Long before the era of fossil fuels, humans may have triggered a massive but mysterious “carbon bomb” lurking beneath the Earth’s surface, a new scientific study suggests. If the finding is correct, it would mean that we have been neglecting a major human contribution to global warming — one whose legacy continues. The researchers, from France’s Laboratory of Climate and Environmental Sciences and several other institutions across the globe, suggest that beginning well before the industrial era, the mass conversion of carbon-rich peatlands for agriculture could have added over 250 billion tons of carbon dioxide to the atmosphere. That’s the equivalent of more than seven years of current emissions from the burning of fossil fuels for energy.

“Globally [peatlands] are only 3 percent of the land surface but store about 30 percent of the global soil carbon,” said Chunjing Qiu, a researcher at the laboratory, a joint institution supported by French government research bodies and the Versailles Saint-Quentin University, and the first author of the study published Friday in the journal Science Advances.

Scientists have long worried about the potential for massive amounts of carbon being released by northern permafrost, where ancient plant remains lie in a kind of suspended animation beneath the surface. But the peat threat is very similar; in fact, peatlands overlap considerably with permafrost regions.

Peatlands are a particular type of wetland, one in which dead plant matter does not fully decay due to the watery conditions, and thus accumulates. In its normal state, peat slowly pulls carbon out of the atmosphere — unless you disturb it.

Carbon dioxide can stay in the atmosphere for hundreds or even 1,000 years, meaning that peatland conversions from long ago, even before we began large-scale burning of fossil fuels, can still be affecting the planet.

“It is almost 10 percent of the carbon that humans have emitted by burning fossil fuels since 1850,” said Philippe Ciais, a co-author of the study and a researcher with the Laboratory of Climate and Environmental Sciences.

What’s difficult is knowing when and where these historical peatland conversions occurred. For example, when a bog was converted to farmland or someone dug a drainage ditch, it’s not necessarily the case that anyone recorded it.