(CNN) 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.

Carbon levels changed enough to cool the Earth by 1610, researchers found. Columbus arrived in 1492

"CO2 and climate had been relatively stable until this point," said UCL Geography Professor Mark Maslin, one of the study's co-authors. "So, this is the first major change we see in the Earth's greenhouse gases."

Before this study, some scientists had argued the temperature change in the 1600s, called the Little Ice Age, was caused only by natural forces.

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