The world’s largest unbroken intertidal system of sand and mud flats could sink beneath the waves by the end of the century due to sea level rise and subsidence caused by gas drills funded by Barclays and other international banks.

The Unesco world heritage site at the Wadden Sea on the Dutch coast stretches over 10,000 sq km. Its saltmarshes, sandy shoals, dunes and mussel beds host millions of migratory birds every year, as well as thousands of basking seals.

But the storm-weathered Frisian coastline may be living on borrowed time. One report this month predicted that it would be submerged by 2100 if current global warming trends and planned gas extractions continue.

“It is more likely that the area will drown than that it will survive,” said Rolf Schuttenhelm, the paper’s author. “The equation is quite simple. If sea level rise and subsidence combined are larger than the maximum speed of sedimentation – and they are – then with time, the drowning of the tidal system becomes inevitable.”

Geese, oystercatchers, spoonbills, pelicans and flamingoes would lose their havens if the flats disappeared. The loss would also sound a death knell for communities which have lived for millennia behind the dykes that dot the Friesland coast.

Seals sunbathe on a sandbank with an oil rig in the background in Texel, Wadden Sea. Photograph: Voorspoels Kurt/Arterra/Alamy

Natural subsidence and gas drills are depressing the Frisian territory, which is mostly below sea level. The North Sea is also rising by 1.8mm a year, a rate likely to accelerate as climate change takes hold.

NAM, a Shell-Exxon partnership, plans to start a gas drilling operation next year and three other wells could soon follow. Local people from 14 villages have formed a movement to resist them.

“It’s all about money,” said Willem Schoorstra, an award-winning Frisian novelist from the group. “We had one talk with NAM and they constantly repeated that they would help us make the clean energy transition. But it’s crap. We don’t buy it. They only want to invest in it when they are drilling. They hold on to their old ways and destroy everything for the sake of money.”

Beneath such talk is an attachment to the low-lying, ancient ground on which Romans, Vikings and Franks have trodden.

“It’s the quietness, the view, the smell,” says Schoorstra, “the birds, the starry skies, the little whispers from the cockles, mudworms and mussels. You can go there twice and the whole landscape has changed. It is where the kids have their first love affairs.”

The Wadden Sea tide ebbs and flows twice a day, bringing sand, silt and nutrients which blanket the mud flats in algae. This holds the sand banks together and provides food for the shellfish and smaller creatures that feed the birds and make the ecosystem’s life cycle turn.

Johan Rispens, a local shrimp fisher, asks: “What will happen to our dykes when they drill for gas under the Wadden Sea? Will they crack? Will there be earthquakes? Why is it even necessary to take the gas from under the sea?”

Michiel Schriers, governor of Friesland’s provincial capital Leeuwarden, holds a protest placard against the gas drilling in the area. Photograph: Arthur Nielsen

Campaigners say some local house prices have fallen by as much as €50,000 (£44,000) each in the last five years due to fears of subsidence and the drilling-related earthquakes that have damaged an estimated 100,000 homes in neighbouring Groningen province.

Michiel Schrier, the Socialist governor of Friesland’s provincial capital Leeuwarden, says: “After the earthquakes in Groningen, people woke up and now every time a contractor wants to take gas from the ground, the communities are revolting and coming together in protest. These movements are getting bigger.”

Gas has been drilled on Ameland island off the Wadden Sea coast since 2006, and Schrier has himself taken part in many of the anti-drill demonstrations.

NAM argues that it can contain subsidence by keeping “a hand on the tap” – to turn their operations off if geological monitoring shows cavities appearing which might not be filled by sedimentation.

But the geological processes at work can take years to become apparent, and NAM’s technical blueprint was dismissed by the government’s mines supervisory board last week for a second time. NAM now has until November to address their report’s shortcomings, after which it could face fines of €500,000 a week.

“We need a proper study and that’s what is not happening at the moment,” said Wouter Van der Zee, the manager of the mines supervision board. “The predicted amount of subsidence and the uncertainty band were just unrealistic.”

If Shell does not correct inadequacies in its methodology, “we may have to advise our minister to reassess the gas production [plans] under the Wadden Sea,” Van der Zee said.

Jeroen Jansen, a senior ecologist for Shell and NAM stressed that boreholes would be drilled from just outside the world heritage site to extract gas from the 200 billion cubic metre field that is directly underneath the mudflats.

The drill was needed “so we can secure a safe, reliable and affordable energy supply in the future,” he said. “We are aware that the Wadden Sea is of high ecological value. If there would be a risk [of collapse], we would not do it.”

Subsidence of 1mm a year under the flats and 3-4mm a year under the salt marshes had taken place since 2006, he said, as a result of the 25 wells drilled in the Wadden Sea area since 1985.



A NAM gas drilling rig near mud flats in the Wadden Sea Islands. Photograph: Frans Lemmens/Alamy Stock Photo

A spokesman for the Dutch government said: “In the transition period towards a fossil fuel-free society, natural gas still has an important role to play. Whether we will eventually extract gas from the Wadden Sea is a different matter. We are still in the midst of a review procedure.”

Gas is seen in the Netherlands as a bridging fuel to a low-carbon future, because it emits just over half as much CO2 as coal. This is still 18 times higher than concentrated solar power and 40 times higher than onshore wind.

In Friesland, the widespread suspicion is that their government listens most attentively when money talks. “The Netherlands is a small country but we are very far from the Hague,” Schrier said. “They have a contract to deliver gas to other countries and it’s a big income for them.”

The issue reached boardroom level earlier this year when Rabobank announced an embargo on new finance for gas exploitation under the Wadden Sea. The bank is a major mortgage provider in the area. ING and ABN Amro, two other major Dutch banks, quickly followed their lead.

However, campaigners say that Barclays, Royal Bank of Canada and the ABP and Delta Lloyd pension funds are still keeping their investment options open.

Candice Macdonald, a spokeswoman for Barclays, said that day-to-day interaction with one of the drilling firms, Tulip Oil, was now being undertaken by Global Natural Resources Investments (GNRI) after a management buy-out of the bank’s natural resources equity office.

“Barclays retains a minority [30%] stake in Tulip,” Macdonald said. “Tulip has ceased all development activities on the Wadden Sea project and has no intention to restart at the current time.”

GNRI did not respond to calls for clarification but the firm’s environmental policy says that they will not back projects that break local laws. In a report on the financing of the gas projects, Barclays did not rule out work on the project resuming, subject to a “clarification of the Dutch mining law”.

Renate de Backere, a world heritage site guide, surveys Wadden Sea mudflats. Photograph: Arthur Nielsen

Back at the Wadden Sea, Renate de Backere, a world heritage site guide surveys the remote muddy wilderness and sighs when asked about the inevitability of subsidence.

“It’s a naturally occurring process but now we humans have created a situation where it’s hardly in control anymore,” she said. “The Wadden Sea is in danger because if there are no mudflats anymore, there will be no places for the seals to rest, the birds will not be able to access food any more, and the ecosystem will become a sea.”