It started with a melting glacier that triggered a massive landslide, which triggered a 650-foot-tall mega-tsunami in Greenland last September. Then something inexplicable happened: a mysterious tremor that rocked the planet for nine days.
Over the past year, dozens of scientists around the world have been trying to figure out what the signal is.
Now they have an answer, according to a new study in the journal Science, and it’s another warning that the Arctic is entering “uncharted waters” as humans push global temperatures higher.
Some seismologists thought the instrument was broken when it started picking up ground vibrations in September, said Stephen Hicks, study co-author and seismologist at University College London.
It’s not a rich orchestra of heights and rumbles you can expect with an earthquake, but more of a monotonous hum, he told CNN. Earthquake signals tend to last for several minutes; this lasted for nine days.
He was confused, “it wasn’t done before,” he said.
Seismologists traced the signal to eastern Greenland, but were unable to pinpoint a specific location. So he contacted his friends in Denmark, who had received reports of a tsunami causing a landslide in a remote area called Dickson Fjord.
The result is a collaboration of almost a year between 68 scientists from 15 countries, who combed through seismic data, satellite and on-the-ground, as well as simulation of tsunami waves to solve the puzzle.
What’s happening is called “cascading hazards,” Svennevig says, and it all starts with human-caused climate change.
For years, the glaciers at the base of the great mountain nearly 4,000 feet above Dickson Fjord have been melting, as many glaciers are in the extremely warm Arctic.
As the glacier thinned, the mountain became increasingly unstable before finally collapsing on September 16 last year, sending enough rock and debris into the water to fill 10,000 Olympic-sized swimming pools.
The subsequent mega-tsunami – one of the highest in recent history – started a wave that became trapped in a bendy, narrow fjord for more than a week, sloshing back and forth every 90 seconds.
The phenomenon, called “seiche,” refers to the rhythmic movement of waves in an enclosed space, similar to water splashing back and forth in a tub or cup. One scientist even tried (and failed) to recreate the impact in his own bathtub.
While seiches are well-known, scientists previously had no idea they could last that long.
“Had I proposed a year ago that a seiche could last for nine days, people would have shaken their heads and said it was impossible,” said Svennevig, who compared the discovery to suddenly discovering a new color in the rainbow.
It is these seiches that create seismic energy in the earth’s crust, scientists have discovered.
This may be the first time scientists have directly observed the impact of climate change “on the ground beneath our feet,” Hicks said. And no place is immune; the signal traveled from Greenland to Antarctica in about an hour, he added.
No one was injured in the tsunami, although centuries-old cultural heritage sites were damaged and an empty military base was destroyed. But this stretch of water is on a commonly used cruise ship route. If someone was there at that time, “the consequences would be devastating,” the study’s authors wrote.
East Greenland has never experienced landslides and tsunamis like this, Svennevig said. This suggests new areas of the Arctic are “coming online” for such climate events, he added.
As the Arctic continues to warm – over the past few decades, the region has warmed four times faster than the rest of the world – mega-tsunamis triggered by landslides could become more common and have potentially deadly consequences.
In June 2017, a tsunami in northwest Greenland killed four people and destroyed homes. The threat goes beyond Greenland, Svennevig said; Similar fjords exist in other regions, including Alaska, parts of Canada and Norway.
What happened in Greenland last September “once again demonstrates the destabilization of large mountain slopes in the Arctic due to amplified climate warming,” said Paula Snook, a landslide geologist at Western Norway University of Applied Sciences who was not involved in the study.
Recent rock avalanches in the Arctic as well as in the Alpine region are “alarming signals,” he told CNN. “We are thawing the ground that has been in the cold, frozen state for many thousands of years.”
There is still a lot of research to be done on rock avalanches, which are also affected by natural processes, said Lena Rubensdotter, a researcher at the Geological Survey of Norway, who was also not involved in the study.
However, he added, “it is logical to assume that we will see more frequent rockfalls on permafrost slopes due to the warming climate in the Arctic region.”
The discovery of natural phenomena behaving in unnatural ways highlights how parts of the world are changing in unexpected ways, Svennevig said.
“This is a sign that climate change is pushing the system into uncharted waters.”
For more CNN news and bulletins create an account at CNN.com