Genghis Khan was good for the environment
An interesting article I stumbled on: Mongols, Vikings, and Romans Connected to Climate from the Discovery Channel, citing a paper in The Holocene, a peer-reviewed scientific journal.
Apparently wide scale elimination of cities or populations reduces the amount of farmland, meaning that the land reverts back to forest land. Forests absorb more carbon dioxide than farmland and thus reduce the impact of global warming.
Besides the Mongol invasions, the study also looked, the Black Plague in Europe, the European conquest of the Americas and the fall of the Ming Dynasty in China. Only the longer lasting events, like the invasion of the Americas and Chinghis Khan’s campaigns had any impact on CO2 levels, but apparently not enough to make much of a difference globally.
Still, an interesting way to look at historical conquests.