A 481-meter tsunami in Alaska's Tracy Arm Fjord, triggered when a mountainside collapsed beside a retreating glacier, shows how warming can quietly prime a tourist spot for disaster

On the morning of August 10, 2025, a large chunk of mountainside came loose above Alaska’s Tracy Arm Fjord and fell into the water below, creating a wave that climbed almost 1,578 feet up the surrounding cliffs. At 481 meters tall, the wave had the second-tallest runup height ever recorded, after the 1958 Lituya Bay wave, a few hundred miles away, according to a study published in Science by University of Calgary geomorphologist Dan Shugar and researchers from the University of Washington. The wave would have been "unsurvivable for any ship of any size," said co-author Gerard Roe, a UW professor of Earth and space sciences.

The good news was that it was too early in the morning for the tour boats, so no one was hurt. The bad news: this fjord is a regular stop on Alaska cruise routes that millions of Americans book every summer, and scientists say a warming planet helped set the disaster in motion.

A wave taller than most skyscrapers
According to the Science study, over 64 million cubic meters of rock fell in seconds, generating a seismic signal comparable to a magnitude 5.4 earthquake. The first wave, which had crossed the fjord at more than 150 miles per hour and was 328 feet tall, struck the opposite shoreline with destructive force. According to the U.S. Geological Survey, kayakers camping near the mouth of the fjord woke to water rushing past their tents and carrying off their gear, while a cruise ship anchored nearby reported that the water suddenly surged and shifted beneath it. It's believed that as many as 20 boats, including big cruise liners, visited the same stretch of fjord later that day, after the wave had passed through.

The trap set by a disappearing glacier
Tracy Arm is fed by two glaciers, Sawyer and South Sawyer, both flowing from a larger ice field that straddles the Alaska-British Columbia border. According to the NASA Earth Observatory, South Sawyer Glacier has been thinning and retreating for decades, and the pace has accelerated dramatically since 2000. Roe explained that the slope itself had been exposed only recently: “It was only in the last few years that the glacier retreated past the bottom of where the hillside failed.” The ice had mechanically held up that slope for years. When it retreated, revealing bare rock that had never before been exposed to open water, the slope lost its support. Researchers note that if the glacier’s ice had still been there, the collapse wouldn’t have caused a tsunami nearly as large.

To determine what portion of that retreat was human-driven, Roe and UW research scientist Mira Berdahl ran a model that compares hundreds of climate simulations to a version of the world without fossil fuel emissions. “With this 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,” said Berdahl. The study’s analysis blames 100% of the industrial-era warming in this part of Alaska on human activity, and Roe notes that rising snowlines and thinning ice are clear signs of a changing climate.

The mountain crumbled without much warning
Big rock avalanches like this one usually give warning, with slopes sagging for weeks, months or years before they finally give way, lead author Shugar noted. “In this case, that didn't happen,” he said. The study found that researchers did detect an increase in low-frequency seismic noise prior to the collapse. Harold Tobin, a UW professor of Earth and space sciences, said that long precursory phase is seldom observed, and could, with enough seismic monitoring, help provide advance warning of similar slides over time.

Why this is important if you're thinking about an Alaska cruise
Alaska’s glacier fjords are among the most popular shore excursions for American cruise passengers. Ships often nose in close to the fjord walls for the best photos, the same walls that researchers now say are growing less stable as glaciers retreat. CNN reported on the new study that at least three major cruise lines have already announced they’re skipping Tracy Arm this season in favor of the nearby Endicott Arm fjord. According to the U.S. Geological Survey, the exposed landslide scar above Tracy Arm remains unstable, and ongoing rockfalls there could trigger smaller future tsunamis in the same channel.

Shugar said the event should be a wake-up call not just for Alaska’s cruise industry, but for anyone working in the proximity of retreating glaciers along the West Coast or in polar regions, where the same setup could play out. The point isn’t that Alaska cruises are suddenly dangerous. It’s that some of the most photogenic parts of these trips are in landscapes that are changing faster than most travelers realize, and the rulebook for spotting the next warning sign is still being written.