When Genghis Khan’s armies swept across Asia and Europe in the early 1200s, the world’s population was around 370 million people. His campaigns and their aftermath killed an estimated 35 to 40 million, roughly one in ten people on Earth at the time.

With entire regions depopulated, farmland and villages were abandoned. Over time, those fields turned back into forests and grasslands that quietly absorbed carbon dioxide from the air. 

As forests spread across abandoned farmland, they absorbed about 700 million tons of carbon from the air. It was a brief but measurable shift in the planet’s atmosphere, long before factories or fossil fuels.

It’s one of the earliest examples of how human activity can alter the planet’s chemistry.

Source: Carnegie Institution for Science (Pongratz & Caldeira, 2011).

The Christian Science Monitor, Scientists calculate Ghengis Khan's carbon footprint, Feb. 08, 2011