Methane, a greenhouse gas second in importance only to carbon dioxide, has built up rapidly in the atmosphere since the Industrial Revolution due to human emissions. It was believed that prior to the 19th century, changes in atmospheric methane were mostly caused by natural emissions from wetlands. New research, led by a team from Utrecht University in the Netherlands, now suggests that earlier human civilizations, such as the Roman Empire and Han Dynasty, had a major impact on methane emissions. By analyzing the isotopes of methane trapped in Greenland ice cores, the researchers tracked changing methane sources over the past 2000 years. They found that the increase in methane emissions during that time is in large part explained by increasing human populations and land-use. While these early human methane emissions were much smaller than emissions caused by burning fossil fuels, they suggest that humans could have influenced greenhouse gasses well before the fossil fuel era.