It was an autumn day in the Shetland Islands of Scotland. Perhaps fishing boats were hauling in catch; maybe children were playing on the beach. Then it came in: a wall of water reaching a height of 25m above normal sea level at some points, high enough to top a seven-storey building. Travelling at a speed of some 35 metres per second (80mph) the wave might as well have been cement.
But unlike the Indian Ocean tsunami in 2004, there were no cameras to record the devastation, no internet to spread the news. Instead, when the water receded, it left behind only imprints on the landscape which would remain undiscovered for another 8,000 years.
Today, geologists, palaeontologists and archaeologists are working to piece together exactly what happened, what its effects were – and how likely a similar event is to occur again.
The tsunami that ripped across the North Sea around 6200BC, steamrolling coastlines from Norway to Scotland, resulted from the sudden collapse of some 180 miles (290km) of the continental shelf near Norway. Displacing some 3,000 cubic kilometres of sediment, the Storegga slide’s impact was felt far beyond Scotland’s shores. The resulting wave reached as far as Greenland, says Dr Jon Hill of Imperial College London in the UK.
The Storegga slide was the largest submarine slide ever known to have produced a tsunami
Even today, it is the third-largest submarine slide we know of in the world and the largest known to have produced a tsunami, says Hill. But it was also just one in a series of enormous events and rapid changes that defined the end of Earth’s last Ice Age, which lasted from 110,000 until about 12,000 years ago.
“This is the period when the world really changes,” says University of Bradford archaeologist Vincent Gaffney. “It’s the formation of the modern world, physically and culturally.”
At its most glaciated point, in the northern hemisphere ice covered all of present-day Canada and the northern United States, as well as much of Asia, Russia, northern Europe and Britain. Then came a period of warming about 20,000 years ago. The reasons for this are still debated, but there is growing evidence that warming from seasonal changes in solar energy may have been exacerbated by a naturally occurring rise in atmospheric greenhouse gases.
As the ice melted, Britain went from being an ice-covered tundra within continental Europe to a forested area with a land bridge larger than Holland – Doggerland – across what now is the southern North Sea.
This was Britain’s first Brexit, when we left the continent
It would have been an extremely attractive place to live: a low-lying plain characterised by lakes, estuaries, forests and salt marshes, according to maps by Gaffney and other researchers that were put together using oil companies’ seismic survey data.
The melting ice had other consequences. In North America, meltwater created an enormous lake roughly the size of the Black Sea. About 8,200 years ago, the last glaciers holding the waters back retreated, causing the lake to burst and flood into the North Atlantic. Over the next 100 years, sea levels rose by 6.6-13.1 feet (2-4m), Hill says.
Rising sea levels ultimately would drown Doggerland, giving it the nickname “Britain’s Atlantis” and making Britain an island once and for all. “This was Britain’s first Brexit, when we left the continent,” Gaffney jokes.
By the time the tsunami hit, sea level rise already had made Doggerland – and Britain – an island, most scientists agree.
But as it collided with Doggerland as well as the coasts of Norway, Scotland and North Sea islands like the Shetlands, the tsunami likely would have devastated Mesolithic coastal settlements.
The tsunami’s impact may have been worse because it likely happened in autumn. As far back as 2000, the University of Dundee’s Dr Sue Dawson found wild cherry stones in tsunami deposits indicating an autumn event, a finding further supported by later finds of fish bones and green moss by Norwegian researchers.
The huge waves that suddenly appeared and swept away people and their belongings must have been terrifying - Knut Rydgren and Stein Bondevik
The timing is important, write the authors of the 2014 study on green moss fragments in tsunami deposits, because hunter-gatherer groups tended to hunt inland in the mountains in summer, returning to the coast in the autumn.
“In late October, Mesolithic people along the North Sea-Norwegian Sea coastline would have been near their seashore settlements. The huge waves that suddenly appeared and swept away people and their belongings must have been terrifying,” write the study’s authors Knut Rydgren and Stein Bondevik. “The loss of stores, infrastructure, and tools must have caused serious problems, and many survivors may have succumbed during the first winter.”
Could this kind of event happen again? Dawson, today one of the world’s primary researchers of Storegga tsunami deposits, says that tsunamis already are a more frequent European event than people think.
“We’re starting to think they’re much more common than we once did,” she says. “The big Storegga slide was the main event, really. But there’s evidence in southwest England of the 1755 tsunami that destroyed the city of Lisbon. In the Shetland Islands, we have evidence for another two more localised tsunamis, one around 5000BC and one around 1500BC. And there are lots of historical documents about big waves and possible things that might be tsunamis, but don’t leave any sediment imprints in the landscape.”
During periods of intense climate change, events like Storegga become more likely, say researchers.
It’s being proposed that these landslides could be more frequent due to climate change
In the last 45,000 years, about 70% of all continental slope failures off western Norway occurred during the period of most intense deglaciation: from 16,000 to 4,000 years ago, says Dr Jennifer Stanford, a palaeoceanographer who is examining submarine landslides in the Norwegian Sea with Southampton's National Oceanography Centre.
“You’re overloading the continental shelf with sediment to the point that it just fails,” Stanford says. “Basically, that was the most likely cause. But we’re still not 100% sure.”
The sheer increase of water volume may also have played a part in destabilising the continental shelf. Or it could have been an earthquake, or methane being degassed – or a combination of a number of causes.
Today, some of these factors may be coming back into play.
“It’s being proposed that these landslides could be more frequent due to climate change,” Dawson says.
“Ocean warming might lead to the melting of the gas hydrates in the ice that contain methane that then ultimately can destabilise sediments. Ice sheet melting can cause a frequency in the big earthquakes. As we move through the 21st Century and as you get a combination of all these changes, it could lead to submarine landslides that may be tsunamigenic. That’s why we’re doing the research we’re doing – to try to determine the frequency and timing of these. And you do see an increase in smaller tsunamis around Greenland today, with the iceberg breakoff leading to rolling and wave generation.”
Maybe people were killed and communities were lost but enough people survived to allow us to continue
If destabilisation led to another Storegga-sized slide, the effect could be enormous, says Hill. “If a similar-sized event went off in Svalbard [a Norwegian archipelago] or Greenland, the impact on the UK would be much, much bigger, because the direction is facing us,” he says. “You might see waves of 10m-plus hitting towns like Edinburgh and Newcastle.”
But, he adds quickly, “the chance of this happening is astronomically small. It’s not likely to happen. But smaller slides may be more likely.”
What is worth remembering, say scientists, is that people adapted, not only to life after the Storegga slide, but to life in an increasingly swamped part of Europe. Doggerland drowned, but settlements – and society – went on.
“One lesson we can learn from our Stone Age ancestors is that they clearly did adapt. Maybe people were killed and communities were lost but enough people survived to allow us to continue into the Neolithic,” Dawson says. “This idea of adapting to climate change is a really important message that we can bring through time up to the present.”
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