In the 1200s, Genghis Khan and the Mongolian army built an empire that stretched from the Pacific Ocean to central Europe, ruling over a fifth of all land on Earth and over 100 million people. And all that conquering reshaped the Earth, reducing carbon dioxide levels enough to offset a year's worth of gasoline usage today.
Stanford researchers considered four of the most traumatic periods in the last 1200 years - the Mongol invasions of 1200 to 1380, the Black Death that killed 25 million Europeans between 1347 and 1400, the European conquest of the Americas between 1519 and 1700 that killed as much as 90% of the native population, and the fall of China's Ming dynasty from 1600 to 16450. All these cataclysmic events involved tremendous loss of life and agricultural devastation, the latter of which can be traced in the historical record through analysis of soil samples.
Still, for all their human carnage and crop destruction, none of these events really affected the Earth itself in a fundamental way. The only exception was the Mongol invasion, which dropped global carbon dioxide levels by about 0.1 part per million. This made forests absorb back about 700 million tons of carbon dioxide, roughly equivalent to the amount released in a year's worth of gasoline demand.
Those might seem like major effects, but the last two hundred years of industrialization has created an entirely new scale. As lead researcher Julia Pongratz explains:
"Since the pre-industrial era, we have increased atmospheric CO2 [or carbon dioxide] concentration by about 100 parts per million, so this is really a different dimension.
Part of the problem is that heavy agriculture and deforestation elsewhere overwhelmed any local effects of these tragedies. The indigenous peoples of the Americas had a relatively small agricultural footprint, which means their mass slaughter and death had little ecological impact. The Black Plague and Ming Dynasties both took place over only about a half-century, which is too short a time-span for trees to regrow and new carbon to be stored.
So then, that just leaves the Mongol invasion as the only pre-industrial event that had even a slight impact on carbon dioxide levels comparable to what is going on right now. But hey, look on the bright side - we've now got a way to solve the carbon crisis. We're just going to need a ton of Mongol hordes...