Researchers at University College in London have published a paper theorizing that the slaughter of Native Americans by whites in the 15th and 16th centuries led to climate change often referred to as "The Little Ice Age."

Temperatures plummeted, the researchers say, after the death of so many indigenous people that the land they were tilling was "reforested," thus leading to an increase in trees and vegetation, which means less CO2 was being pumped into the atmosphere.

CNN:

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.

The notion that white men killed 56 million Native Americans is beyond speculation; it's fantasy. First and foremost, no one has been able to come up with a figure for the number of Native Americans in the Americas before Columbus. Most tribes did not have a written language. Certainly there were complex civilizations like the Aztecs and Incas, and agriculture was fairly widespread at the time of Columbus. But trying to get a grasp on the sheer numbers of inhabitants in North, Central, and South America is a fool's errand.

From the research paper:

We summarize the existing evidence for pre-Columbian populations using seven geographical regions: the Caribbean, Mexico, Central America, the Inca Territory, Amazonia and contiguous forested area, North America and the Rest of the Americas. The Inca Territory at contact encompassed Peru, Bolivia, Ecuador, Colombia, Chile and parts of north-western Argentina; the Rest of Americas is composed of Venezuela, Uruguay, Paraguay and the rest of Argentina. We complied 129 estimates from 82 studies. 

They looked at 82 studies from people who had no idea what they were talking about. They are - they have to be - wild guesses. But academics have no other choice but to try and grapple with the numbers as best they can by building on the work of others - even though that work is fatally flawed.

That said, is it really "genocide" when the overwhelming majority of Native Americans who were killed by European diseases never laid eyes on a white man? The notion of spreading germs and viruses was unknown until the late 19th century. Almost all Native Americans who died in the 100 years after Columbus landed lost their lives because they had no immunity to diseases like smallpox, measels, mumps, and scarlett fever. Europeans still caught these diseases but after nearly a milenium of exposure to them, white people had developed a partial immunity in that when they got sick, they had a pretty good chance of surviving.

Not so Native Americans who suffered the full force of the bugs' effects, wiping out entire civilizations. The diseases spread because of the extraordinarily complex trade routes that snaked their way up and down the western hemisphere. In short, Native Americans never knew what killed them.

As for the idea that carbon levels fell because a few million acres of land were reclaimed by nature, this doesn't even come close to the effect that one or two erupting volcanoes would have on CO2 levels. There has been much speculation about the causes of the Little Ice Age, including a change in ocean currents and the lack of sunspots. But our understanding of climate is so limited that trying to shoehorn a theory about climate change into the death of millions of people just doesn't hold water.

The virtue of this study is that it presses all the right social justice buttons. Evil white men? Check. Human caused climate change? Check. Genocide? Check. All the boxes might be marked, but that doesn't make this study any more factual.