Scientists say they have evidence that the powerful greenhouse gas methane¹ is escaping from the Arctic sea bed.

科学家发现温室效应气体甲烷总北极海岩床渗出的证据。

Methane bubbles observed by sonar, escape from sea bed as temperatures rise

Researchers say this could be evidence of a predicted positive feedback effect of climate change.

As temperatures rise, the sea bed grows warmer and frozen water crystals in the sediment²（沉淀物） break down, allowing methane（甲烷） trapped inside them to escape.

The research team found that more than 250 plumes³ of methane bubbles are rising from the sea bed off Norway.

The joint⁴ British and German research team detected the bubbles using a type of sonar（声纳） normally used to search for shoals（鱼群） of fish. Once detected, the bubbles were sampled and tested for methane at a range of depths.

Writing in Geophysical Research Letters, the team says the methane was rising from an area of sea bed off West Spitsbergen, from depths between 150 and 400m.

The gas is normally trapped as "methane hydrate（水化物）" in sediment under the ocean floor.

"Methane hydrate" is an ice-like substance composed of water and methane which is stable under conditions of high pressure and low temperature.

As temperatures rise, the hydrate breaks down. So this new evidence shows that methane is stable at water depths greater than 400m off Spitsbergen.

However data collected over 30 years shows it was then stable at water depths as shallow as 360m.

Ocean has warmed

Temperature records show that this area of the ocean has warmed by 1C during the same period.

The research was carried out as part of the International Polar Year Initiative, funded by Britain's Natural Environment Research Council (NERC).

The team says this is the first time that this loss of stability associated with temperature rise has been observed during the current geological period.

Professor Tim Minshull of the National Oceanography Centre at Southampton told BBC News: "We already knew there was some methane hydrate in the ocean off Spitsbergen and that's an area where climate change is happening rather faster than just about anywhere else in the world."

"There's been an idea for a long time that if the oceans warm, methane might be released from hydrate beneath the sea floor and generate a positive greenhouse effect."

"What we're trying to do is to use lots of different techniques to assess whether this was something that was likely to happen in a relatively⁵ short time scale off Spitsbergen."

However methane is already released from ocean floor hydrates at higher temperatures and lower pressures - so the team also suggest that some methane release may have been going on in this area since the last ice age.