Massive leakage of methane from thawing shoreline in the Arctic would devastate the world's climate and economy, a trio of scientists warned on Wednesday.
Billions of tonnes of this potent greenhouse gas are locked in the shallow frozen shelf of the Arctic Ocean, which warms when summer sea ice retreats as a result of the greenhouse-gas effect, they said in a contribution to Nature.
The team modelled what would happen if 50 billion tonnes, or gigatonnes (Gt), of methane escaped over a decade from the floor of the East Siberian Sea, covering two million square kilometres (772,200 square miles) of the Arctic Ocean off northeastern Russia.
"The methane release would bring forward the date at which the global mean temperature rise exceeds two degrees Celsius (3.6 degrees Fahrenheit) by between 15 and 35 years," said Chris Hope of Cambridge Judge Business School, part of England's University of Cambridge.
Gail Whiteman, a professor of sustainability, management and climate change at Erasmus University in Rotterdam, the Netherlands, described the threat as an "invisible time bomb".
"The mean impacts of just this one effect -- $60 trillion -- approaches the $70 trillion value of the world economy in 2012."
The high cost is explained by damage to the climate system, reflected in worse floods, droughts, storms and heat stress, she said.
Eighty percent of the effects would occur in poorer economies in Africa, Asia and South America, according to their model, called PAGE09.
The estimates are based on how the added methane would affect two trends -- one for existing greenhouse-gas emissions, which are very high, and the other for lower emissions giving a more than one-in-two chance of meeting the UN's 2C warming target.
In an email exchange with AFP, Hope said that if the 50 Gt were released over 20 years, from 2015-2035, the cost would be around $64.5 trillion.
If the release were spread over 30 years, from 2015-2045, it would be $66.2 trillion.
"This is because more of the methane remains in the atmosphere in the period when impacts are expected to be higher in the latter half of this century," he said.
If 25 Gt were released, the cost in all scenarios would be roughly halved.
Scientists have long worried about methane locked up in shoreline sediments and also in permafrost on land.
Methane is 25 times more effective than carbon dioxide (CO2) in trapping solar heat.
The big concern is that the methane, if released to the atmosphere, adds to global warming, which thus accelerates the thaw, adding more gas and amplifying the temperature rise -- a "positive feedback," or vicious circle in climate terms.
But evidence for such a threat is sketchy, research into it is meagre and the conclusions often contested. Some experts also say there could be as-yet unknown mechanisms in permafrost thaw that may limit the methane leakage.
In 2008, Russian scientists writing in the journal Geophysical Research said they estimated 540 Gt of methane to be stored in the Siberian Arctic shelf. Of this, up to 50 Gt could be considered as being "highly possible for abrupt release at any time".
In 2010, another Russian team, reporting in Science, said they had found large amounts of leakage from perforated permafrost on the shelf.
In contrast, in 2011, another team, also Russian, said they saw evidence of outgassing in the East Siberian Sea. But they attributed this to a long-delayed consequence of the end of the last Ice Age, not from recent changes in the Arctic Ocean.