By Richard Black
Environment correspondent, BBC News website
A 55-million-year-old British bog uncovered by the Channel Tunnel rail link is giving scientists insights into an ancient period of global warming.
The ancient bog emerged during digging for the Channel Tunnel link
The researchers found methane released from the bogs played a major role in the Palaeocene-Eocene Thermal Maximum, when temperatures suddenly rose.
They suggest in the journal Nature that this is likely to clarify the role of bogs in present-day global warming.
Methane released by warmer conditions could make temperatures rise faster.
"This is the first time that we have seen evidence from the geological record of methane cycling in response to a warming event," said study leader Richard Pancost from Bristol University.
"It provides insight into how some ecosystems could respond to rapid warming-induced changes in climate, and, therefore, how they could respond to warming in the future."
The Palaeocene-Eocene transition about 55 million years ago saw the fastest period of warming documented in Earth's geological history. The average global temperature rose by about 5C in a few thousand years, perhaps even faster.
The principal theory is that something triggered the release of vast amounts of methane from the sea bed. As methane is a potent greenhouse gas, the planet's surface swiftly warmed.
The new research focussed on the Cobham Lignite, an ancient bog discovered during excavations for the Channel Tunnel rail link.
Scientists looked for hopanoids, chemicals made by bacteria which survive remarkably well over time.
At the Palaeocene-Eocene transition, they found the ratio of two carbon isotopes changed in the hopanoids - a change which is very probably down to an increase in methane in the atmosphere.
But this could not have been the initial burst of methane released by the oceans, if indeed that theory is correct. Methane degrades rapidly in the atmosphere; yet the methane enrichment recorded in the hopanoids endured for tens of thousands of years.
Instead, the scientists believe, the chemicals are documenting a long-term release of methane from the bog itself which was stimulated by the temperature rise.
"We think what we were seeing was a response to the warming, a positive feedback mechanism which could amplify climate change," explained Dr Pancost.
With billions of tonnes of carbon locked up in bogs, there is concern that rising temperatures could cause a similar feedback loop in the near future.
In Britain, bodies such as the National Trust have appealed for better conservation of bogs to keep the carbon stored away.
The Bristol team is cautious about extrapolating from events 55 million years ago, when temperatures and conditions were very different, to the present - particularly after having investigated just a single site.
But, says Richard Pancost: "A lot of temperate and polar wetlands are going to be wetter, and of course warmer as well.
"That implies a switch to more anaerobic conditions which are more likely to release methane. That's what's predicted, and that would be a positive feedback - and we have evidence now that this is what happened."