"The pre-industrial time was not a natural time for the climate - it was already influenced by human activity," she said. "When we do future climate predictions we have to think about what is natural and what did we add. We have to define what is really natural," she said.

The scientists, in the Netherlands, Switzerland, Denmark, the United States and France, noted a second rise in methane in Medieval times, coinciding with a warm period from 800 to 1200 that also saw Europe's economy emerge from the Dark Ages.

That spike might be because population growth in Asia and Europe led to more deforestation for farming.

Rates then fell, perhaps partly because factors such as the Black Death cut the population.

Methane levels rose a third time around the start of a cool period known as the Little Ice Age in the 1500s, perhaps also reflecting strong population growth after the plague.

The scientists used variations in the chemical make-up of methane in the ice to try to distinguish background natural sources from man-made emissions.

Ice cores from Greenland - made up of layers of compacted snow that give a year-by-year record - found concentrations of methane rose from about 600 parts per billion around 2,000 years ago to above 700 ppb by 1800.

They are now at about 1,800 ppb. Methane is the second most important greenhouse gas behind carbon dioxide, emitted by human burning of fossil fuels.

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