Colonisation of the Americas at the end of the 15th Century killed so many people, it disturbed Earth's climate.
That's the conclusion of scientists from University College London, UK.
The team says the disruption that followed European settlement led to a huge swathe of abandoned agricultural land being reclaimed by fast-growing trees and other vegetation.
This pulled down enough carbon dioxide (CO₂) from the atmosphere to eventually chill the planet.
It's a cooling period often referred to in the history books as the "Little Ice Age" - a time when winters in Europe would see the Thames in London regularly freeze over.
"The Great Dying of the Indigenous Peoples of the Americas led to the abandonment of enough cleared land that the resulting terrestrial carbon uptake had a detectable impact on both atmospheric CO₂ and global surface air temperatures," Alexander Koch and colleagues write in their paper published in Quaternary Science Reviews.