tents and carrying away some of their gear. A cruise ship anchored near the mouth of the fjord described large waves rolling through and shifting currents. These reports allowed researchers to triangulate the landslide, but the authors say there were very few advance warning signs.

“Normally with these gigantic rock avalanches, they often give some sort of warning signs in the weeks, months or years prior when the slope is slowly moving down the mountain. It’s sagging and then it catastrophically gives way in a rock avalanche,” said lead author Dan Shugar, associate professor of Earth, energy and environment at the University of Calgary. “In this case, that didn’t happen.”

The researchers did note an increase in low frequency seismic noise before the landslide.

“The long precursory phase of seismic activity before the landslide is fascinating, and to my knowledge, rarely observed,” said Harold Tobin, a UW professor of Earth and space sciences. “Given its duration and the relative ease of detection, this type of signal could conceivably provide advance warning of large slides if enough seismic monitoring can be deployed.”

Until that happens though, it will be difficult to predict the behavior of changing terrain.

The unexpected event presents challenges when it comes to disaster reduction in high-risk areas, Shugar said. Cruise ship companies, captains and other stakeholders should pay close attention, particularly in areas on the West Coast and in polar regions where glaciers are thinning due to the changing climate.

This study was funded by Natural Sciences and Engineering Research Council, Alberta Innovates, Canadian Space Agency, U.S. Geological Survey Landslide Hazards Program, the U.S. National Science Foundation, NERC, the Eric and Wendy Schmidt Foundation, and the Carlsberg Foundation.

This story was adapted from a press release by the University of Calgary. 

For more information, contact Roe at groe@uw.edu.