A Dutch government scientist has proposed building two mammoth dams to completely enclose the North Sea and protect an estimated 25 million Europeans from the consequences of rising sea levels as a result of global heating.

Sjoerd Groeskamp, an oceanographer at the Royal Netherlands Institute for Sea Research, said a 475km dam between north Scotland and west Norway and another 160km one between west France and south-west England was “a possible solution”.

In a paper to be published this month in the American Journal of Meteorology, Groeskamp and Joakim Kjellsson of the Geomar centre for ocean research in Kiel, Germany, say the idea is affordable and technically feasible – if intended more as “a warning of the immensity of the problem hanging over our heads”.

Based on existing projects, the scientists estimate the cost of building a so-called North Sea Enclosure Dyke at between €250bn and €500bn. Spread over 20 years, the annual cost to the 14 countries that would be protected by it would amount to just over 0.1% of their combined GDP, they calculate.

Groeskamp said it also appeared technically viable. The depth of the North Sea between France and England rarely exceeded 100 metres, he said, while between Scotland and Norway it averaged about 127 metres, peaking at just over 320 off the coast of Norway.

“We are currently able to build fixed platforms in depths exceeding 500 metres, so such a dam seems feasible,” he said.

International experts agreed that the plans looked theoretically viable. “I guess it depends on what timescale we’re thinking of,” said Hannah Cloke, a professor of hydrology at the University of Reading.

“If you look back hundreds and hundreds of years, then we’ve made some significant adaptations to our landscape, and the Netherlands is an example of that … We can, as humans, do amazing things.”

She added that it was “good that we’re thinking outside the box. I think it is really important that we keep thinking about these ideas, because the future looks very scary. If you look back into the 1940s in the UK, the Thames Barrier probably seemed equally ridiculous. It depends what happens in the next 20-30 years, how bad it gets, and then perhaps we will need something like this.”

However, Cloke cautioned that a dam may not be the best use of the money. “Maybe we should be thinking about making populations resilient to flooding in different ways, and also think about what we can do to stop the climate getting worse – invest in keeping ourselves safe in the long term.”

The authors acknowledge that over time, their project would eventually turn much of the North Sea into a vast tide-free freshwater lake, radically changing its ecosystem. “We estimated the construction costs by extrapolating the costs for large dams in South Korea,” Groeskamp said.

“But in the final calculation, we must also take into account factors such as the loss of income from North Sea fishing, the increased costs for shipping across the North Sea, and the costs of gigantic pumps to transport all of the river water that currently flows into the North Sea to the other side of the dam.”

However, the costs and consequences of doing nothing about rising sea levels would ultimately be “many times higher”, they warned. The project “makes it almost tangible what the consequences of rising sea levels will be”, Groeskamp said.

“A rise of 10 metres by the year 2500 is predicted, according to the bleakest scenarios. This dam is therefore mainly a call to do something about climate change now. If we do nothing, this extreme dam might just be the only solution.”

While experts have criticised Boris Johnson’s plan to build a 20-mile, £15bn bridge between Scotland and Northern Ireland as fraught with possibly insurmountable technical difficulties, the Dutch have been successfully protecting themselves against the sea with dykes for centuries.

Their original 32km Enclosure Dyke, or Afsluitdijk, officially opened in 1933, sealed off a large North Sea inlet that became the freshwater IJsselmeer lake, and subsequently allowed the largest land reclamation project in history.

The Zuiderzee works – of which the Afsluitdijk was a key part – and Delta works, a vast series of dams, sluices, locks and storm-surge barriers protecting the south-west Netherlands from the sea, are widely seen as marvels of hydraulic engineering.

The Intergovernmental Panel on Climate Change predicts that sea levels will rise by 30cm-60cm by 2100, even if the Paris climate accord pledges are met. If emissions continue on their present trends, it foresees an 84cm rise by 2100 and up to 5.4 metres by 2300.

The threat is particularly acute in the Netherlands, about a third of which lies below sea level. Last year, the Dutch government assembled a committee of specialists to monitor the threat closely and devise possible responses.

The government has warned that while it expects its own defences to hold until about 2050, bolstering them further could take years: the past three decades of Dutch sea defence works are designed to cope with a rise of only 40cm.

• This article was amended on 13 February 2020 to clarify that the estimated cost of the project as a percentage of the GDP of the countries affected is annual and spread over 20 years.