The Mongol invasion of Asia in the 1200s took enough carbon dioxide out of the atmosphere to offset a year's worth of the world's gasoline demand today, according to a new study. But even Genghis Khan couldn't create more than a blip in atmospheric carbon compared to the overwhelming effect of agriculture.

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The study, published online Jan. 20 in the journal The Holocene, looked at land use and carbon dioxide in the atmosphere between the years 800 and 1850. Globally at the time, humans were cutting down forests for agriculture, driving carbon into the atmosphere (vegetation stores carbon, so trees and shrubs are what scientists call "carbon sinks"). But in some regions during certain times, wars and plagues culled the population, disrupting agriculture and allowing forests to regrow.

The question, said Julia Pongratz, a postdoctoral researcher at the Carnegie Institution's Department for Global Ecology at Stanford University, was whether this regrowth could have locked up enough carbon to make a difference in global atmospheric carbon dioxide.

"We wanted to check if humans had an impact on carbon dioxide by increasing it by deforestation, but also by decreasing it," Pongratz told LiveScience.

Catastrophes and carbon

Pongratz and her colleagues used a detailed reconstruction of historical agriculture to model the effect of four major wars and plagues in the 800 to 1850 time period: the Mongol takeover of Asia (from about 1200 to 1380), the Black Death in Europe (1347 to 1400), the conquest of the Americas (1519 to 1700) and the fall of the Ming Dynasty in China (1600 to 1650).

All of these events led to death on a massive scale (the Black Death alone is thought to have killed 25 million people in Europe). But Mother Nature barely noticed, the researchers found. Only the Mongol invasion had a noticeable impact, decreasing global carbon dioxide by less than 0.1 part per million. This small amount required that the forests absorb about 700 million tons of carbon dioxide, which is the amount emitted annually by worldwide gasoline demand today. But it was still a very minor effect, Pongratz said.

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