A mountainside in Greenland collapsed catastrophically into the ocean, last year. The landslide triggered a wave as tall as the Statue of Liberty. This wave — a tsunami — swept 80 kilometers (50 miles) down a fjord there. Its rapid and violent motion ripped up the coastline. And its vibrations were so violent that they were detected worldwide.

Indeed, they rang the planet like a seismic bell every minute and a half — for nine straight days.

The good news: No one was hurt. Large tourist ships ventured into that same fjord on the days before and after the tsunami. They were ferrying people on and off a beach with small boats, according to Kristian Svennevig.

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“It was pure luck that no one was there when it happened,” he says. The powerful wave would have easily capsized the small boats and swept the people away.

Svennevig is a geologist with the Geological Survey of Denmark and Greenland (GEUS). He’s based in Copenhagen, Denmark. He led a team that investigated the event. They now believe that Earth’s warming climate set off the massive landslide that started it all. They described their findings September 13 in Science.

Aside from those caused by earthquakes and volcanoes, “most landslides around the world you can tie to climatic factors,” says Svennevig. And as Earth keeps warming, he worries, this dangerous event could be a warning of worse things to come.

Diagnosing the mystery ringing

Saturday, September 16, 2023, started normally enough. But then at 11:35 a.m., sensors in Asia and North America picked up a sharp seismic signal. They detected waves rippling from a remote part of Greenland’s east coast.

Earthquakes typically cause such vibrations. But explosions, tsunamis and volcanoes can, too. Initially, the source here was unknown.

The next day, tourists on a cruise ship noticed damage to an abandoned research station on east Greenland’s Ella Island. Equipment that was stored on land, including a boat and a truck-size shipping container, had been washed into the sea.

Svennevig heard these reports when he arrived at work Monday morning. Suspecting a tsunami, he looked through recent satellite images of the area. He hoped they would offer clues.

And they did.

For many kilometers along Dickson Fjord, vegetation had been erased from the shorelines. At Ella Island, the plants had vanished to heights as much as four meters (13 feet) above the water. Further west and inland, plants were missing to higher and higher coastal elevations.

A huge wave must have come from the west, Svennevig realized. It would have started out tall and gradually shrunk as it rolled east.

That afternoon, he and several colleagues homed in on its source.

Spied by eyes in the sky

Two satellite photos taken September 15 and 17 showed that a small mountain ridge near the west end of the fjord had vanished.

A frozen glacier had long sat at the foot of this ridge, some 72 kilometers (45 miles) from Ella Island. Somehow, the lower reaches of that glacier had been pulverized.

The collapsed ridge had smashed down onto the glacier below. It released some 25 million cubic meters (880 million cubic feet) of rock and shattered ice. The volume of this debris was equal to 10,000 Olympic-sized swimming pools. This rock and ice tumbled off the end of the glacier and into the fjord’s water.

The resulting splash threw water 200 meters (650 feet) up the nearby mountain slopes. The wave it triggered was 110 meters (360 feet) tall. It may have sped along the fjord at 160 kilometers (100 miles) per hour or more!

But there wasn’t just a single wave.

This explosive event provoked a repeating wave called a seiche (SAYSH). It resembled the back-and-forth sloshing of water in a bathtub. Except here it was way bigger — echoing through the fjord every 90 seconds for nine straight days.

A loss of icy ‘glue’

Though sudden, this catastrophe was caused by changes that have been slowly happening for decades. Earth’s warming climate has shrunk the glacier by Dickson Fjord over time. No one knows how quickly it was losing ice. But other glaciers in this part of Greenland have thinned by as much as 150 meters (490 feet) since 1900. As the glaciers thinned, they laid bare the fragile mountain slopes that their ice had previously supported.

And this appears to be why the ridge at Dickson Fjord collapsed, Svennevig says. It lost that icy support.

He fears such disasters could become more common as Greenland and other Arctic sites warm.

Since 2017, Svennevig has used satellite images to map more than 500 landslides along Greenland’s coasts. Some happened thousands of years ago. Others crumbled as recently as 2021.

In warmer parts of the world, heavy rains often trigger landslides. But a warming of permafrost caused many of the recent ones in Greenland. And here’s how.

As long as the temperature inside a mountain is no warmer than -10° Celsius (14° Fahrenheit), the ice within it acts as cement. It glues bits of rock together. But if the permafrost warms to around -1 °C (30 °F), that ice will change. “It actually starts to deform, like plastic or Play-Doh,” Svennevig explains. Now individual rock bits no longer stick together. This can allow part of the mountainside to collapse.

Indeed, at the sites of new landslides, he often sees rectangular blocks of frozen rock and gravel within the wreckage. Those house-sized icy blocks slowly thaw and crumble.

The role of a warming climate

Elsewhere, melting ice has caused similar disasters.

In 2016, two mountain glaciers in Tibet suddenly collapsed. Gradual warming weakened the Aru-1 and Aru-2 glaciers until their ice could no longer hold onto the mountain. The debris from this disintegration sent enough snow and ice cascading into a valley to fill 60,000 Olympic swimming pools.

The ice and snow traveled up to eight kilometers (five miles) from its source. Along the way, it killed nine herders and hundreds of livestock.

Melting Himalayan glaciers create a different hazard, too. Vast lakes of meltwater have formed behind huge rock piles at the ends of the glaciers. These lakes can suddenly burst out of their fragile natural dams. The resulting “outburst floods” can gush tens of kilometers down through any valley below, sweeping away people, homes, bridges and animals. 

But landslide-tsunamis are especially worrisome because they often happen at sites with more people.

In 1936, one roared through Lake Lovatnet in Norway. It swept away two villages, killing 74 people. And in 1958, a wave in Lituya Bay, Alaska, struck three large fishing vessels. One capsized and sank. One was tossed onto a hillside and destroyed. The third was tossed over a narrow strip of land and into deeper water; that boat, its captain and his 10-year-old son somehow survived.

More recently, a 2017 landslide-tsunami flooded a village in western Greenland, killing four.

But the risk of such an event around cruise ships that venture into fjords could prove especially catastrophic.

“That is something that we definitely think a lot about,” says Katherine Barnhart. She’s a geologist at the U.S. Geological Survey in Golden, Colo. She studies landslide and tsunami risks in Alaska’s Prince William Sound. Much like Dickson Fjord in Greenland, steep mountains line its narrow, branching bay. And here, too, glaciers are shrinking.

Detailed studies like the one Svennevig just published will help Barnhart accomplish a difficult task: building a computer program that can predict the size of a tsunami that a potential landslide might cause. “Building up an inventory of events that have happened is really valuable,” she explains.

Worse to come?

The latest discovery leads Svennevig to worry that things could become worse. Far worse.

Two years ago, one of his collaborators noticed something strange. At the time, Matthew Owen was a marine geologist at Aarhus University in Denmark. He was building sonar maps of the seafloor in Vaigat strait. It’s a narrow ocean channel off western Greenland. These 3-D seafloor maps were being made to help scientists predict how future tsunamis would move through the water.

As Owen assembled the maps, he noticed several large mounds on the bottom of the channel. Each covered many square kilometers. They looked like the mounds that a landslide would drop — except much bigger. Strewn along the seafloor were rectangular rocks the size of skyscrapers.

“Some of these individual blocks were bigger than the recent landslides” that killed people, recalls Owen. (He now works at Global Maritime, a company in England that builds off-shore wind-energy farms.)

When Owen showed these maps to Svennevig, they noticed something important. Each mound sat in front of a massive valley carved into the mountains above. Until then, no one had dared to think that those gouged valleys might be due to landslides. “They were just too big to comprehend,” says Svennevig.

Some were up to 100 times larger than the recent landslides. One had dumped up to 8.4 cubic kilometers (2 cubic miles) of rock onto the seafloor. That’s enough to build 2,800 stacks of rock the size of the Empire State Building.

Those landslides would have triggered enormous waves — many hundreds of meters high. Even 30 to 100 kilometers (18 to 60 miles) from where they formed, he found, waves had tossed rock and gravel as high as 70 meters (230 feet) up onto the land. At a similar distance, recent tsunamis like the one at Dickson Fjord had risen only about one-twentieth that high.

These gigantic landslides likely happened at the end of the last Ice Age, some 7,000 to 10,000 years ago. That’s when glaciers were retreating. And this rapid change destabilized the mountains.

The Arctic’s present-day warming could set the stage for a repeat of this, says Svennevig. “We should prepare for a future where these giant landslides can become a reality again.”