Centuries before the Industrial Revolution or the recognition of global warming, the ancient Roman and Chinese empires were already producing powerful greenhouse gases through their daily toil, according to a new study. The burning of plant matter to cook food, clear cropland and process metals released millions of tons of methane gas into the atmosphere each year during several periods of pre-industrial history, according to the study, published in the journal Nature.

Although the quantity of methane produced back then pales in comparison with the emissions released today — the total amount is roughly 70 times greater now — the findings suggest that man’s footprint on the climate is larger than previously realized. Until now, it was assumed by scientists that human activity began increasing greenhouse gas levels only after the year 1750.