According to some climatologists, the cold spell known as the Little Ice Age, from roughly 1500 to 1850, may well have been due to the reduction of CO2—a greenhouse gas—brought about by the die-off of North America’s indigenous fire farmers. [James C Scott, Against the Grain]

This theory is probably too neat to be true, but that doesn’t stop it being fun. Scott is largely drawing on (and perhaps slightly exaggerating) the work of William Ruddiman

He is interested in how humans before and outside of sedentary, grain-based ‘civilization’ shaped their landscape with fire. By burning vegetation, you can herd animals into a spot where they are easy to kill. The plants which grow back first after fire might also be more human-friendly: less huge trees, more bushes with fruits or berries.

So before Columbus, humans in the Americas were burning the landscape every year. Then Europeans came, brought smallpox, and killed off a the majority of them. So the forests stopped being burned, leading to the reforestation forest of an area the size of Venezuela. This decreased CO2 in the atmosphere, which cooled the temperature, causing the Little Ice AGe.