So many Native Americans were killed during European colonization that it may have contributed to the Little Ice Age.
As European colonizers headed to the Americas, they caused what is known as "Great Dying". In 1492 CE, the population of Native Americans was estimated to be 60.5 million by a team from University College London, based on historical records including sizes of armies, census data, and archaeological finds. Over the next century, the team suggests newly-introduced pathogens killed around 90 percent of the population.
This had a knock-on effect on the carbon cycle, as previously-farmed land turned to forests and savannas, which pull in more carbon than farmland. It was enough, according to a paper published in Quaternary Science Reviews in 2019, to cool the planet.
"The carbon uptake that is thought to have occurred following the arrival of epidemics in the Americas may have reduced atmospheric CO2 levels and led to a decline in radiative forcing that may then have contributed to the coldest part of the Little Ice Age," the team wrote in their paper.
"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."
According to the team's calculations, 56 million hectares (138 million acres) of farmland became disused in this time. A drop in CO2 is seen in the ice core record around the time of the Little Ice Age. The team believes that the disused land could account for levels of CO2 falling by around 7-10 parts per million (ppm).
"To put that in the modern context - we basically burn (fossil fuels) and produce about 3 ppm per year. So, we're talking a large amount of carbon that's being sucked out of the atmosphere," Professor Mark Maslin, who co-authored the study, told the BBC.
"There is a marked cooling around that time (1500s/1600s) which is called the Little Ice Age, and what's interesting is that we can see natural processes giving a little bit of cooling, but actually to get the full cooling - double the natural processes - you have to have this genocide-generated drop in CO2."