Comment That's not what happened - read the article (Score 2) 420
Per the article, that's not what happened.
Pre-Columbus, the Native population of the Americas was many, many times larger than most people imagine - on the order of 80 million people. This population actively cleared land via slash-and-burn agriculture and generally comported themselves the way humans do (contrary to the popular imagination of Avatar-esque tiny populations living in perfect harmony with nature)
When Columbus made contact, he passed on smallpox and diptheria, and the subsequent wave of epidemics wiped out 90% of the Native population. Along with this, most of the previously cleared land was reforested, and the theory is that the reforestation pulled out enough atmospheric CO2 to cause a temperature drop due to lack of greenhouse effect.
Note that the tiny Native populations encountered by later European explorers were the remnents of the mass pandemic extinctions that played out "offstage" from European observation.
DG