How Climate Change in the Arctic Could Cause Tsunamis in the UK

Every few years the government of the United Kingdom produces a new edition of the National Risk Register of Civil Emergencies: a 50-odd page document listing all the bad things that could possibly happen to the country.

Disasters are ranked according to both their potential for damage and the likelihood that they will occur in the next five years. An influenza pandemic, for example, is considered both likely and serious, while wildfires are both less damaging and about a hundred times less likely. Disasters sometimes change rank depending on new information—between 2013 and 2015 "public unrest" became less likely, somehow—but the overall list of disasters is generally the same each time. Tsunamis, probably owing to the UK's lack of seismic activity, have never made it into the register.

But that could change in future editions. A group of over 30 UK scientists, representing 11 different universities and research institutes, are busy studying a lesser-understood type of tsunami, caused by enormous underwater landslides instead of earthquakes. These "submarine slides" have happened in the North Atlantic in the distant past, and there's a possibility that, owing to climate change, the rapidly warming conditions in the Arctic could make them more likely in the future.

"They're not on the risk register right now. But they could be more frequent than currently expected, based on the historical record," said Peter Talling of the National Oceanography Center, who heads the project, known as the Landslide-Tsunami Consortium.

The group has sent teams to dig up sediment in Iceland, Norway, and across North England and Scotland looking for evidence of past tsunami activity; it sent a research ship to the northern Arctic to search for undersea evidence of ancient landslides and deposits of sediment that could cause future slides; and it's engaged a disaster modelling group to predict the size and trajectory of tsunamis that could reach the shores of UK—and ultimately, the kind of damage they would do.

"We're trying to provide a complete scientific basis to make decisions for something like the risk register," said Talling.

So how does a landslide cause a tsunami? With an earthquake like the one that caused the tsunami that struck Japan in 2011, the sudden, violent movement of a chunk of the Earth's crust displaces a vast column of water. An underwater landslide also displaces water: by causing an enormous amount of the loose sand and debris that covers the ocean floor to shift in a very short time. The scary thing is, unlike a slide on dry land, an underwater landslide can happen on almost any surface. Talling likes to point out that in the ocean, the downward slope needed to move thousands of tons of material is only one or two degrees—"about the same as a football pitch, nearly flat," he said.

But it can produce a tsunami all the same, and landslide tsunamis have hit the UK in the past. Talling noted that the Storegga slide, a landslide that moved over 3000 square kilometers of sea sediment off the coast of Norway over 8,000 years ago, generated a wave that travelled over 800 km south to cause six meter swells on the Scottish coast, devastating the landscape at that time.

Sue Dawson, a geologist at the University of Dundee, has led teams digging all over the northern UK looking for the geological record of those ancient waves. She said that in the Shetland Islands, north of mainland Scotland, they see evidence of at least three tsunami events over the past 8,000 years—one as recently as 1,500 years ago—with ocean sediment layered into the black peat that slopes down to the coast. "The sediment runup is in excess of 20 meters (above sea level)—that's akin to the Indian Ocean event," she said, referring to the 2004 tsunami that caused over 230 000 deaths in that region.

How likely is a North Atlantic tsunami in the future? According to James Hunt at the National Oceanography Center, climate change could tilt things significantly. "Offshore Norway has been known for a long time as an area susceptible to large submarine landslides. I don't want to call it a hotspot, but it's glacially influenced," he said. "It has some level of seismicity because it is undergoing glacial retreat."

Hunt said that as a glacier melts, the Earth beneath it is relieved of an enormous weight very quickly, and—like the springs on a trampoline, or a class at the dismissal bell—it releases some of the tension and strain, possibly as seismic activity that could cause landslides.

Glacial melting also flushes previously trapped water and sediment into the ocean. According to Hunt, these rapid deposits can significantly affect the stability of the ocean floor. "The sediment isn't like pouring sand on a desk; there's fluid in between the grains, so as you dump a lot of this sediment and water very quickly, the pressure felt at the bottom of the pile can't escape. The pressure is trapped. So if there's a strain—say an earthquake—the probability of [the sediment layer] failing is higher," he told me.

And glacial melting isn't the only climate change-related worry for the team. There's also an entire group working on gas hydrates: hard frozen crystals found within the sediment layers that may melt into gas as the ocean warms. This would cause an abrupt pressure shift and, as with everything else, could destabilize the sediment and cause a landslide.

Talling has previously pointed out that the Storegga landslide and tsunami also coincided with the last major global climatic shift, a rapid cooling around Greenland and the North Atlantic called the 8.2 kiloyear event. Both he and Hunt, though, are cautious about drawing any conclusions about how the warming ocean could affect landslides at this stage.

"I don't think now, even with the wealth of data we are generating, we can reliably say that climate change is going to have a positive or negative effect on landslides. What our project has tried to address is providing more empirical data so that we can have these discussions," said Hunt.

To provide that data, both Talling and Hunt took part in an expedition to the Arctic in 2014. Using a Dutch research vessel, they took core samples all over the sea between Norway and Greenland, looking for large sediment deposits and places where previous slides had occurred.

"With a bit of prior knowledge, we can also prospect for areas that are prone to failure," said Hunt. He said that the sampling project has two main aims: to produce maps of the sediment on and underneath the sea floor, and to get an idea of the frequency and volume of landslides in the area. "If we're able to sample the landslide, the material we get back tells us a lot about the process of how the landslide occurred, whether it ran very quickly. We can get a record of how many occurred, and how big they are."

They are in the process of analysing the data for frequency now. The information is also shared with a team at Imperial College London, in the Applied Modelling and Computation Group, whose job is to generate models of possible tsunamis, including where they go and what happens if they hit land.

"Once they have identified plausible submarine landslide scenarios within the Arctic basin then we come and simulate those scenarios using our numerical models," said Simon Mouradian, a post-doc in the modelling group. "They provide us with locations, directions, slide inclinations, and the potential volume of sediment displaced in each location."

The group uses a software they developed called Fluidity to simulate the whole process. In an animation from the model, a landslide near Svalbard generates a series of waves that radiate outward from the North Atlantic, appearing to splash gently around the shores of Scandinavia and the UK.

Of course, in reality there would be nothing gentle about it. In a conference presentation earlier this year, the group noted that, according to their models, a landslide event three times smaller than the Storegga slide could still produce large enough waves to threaten the coast, and that waves could reach as far south as Hartlepool, in north east England.

It's also part of the computation group's job to asses the likely damage from such an impact, no matter how unlikely. "We are working with the reinsurance industry, they provide us with exposure data for assets," said Mouradian. "For some locations, our preliminary results show that the impact of the tsunami from some landslide scenarios can cause several millions of pounds of direct damages."

All of the work is still ongoing. The project is funded with a £2.3 million grant from the Natural Environment Research Council, and plans to wrap up its four year investigation later this year. Everyone I spoke with mentioned that they were planning a publication specific to their own field from the data they have gathered, but the hope is that together they will also be able to provide a much wider look at the threat of tsunamis to the UK, using the data from the past to inform models about the future.

"The UK exerts a legal requirement on certain key infrastructure providers to address hazards events that occur once every 10,000 years or less," Talling told the journal Nature Climate Change in 2013, in the early days of the project. For the National Risk Register, all the listed disasters have at least a 1 in 2,000 chance of occurring within a five-year window.

I asked Hunt about the possibility of tsunamis making the next year's register.

"I don't think it would be our place to 'get' a particular hazard onto the risk register," he told me in an email, noting that the group's job was simply to provide the best available information to inform policy makers. He did say that the group is in consultation about having submarine landslides and tsunamis considered for inclusion, and that although the data isn't complete or published yet, the evidence "would incline us to believe these hazards should be considered."