Experts have long struggled to quantify the extent of the slaughter of indigenous American peoples in North, Central, and South America. That's mostly because no census data or records of population size exist to help pinpoint how many people were living in these areas prior to 1492.

To approximate population numbers, researchers often rely on a combination of European eyewitness accounts and records of "encomienda" tribute payments set up during colonial rule. But neither metric is accurate — the former tends to overestimate population sizes, since early colonizers wanted to advertise riches of newly discovered lands to European financial backers. The latter reflects a payment system that was put in place after many disease epidemics had already run their course, the authors of the new study noted.

So the new study offers a different method: the researchers divided up North and South America into 119 regions and combed through all published estimates of pre-Columbian populations in each one. In doing so, authors calculated that about 60.5 million people lived in the Americas prior to European contact.

Once Koch and his colleagues collated the before-and-after numbers, the conclusion was stark. Between 1492 and 1600, 90% of the indigenous populations in the Americas had died. That means about 55 million people perished because of violence and never-before-seen pathogens like smallpox, measles, and influenza.

According to these new calculations, the death toll represented about 10% of the entire Earth's population at the time. It's more people than the modern-day populations of New York City, London, Paris, Tokyo, and Beijing combined.

The disappearance of so many people meant less farming

Using these population numbers and estimates about how much land people used per capita, the study authors calculated that indigenous populations farmed roughly 62 million hectares (239,000 square miles) of land prior to European contact.

That number, too, dropped by roughly 90%, to only 6 million hectares (23,000 square miles) by 1600. Over time, trees and vegetation took over that previously farmed land and started absorbing more carbon dioxide from the atmosphere.

Julia Pareci of the indigenous Pareci community stands iin front a corn field planted within an Indian reservation, near the town of Conquista do Oeste, Brazil. Ueslei Marcelino/Reuters

Carbon dioxide (CO2) traps heat in the planet's atmosphere (it's what human activity now emits on an unprecedented scale), but plants and trees absorb that gas as part of photosynthesis. So when the previously farmed land in North and South America — equal to an area almost the size of France — was reforested by trees and flora, atmospheric carbon-dioxide levels dropped.

Antarctic ice cores dating back to the late 1500s and 1600s confirm that decrease in carbon dioxide.

That CO2 drop was enough to lower global temperatures by 0.15 degrees Celsius and contribute to the enigmatic global cooling trend called the "Little Ice Age," during which glaciers expanded.