Bell ringer

Sometimes we dodge a bullet.

In September of 2023, seismologists all over the world recorded an odd periodic signal that lasted about nine days.  It was strongest in Europe and eastern North America, but was recorded in places far distant.  The first pulses of the signal had the highest amplitude, and it gradually faded in intensity afterward; the effect was very much like the sound waves generated by a struck bell, which begin loud and eventually diminish into silence once the metal stops vibrating.

It took a while for the geologists to figure out what caused the signal, and when they did, it caused a few gasps -- and then sighs of relief.

The climate-change-induced warmup in the polar regions has caused a huge loss of ice mass in Greenland and Antarctica, and the main associated hazard we've been warned about is sea level rise.  But the September 2023 event highlights another potential problem.  The source of the seismic signal was the collapse of a 1.2-kilometer-high mountain peak into remote Dickson Fjord in Greenland, triggered by the thinning of an ice wall that had held back the rock and debris.  When the estimate 25 million cubic meter landslide hit the water, it triggered a tsunami over a hundred meters high that proceeded to slosh back and forth across the fjord about once every ninety seconds, creating a vibration in the Earth's crust that was picked up on seismometers thousands of kilometers away.

Dickson Fjord, Greenland [Image licensed under the Creative Commons Bjoertvedt, Dicksonfjorden IMG 8800, CC BY-SA 3.0]

The reason I call this "dodging a bullet" is twofold.  First, Dickson Fjord is far away from human settlements; the only damage was to an at-the-time-unoccupied patrol station on Ella Ø, an island seventy or so kilometers away, where the tsunami height was about four meters.  Second, Dickson Fjord is narrow, with a lot of twists and turns, so most of the energy of the tsunami was expended by the sloshing of water back and forth across the inlet; little of the energy made it out of the mouth of the fjord into the north Atlantic.

The analysis of the seismograph data, and their cause, were the subject of a paper in Science this week.

"When I first saw the seismic signal, I was completely baffled," said Stephen Hicks, geologist at University College London, who co-authored the study, in an interview with Science Daily.  "Even though we know seismometers can record a variety of sources happening on Earth's surface, never before has such a long-lasting, globally-traveling seismic wave, containing only a single frequency of oscillation, been recorded.  This inspired me to co-lead a large team of scientists to figure out the puzzle.  Our study of this event amazingly highlights the intricate interconnections between climate change in the atmosphere, destabilization of glacier ice in the cryosphere, movements of water bodies in the hydrosphere, and Earth's solid crust in the lithosphere.  This is the first time that water sloshing has been recorded as vibrations through the Earth's crust, traveling the world over and lasting several days."

It's sobering to think of what would have happened had the landslide occurred in a cliff facing the open ocean, and not in a narrow, remote fjord.  Without anything to damp the oscillations and act as a shock absorber to dissipate the energy of the landslide, the displaced water would have created an unimpeded tsunami that might have wrought havoc on populated coastlines.

We lucked out.

Addressing climate change should be a priority not only for the obvious reasons -- mitigating extremes of weather, slowing down sea level rise, and minimizing the impacts on biodiversity and agriculture.  But the warming Earth increases our risks of other, more sudden and unexpected, hazards, ones that are impossible to predict and therefore way harder to protect ourselves against.  Decreasing our reliance on fossil fuels isn't going to be an easy fix, but in the long run, the dangers of accelerating climate change far outweigh the problems created by our efforts to slow it down.

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