As the 15th century drew to a close, some 60 million people lived across the Americas, sustaining themselves with the bounty of the vast lands they inhabited.

But with the arrival of the first European settlers, waves of new diseases, along with warfare, slavery and other brutality would kill off around 56 million people, or around 90 percent of the indigenous population.

Now, scientists from the University College London (United Kingdom) argue in a new study that this “Great Dying” that followed European colonization of the Americas may have actually affected Earth’s climate.

Their version of events, published in Quaternary Science Reviews, goes like this: After so many indigenous people died, no one was left to tend many of their fields, and trees and other vegetation quickly reclaimed huge expanses of land previously used for agriculture. As a result, enough carbon dioxide (CO₂) was removed from the atmosphere to actually cool down the planet, contributing to the coldest part of the mysterious period that historians have called the Little Ice Age.

Universiy College of London/CC BY 4.0
A map highlighting regions known to have been affected by disease outbreaks by 1600 CE and pre-Columbian land use.

During this period, which peaked in the early 17th century, the Thames River in London consistently froze over, and harsh winters and cold summers across the globe were blamed for causing famines, encouraging witch hunts and even sparking wars.

The scientists drew on existing population records to estimate just how many people were living in the Americas around 1492—the year, of course, that Christopher Columbus arrived in the Caribbean. They found that while 60 million (or around 10 percent of the world’s total population) lived across the Americas at the tail end of the 15th century, that number would be reduced to just 5 or 6 million in the decades that followed European colonization. The rest had been wiped out by conflict or disease, including smallpox, measles, influenza and the bubonic plague.

With few people left to manage the fields, the scientists estimated that some 56 million hectares of land previously used for agriculture—an area about the size of modern-day France—would have fallen into disuse. If forest, savannah and other vegetation had quickly covered that land, they found, the additional vegetation would have pulled enough (CO₂) out of the atmosphere to lower the concentration of that gas by 7-10 parts per million (ppm), or 7-10 molecules of CO₂ in every 1 million air molecules.

To put this into perspective, study co-author Mark Maslin told BBC News that today’s burning of fossil fuels produces about 3ppm of CO₂ per year. “So we're talking a large amount of carbon that's being sucked out of the atmosphere,” he clarified.

The scientists also point to the ice core record from Antarctica to support their theory, as air bubbles trapped in the ice do show a fall in carbon dioxide around 1610, at the beginning of the Little Ice Age.

“Human actions at that time caused a drop in atmospheric CO₂ that cooled the planet long before human civilization was concerned with the idea of climate change,” the authors of the new study wrote in Newsweek. The new study suggests that human impact on the planet stretches back centuries before the Industrial Revolution began, with the collapse of farming in the Americans hundreds of years ago.