A seismic hum signals a new era of climate uncertainty

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The writer is a science commentator

Seismologists initially thought their instruments were faulty. The signal, detected by monitoring stations in September 2023, did not match the rich tapestry of frequencies characteristic of an earthquake or volcanic eruption. Besides, no such events had been reported.

Yet there it was, plain to see on monitors world over: a single-frequency global hum, pulsing every 92 seconds, for nine days. On online academic message boards, the mysterious signal was ascribed to a USO, or unidentified seismic object.

A year on, scientists think they have cracked the puzzle of what shook the Earth: a climate change-induced landslide in eastern Greenland triggered a tsunami, which sloshed back and forth in a narrow fjord for nine days. The rhythmic slamming of water between the tall, steep parallel banks created detectable vibrations in the Earth’s crust.

The saga shows how climate change is reconfiguring our world in hidden ways — and pushing scientists to the edge of their investigative capabilities. “Our existing [scientific] methods were not set up to deal with this because nothing like it has been seen before,” says Stephen Hicks, a computational seismologist at University College London, who, together with Kristian Svennevig from the Geological Survey of Denmark and Greenland, led an effort to decipher the signal’s backstory. The analysis, published last week in the journal Science, took months of collaboration between 68 scientists across 15 countries, who drew on satellite imagery, ground observations, massive computing power and input from the Danish military.

The headline is not that climate change is happening, nor even that it is happening faster than anticipated. Rather, it is that the consequences are not always apparent, predictable, measurable or explicable. We are moving into a new era of climate uncertainty.

The signal, first picked up on September 16 2023, reached geographically scattered seismometers at different times, allowing the source to be traced back to eastern Greenland. The vibrations wobbled Antarctica, on the other side of the world, within an hour. After an initial seismic burst, one enigmatic, single-frequency signal persisted.

A string of before-and-after satellite images of the region revealed a massive dust cloud and subsequently altered landscape above an inland waterway called Dickson Fjord. A mountain had collapsed, triggering a landslide that, using subsequent measurements from satellites and drones, dumped 25mn cubic metres of rock and ice into the fjord.

This coincided with reports of a tsunami that had inundated a military site and research station about 70km away. The Danish navy photographed the aftermath while sailing in the fjord several days later: a 200m-high black line etched by debris on a nearby glacier marked the height of the initial tsunami.

The rock-ice avalanche tallies with the initial seismic burst — but what caused the longer-lived vibration? By modelling the material entering the water and using the fjord’s width, depth and shape, scientists were able to simulate a plausible scenario.

The collapsed material plummeted into the nearly 3km-wide fjord on one bank, triggering a tsunami that set up a standing wave — called a seiche — sloshing to and fro across the fjord. Such waves, whether in a lake or a bathtub, tend to dissipate quickly but this happened in a stretch of water constricted by a glacier at one end and a tight bend at the other. Essentially, the standing wave was trapped, unable to empty its energy into the ocean around 200km away.

Simulations suggest the water would have swayed from one bank to the other about every 87 seconds, a reasonably close match to the observed signal. The modelling also indicates the seiche would have decayed slowly.

The real landslide, unseen by human eye, was caused by glacial thinning at the foot of the mountain, which destabilised the precipitous terrain. A landslide-tsunami is a first for eastern Greenland, and it happened on a popular tourist route for cruise ships. The Danish authorities, Hicks told me, are assessing the safety of the area.

Scientists have combed through historical seismic data — and have, disturbingly, turned up similar, single-frequency signals in the same region as far back as 2016. This frozen corner of the world may have been creaking before last year’s cataclysmic event, with the odd landslide prompting smaller standing waves that have hitherto gone unnoticed.

The melting landscape is quivering. We should be rattled.