A record of the atmosphere trapped in Greenland's ice found the level of heat-trapping methane rose about 2,000 years ago and stayed at that higher level for about two centuries.
Methane was probably released during deforestation to clear land for farming and from the use of charcoal as fuel, for instance to smelt metal to make weapons, lead author Celia Sapart of Utrecht University in the Netherlands told Reuters.
"Per capita they were already emitting quite a lot in the Roman Empire and Han Dynasty," she said of the findings by an international team of scientists in Thursday's edition of the journal Nature.
Rates of deforestation "show a decrease around AD 200, which is related to drastic population declines in China and Europe following the fall of the Han Dynasty and the decline of the Roman Empire," the scientists wrote.
Mankind's emissions 2,000 years ago, when the world population was an estimated 300 million, were discernible but tiny compared with current levels caused by a population of 7 billion.
Sapart estimated that methane emissions until 1800 were about 10 percent of the total for the past 2,000 years, with 90 percent since the Industrial Revolution.
Methane is generated from human sources including burning of forests and fossil fuels, rice paddies, livestock or landfills. Natural sources include wetlands, wildfires or mud volcanoes.
The findings by Sapart's team questioned the view by a U.N. panel of climate scientists that man-made climate change started with the surge in use of fossil fuels during the Industrial Revolution.
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