European settlers in Americas cooled Earth's climate

European colonizers killed so many Native Americans through conflict and disease that they cooled Earth's climate, according to a recently published study.

At least 56 million indigenous Americans were killed by European settlers over about a 100-year span ending in 1600 in South, Central and North America. The mass slaughter left vast swaths of farmland unattended, resulting in mass reforestation that eventually reduced carbon-dioxide levels in the atmosphere.

Surface air temperatures dropped between the late 1500s through the early 1600s -- during which time the coldest parts of what is known as the Little Ice Age were recorded -- by 0.15 degrees Celsius.

"The Great Dying of the Indigenous Peoples of the Americas resulted in a human-driven global impact on the Earth System in the two centuries prior to the Industrial Revolution," the University College London, or UCL, study says.

The Industrial Revolution is generally seen as the start of a massive uptick in carbon dioxide levels prompted by the burning of fossil fuels

UCL Geography Professor Mark Maslin, one of the study's co-authors, told CNN the study proves for the first time "that the only way the Little Ice Age was so intense is ... because of the genocide of millions of people."

"The really weird thing is, the depopulation of the Americas may have inadvertently allowed the Europeans to dominate the world," Maslin said. "It also allowed for the Industrial Revolution and for Europeans to continue that domination."