Two interpretations of this recently promoted proposition:
European colonization of the Americas killed 10% of world population and caused global cooling,
While Europe was in the early days of the Renaissance, there were empires in the Americas sustaining more than 60m people. But the first European contact in 1492 brought diseases to the Americas which devastated the native population and the resultant collapse of farming in the Americas was so significant that it may have even cooled the global climate.
Lauren Kent, CNN, February 1st 2019
When Europeans arrived in the Americas, they caused so much death and disease that it changed the global climate, a new study finds.
European settlers killed 56 million indigenous people over about 100 years in South, Central and North America, causing large swaths of farmland to be abandoned and reforested, researchers at University College London, or UCL, estimate. The increase in trees and vegetation across an area the size of France resulted in a massive decrease in carbon dioxide (CO2) in the atmosphere, according to the study.
