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.