By Helen Briggs
BBC News science reporter
Humans were influencing the climate long before the Industrial Revolution, new research suggests.
Law Dome, Antarctica, where the ice cores came from. (Image: Science)
Levels of methane rose steadily in the atmosphere in the first millennium, according to an analysis of gases trapped in ice beneath Antarctica.
Much of the greenhouse gas came from huge fires lit by humans as they cleared land for settlements and farming, researchers report in Science.
But natural climate change would have contributed to the emissions, they say.
Greenhouse gas emissions have risen to record levels over recent centuries but little is known about the atmosphere in pre-industrial times.
Now, using a new technique, scientists have been able to analyse traces of methane trapped in air bubbles within cores of 2,000-year-old Antarctic ice.
Dr Ferretti with the ice core evidence. (Image: Science)
The chemical fingerprint of stable types, or isotopes, of carbon atoms gives a record of methane in the atmosphere over the course of history, and where it came from.
It appears that much of the gas came from the burning of biomass - the likes of wood and grass - rather than other known sources of methane, such as the burning of fossil fuels, or natural emissions of methane from swamps and wetlands.
"Fire has been known to mankind for hundreds of thousands of years - even though the human population was very small, they set off large fires on a regular basis," lead researcher Dominic Ferretti, of the National Institute of Water and Atmospheric Research Limited in Wellington, New Zealand, told the BBC News website.
"It shows that in pre-industrial times there were much higher levels of methane from wood and grassland fires than we ever thought before.
"The end result is that in the future, with climate change and inevitably warming, we are likely to experience more wild fires in the bush in many areas of the planet as it becomes warmer and drier."
The research adds to a body of evidence that human settlers torched vast areas of jungle and grassland to clear land for farming and settlements.
In the Americas, large swathes of grassland appear to have been burnt every year, for farming or to drive animals into the path of hunters.
Large-scale fires were also lit in the Amazon jungle, to produce charcoal for improving the fertility of the soil.
Early settlers cleared large swathes of grass and shrubland
The data suggests that methane emissions from burning tailed off by about 1700.
The researchers say this may have been due to a natural trend toward cooler and wetter conditions, as well as the decline in the indigenous population in the Americas because of the introduction of diseases by European explorers.
With the advent of industry, however, emissions started to rise once again, far exceeding the levels seen between AD 1 and 1000.