(CNN) – It all started with a melting glacier that caused a huge landslide, which triggered a 200-meter-high megatsunami in Greenland last September. Then something inexplicable happened: a mysterious vibration that shook the planet for nine days.
Over the past year, dozens of scientists around the world have tried to figure out what that signal was.
Now they have an answer, according to a new study published in the journal Science, which provides another warning that the Arctic is entering “uncharted waters” as humans push global temperatures ever higher.
Some seismologists thought their instruments were broken when they began detecting vibrations in the ground in September, said Stephen Hicks, a co-author of the study and a seismologist at University College London.
It wasn’t the rich orchestra of high pitches and rumblings that an earthquake might produce, but rather a monotonous hum, he told CNN. Earthquake signals tend to last minutes; This lasted nine days.
Hicks was baffled, it was “completely unprecedented,” he said.
Seismologists tracked the signal to eastern Greenland, but were unable to identify a specific location. So they contacted colleagues in Denmark, who had received reports of a tsunami caused by a landslide in a remote part of the region called Dickson Fjord.
The result was a nearly year-long collaboration between 68 scientists from 15 countries, who reviewed seismic, satellite and ground data, as well as simulations of tsunami waves to solve the puzzle.
What happened is called a “cascading danger,” Svennevig said, and it all started with human-caused climate change.
For years, the glacier at the base of a massive mountain nearly 1,200 meters high above Dickson Fjord had been melting, like many glaciers in the Arctic as it warms at an accelerated rate.
As the glacier shrank, the mountain became increasingly unstable until it finally collapsed on September 16 last year, sending enough rock and debris into the water to fill 10,000 Olympic-size swimming pools.
The subsequent megatsunami—one of the largest in recent history—triggered a wave that was trapped in the winding, narrow fjord for more than a week, swaying back and forth every 90 seconds.
The phenomenon, called “seiche,” refers to the rhythmic movement of a wave in an enclosed space, similar to water splashing back and forth in a bathtub or cup. One of the scientists even tried (and failed) to recreate the impact in his own bathtub.
Although seiches are well known, scientists had no idea they could last so long.
“If I had suggested a year ago that a seiche could last nine days, people would have shaken their heads and told me it’s impossible,” said Svennevig, who compared the discovery to suddenly finding a new color in a rainbow.
It was this seiche that created seismic energy in the Earth’s crust, scientists discovered.
It is perhaps the first time scientists have directly observed the impact of climate change “on the ground beneath our feet,” Hicks said. And no place was immune; The signal traveled from Greenland to Antarctica in about an hour, he added.
No one was injured in the tsunami, although it swept away centuries-old cultural heritage sites and damaged an empty military base. But this stretch of water is on a route commonly used by cruise ships. If there had been any there at the time, “the consequences would have been devastating,” the study authors wrote.
East Greenland had never experienced a landslide and tsunami like this before, Svennevig said. It shows that new areas of the Arctic are also experiencing these types of climate events, she added.
As the Arctic continues to warm—in recent decades, the region has warmed four times faster than the rest of the world—megatsunami caused by landslides may become more common and deadly.
In June 2017, a tsunami in northwest Greenland killed four people and leveled homes. The threat goes beyond Greenland, Svennevig said; Similarly shaped fjords exist in other regions, including Alaska, parts of Canada and Norway.
What happened in Greenland last September “demonstrates once again the continued 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 did not participate in the study.
Recent rock avalanches in the Arctic, as well as in alpine regions, are “an alarming sign,” he told CNN. “We are thawing soils that have been in a cold, frozen state for many thousands of years.”
There is still much to be investigated about 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 rock collapses on permafrost slopes as the climate warms in Arctic regions.”
The discovery of natural phenomena behaving in seemingly unnatural ways highlights how this part of the world is changing in unexpected ways, Svennevig said.
“It’s a sign that climate change is pushing these systems into uncharted waters.”
