It is one of the nightmare scenarios of global warming.

As the temperature rises, vast areas of permafrost begin to thaw, allowing dead vegetation and the bodies of countless millions of animals to decompose.

The associated release of greenhouse gases like methane and carbon dioxide adds to global warming, causing more permafrost to melt in a vicious upward spiral.

Now a new scientific paper has found the frozen wastes of places like Siberia and Canada are much more susceptible to warming than previously thought.

The researchers estimated that about four million square kilometres of land – an area larger than India – will melt for every degree of global warming.

And that figure is about 20 per cent higher than the one currently used in climate model projections.

In total, about 15 million square kilometres on land is permafrost, which is defined as an area that remains below zero degrees Celsius for at least two years.

Writing in the journal Nature Climate Change, the researchers said the “loss of permafrost … could provide very significant feedbacks on climate change”.

And they added it would also “radically change” the biochemistry and hydrology of the land itself, potentially causing the ground to become unstable, putting buildings and roads at risk.

There is already significant evidence that the tundra is starting to break up with a massive crater known as the Gateway to the Underworld growing in Siberia at an alarming rate.

There has also been concern that smallpox – eradicated in 1977 – could return after the virus’s DNA was found in the frozen corpses of historic victims.

However the researchers, from the UK, Sweden and Norway, said the effect of thawing permafrost could be significantly limited if the world managed to prevent global warming from going higher than 1.5C above pre-industrial levels, set as an aspiration by the Paris Agreement on climate change alongside the target of 2C.

Last year, the world was about 1.1C above pre-industrial levels, partly boosted by the natural El Nino effect.

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The paper’s lead-author, Dr Sarah Chadburn, of Leeds University, told The Independent that previously the level of permafrost thawing had been poorly understood.

Their study was the first to come up with an estimate based on actual observations on the ground, rather than purely using computer models.

“We are much more confident that we have understood it,” she said.

Asked to describe the significance of the 20 per cent increase in thawed permafrost, Dr Chadburn said: “I think we have sort of known it [the issue of melting permafrost] is big for a long time.

“It’s 20 per cent more, it was already big. I’ve already for the last few years … had a few moments when I’ve been thinking ‘oh gosh’.

“We spend all our lives thinking about it, so it doesn't shock us any more, but pinning down the answers really more makes us feel hopeful that we will be able to better inform people.”

However it remains uncertain just how much greenhouse gases will be emitted from the decomposing material.

“It’s another big step to say how much carbon would then be released into the atmosphere,” Dr Chadburn said.

“We have a rough idea of how much carbon is there, but how much will decompose is very uncertain, scientifically.”

She added in an email that previous studies had found there was “a ‘small’ risk that there could be a ‘very large’ carbon release from permafrost and a lot of extra warming, up to 0.6 degrees” over several centuries.

However Professor Peter Wadhams, of Cambridge University, who was not involved in the researcher, said there was a far bigger threat posed by methane trapped in the frozen sea bed of the Arctic, which he said was already being released as the sea ice on the surface melts.

He was one of the authors of a paper in the journal Nature which estimated there could be what he described as a “catastrophic methane pulse” that could result in warming of 0.6C in just five years.

Describing the loss of permafrost on land as a “slow process”, Professor Wadhams argued the real danger was from the sea.

“We are getting pure methane coming out, which gets a huge climate change boost and the threat offshore is immediate and very serious,” he said.

“You can see methane plumes coming up from the sea bed. When it comes out into the atmosphere at the top of that plume it could be set fire to.