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 Thursday 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.
"The quantities are much smaller, because there were fewer people on Earth," said study leader Celia Sapart, an atmospheric chemist at Utrecht University in the Netherlands. "But the amount of methane emitted per person was significant."
Ms. Sapart's conclusions were based on an analysis of ice core samples from Greenland. The layered ice columns, which date back 2,000 years, contain tiny air bubbles from different periods of history, and provide scientists with a view into the atmosphere's changing chemistry.
The first period of methane production captured in the ice cores -- roughly from the years A.D. 1 to 300 -- encompassed the tail ends of the Roman Empire and the Han Dynasty, when charcoal was the preferred form of fuel. The second period of elevated methane emissions occurred during what's known as the Medieval Climate Anomaly, from roughly 800 to 1200, and a third was found during the Little Ice Age between 1300 and 1600.
Methane is one of a few gases that trap heat in the atmosphere and contribute to global warming. It forms naturally when plant and animal matter decomposes in airless environments, and it's also released when vegetation burns. However, when methane is produced by burning, it contains heavier carbon isotopes than methane generated through decomposition.
By using a mass spectrometer to study the air trapped in the ice cores, Ms. Sapart and her colleagues were able to determine the ratio of methane produced by burning and by decomposition. The study notes that not all cases of burned vegetation were the result of human activity; forest fires, particularly in times of drought, would also contribute to so-called pyrogenic methane production. The research team used mathematical models to account for this naturally burning vegetation and other fluctuations in atmospheric methane content.
"The results show that between 100 B.C. and A.D. 1600, human activity may have been responsible for roughly 20-30 [percent] of the total pyrogenic methane emissions," the authors wrote.
The research appeared to be the result of very careful and very difficult examination of carbon isotopes and could impact global warming estimates for the pre-industrial period, according to Ed Dlugokencky, a methane expert at the National Oceanic and Atmospheric Administration's Earth System Research Laboratory in Boulder, Colo.
"The study gives further evidence for a contribution to the global methane burden from anthropogenic sources," said Mr. Dlugokencky, who was not involved in the study.
Ms. Sapart said that though the study helped answer questions about the past, there were still plenty that remained about the future. Of particular concern is the melting of permafrost in the Arctic regions, where methane trapped in the frozen earth and ice is allowed to escape into the atmosphere.environment