Scientists are interested in this period because the Earth warmed fairly quickly as increased amounts of carbon dioxide entered the atmosphere at a pace similar to what is happening today, said Richard Pancost, a geochemist at Bristol University, who led the study.

Greenhouse gases such as methane and carbon dioxide are widely blamed for global warming. Scientists say average temperatures will rise by 2-6 degrees Celsius by the end of the century, causing droughts, floods and violent storms.

"It is in the same ballpark of what we have done over the past 100 years and what we might do over the next 100 years," Pancost said in a telephone interview.

The team analysed sediments taken from a wetland in southeast England that was unearthed during construction of a rail link between London and Paris.

This section of exposed rock offered a clear sediment record of changes in vegetation and indicated how global warming affected the area tens of millions of years ago, Pancost said.

The researchers looked at molecular fossils that came from bacteria and found that as temperatures rose, the organisms switched to a diet of methane -- probably because there was more of it around, Pancost said.

"Methane is a very powerful greenhouse gas," he said. "So if the processes at (the bog) were widespread, then the increase in methane emissions could have caused further warming, amplifying the climate change at this time."

The bog became part of a vicious cycle -- warmer temperatures caused higher emissions of methane, which drove temperatures even higher, he said.

"The main event made it warmer and wetter," Pancost said. "What we are talking about is a response to the system."

The researchers, who published their findings in the journal Nature, cautioned that the data was only from a single site but said it nevertheless shows how some ecosystems might respond to rapid climate change in the future.

This means that warming could occur both because of more carbon pumped into the atmosphere through the burning of coal and oil and from the biological response of the individual ecosystems, the team said.