Just kill a lot of people:
Unlike modern day climate change, however, the Mongol invasion cooled the planet, effectively scrubbing around 700 million tons of carbon from the atmosphere. [...] Over the course of the century and a half run of the Mongol Empire, about 22 percent of the world’s total land area had been conquered and an estimated 40 million people were slaughtered by the horse-driven, bow-wielding hordes. Depopulation over such a large swathe of land meant that countless numbers of cultivated fields eventually returned to forests.
As an alternative to mass slaughter, imagine the whole planet turned vegetarian leading to a huge decline in the world’s population of cows, pigs, chickens, etc. The smaller number of animals would lead to a direct decline in emissions, but more importantly you wouldn’t need nearly as much farmland to grow the grain to feed the animals. Then you’d have a lot of reforestation.