Methane that was once frozen beneath the Arctic seabed is being released at an ever-increasing rate as the waters warm.
Scientists have long predicted that this would happen but what surprised researchers investigating the phenomenon was the quantity of methane already being released.
Prof Tim Minshull, head of the University of Southampton's School of Ocean and Earth Science based at the National Oceanography Centre (NOC), said: "Our survey was designed to work out how much methane might be released by future ocean warming; we did not expect to discover such strong evidence that this process has already started."
The research team found more than 250 plumes of methane rising from the seabed in the limited area they examined.
The potent greenhouse gas is being released from methane hydrate, an ice-like substance that has until now been stable under the high pressures and low temperatures of the arctic sea floor.
Today, the substance remains stable at depths of greater than 400m, but 30 years ago it was stable in waters as shallow as 360m.
This is the first time that such behaviour in response to climate change has been observed in the modern period.
While most of the methane currently released from the seabed is dissolved in the seawater before it reaches the atmosphere, methane seeps are episodic and unpredictable and periods of more vigorous outflow of methane into the atmosphere are possible.
Methane dissolved in the seawater contributes to ocean acidification.
Graham Westbrook, Professor of geophysics at the University of Birmingham, said: "If this process becomes widespread along Arctic continental margins, tens of megatonnes of methane per year - equivalent to 5-10% of the total amount released globally by natural sources, could be released into the ocean."
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