The Inca ruler Atahulpa supplicant before the conquistador Francisco Pizarro in 1533 - Hulton Archive

The “Little Ice Age” of the sixteenth and seventeenth centuries was triggered by the genocide of indigenous people in the Americas by European settlers, new research shows.

Scientists have long wondered what caused the drop in temperatures so severe it sometimes caused the River Thames to freeze over.

Now, new analysis by University College London (UCL) argues that so many people were slaughtered or died of disease that the amount of agricultural land dramatically reduced, in turn sucking carbon dioxide (CO²) from the atmosphere.

Known as the “Great Dying”, the upheavals following the first contact with Europeans in 1492 is thought to have slashed the population of 60 million living across the Americas down to five or six million within just 100 years.

Published in Quaternary Science Reviews, the study found that much of the land previously cultivated by indigenous civilisations would have fallen into disuse, becoming swallowed up by forest and grassland.

It estimates that an area of 56 million hectares, roughly the size of modern-day France, would have been rewilded in this way.

The scale of the change is believed to have drawn an amount of CO² from the atmosphere equivalent to two years’ fossil fuel emissions at the present rate.

Winter landscape with iceskaters, c. 1608, Hendrick Avercamp Credit:

Professor Mark Maslin, from UCL’s school of Geography, said: “There is a marked cooling around that time which is called the Little Ice Age, and what’s interesting is that we can see natural processes giving a bit of cooling, but to actually get the full cooling - double the natural processes - you have to have this genocide-generated drop in CO².”

The research team examined historical population data, using it to model the reduction of land devoted to agriculture.

They combined this with evidence of a cold period found in ice core records from Antarctica which contained trapped air bubbles.

This was supported by records of charcoal and pollen deposits in the Americas.

Ed Hawkins, professor of climate science at Reading University, said: "Scientists understand that the so-called Little Ice Age was caused by several factors - a drop in atmospheric carbon dioxide levels, a series of large volcanic eruptions, changes in land use and a temporary decline in solar activity.

"This new study demonstrates that the drop in CO² is itself partly due the settlement of the Americas and resulting collapse of the indigenous population, allowing regrowth of natural vegetation. “It demonstrates that human activities affected the climate well before the industrial revolution began."