The last time humans engineered the climate, they changed history

We're a horrible species, with little regard for the resources we have available to us. Almost 500 years ago:

The deaths of some 55 million native people to pandemic disease over less than a century, representing 90% of the inhabitants of North and South America — and 10% of the world population.

The effects of losing so many people?

A reforestation of their now-fallow farms, covering a combined area about the size of France, causing a massive and sustained amount of carbon to be sucked from the atmosphere.