Quirks and Quarks

50 million deaths in the New World drove cooling in the Little Ice Age

Massive depopulation of the Americas after European contact has been linked to the climate change in the 1500s and 1600s

European contact led to massive depopulation, which in turn led to climate change

Massive depopulation of the Americas after European contact was found to be linked to the climate change in the 1500s and 1600s, according to a new study. (George Catlin)

A research team in the UK has found a connection between climate change in the 1500s and 1600s and the massive depopulation of the Americas after European contact.

They estimate that a century of warfare, slavery, disease and social collapse led to the death of as many as 55 million people — about 90 per cent of the population of the Americas at the time.

As the population collapsed, vast swathes of forest grew on abandoned agricultural land and sucked carbon dioxide out of the atmosphere. This contributed to the "Little Ice Age" that was cooling the planet at the time.

Population crash

Part of the key to this study was a new estimate of the population of the Americas at the time of conquest. There were no written population records before Europeans arrived, or for that matter, until well after Europeans were established.

Previous estimates were made based on colonial records from the period after the population collapse, which resulted in some estimates of pre-contact population of as few as ten million.

Christoper Columbus arrives in America. (L. Prang & Co., Boston)

This new study by Alexander Koch, a Ph.D student in the Geography department at University College London, and his colleagues, combined previous regional population estimates and arrived at a population estimate of about 60 million people at the time of European contact.

That would have rivaled the population of Europe at the time. Most of these people would have lived in densely populated and highly developed areas of Mexico and Central America. 

The route of the Spanish conquest of the Aztec Empire in central Mexico. Used by Hernán Cortés and troops, from 1519 to 1521 and 'victory' at Tenochtitlan. (Yavidaxiu, CC-BY-3.0 )

Columbus' arrival in 1492, however, inaugurated better than a century of bloody conquest, exploitation and multiple lethal waves of new European diseases. Smallpox, measles, influenza and bubonic plague swept through the population, resulting in what's become known as "The Great Dying."

"You might get ill from smallpox but you might not die. But a third of the population around you died. And so the next wave of influenza might come and because the population was so weak, it just wipes them out gradually over time," Koch told Quirks & Quarks host Bob McDonald. "And it's really not helped by having European forces kidnapping the healthy population and putting them in the mines."

Abandoned land and new forests

With 90 per cent of the population dead, vast amounts of agricultural land was abandoned and new forests replaced pastures and fields. According to Koch, evidence for this can be seen in pollen records preserved in sediment. They saw pollen from agricultural plants like maize and quinoa giving way to trees and other forest plants.

Vast amounts of agricultural land were abandoned when a large percentage of the native population died after the Europeans arrived in the Americas. This caused vast swathes of forest to grow on the abandoned agricultural land, which sucked carbon dioxide out of the atmosphere. ( JOHAN ORDONEZ/AFP/Getty Images)

Reduced amounts of charcoal in soil deposits also provided evidence, said Koch, because pre-contact farmers used fire to clear land and prepare it for planting. "If there's more charcoal in the soil you have, it's most likely from from humans rather than natural burning in the tropics."

Altogether, Koch and his collaborators estimate that this resulted in a huge amount of agricultural land returning to forest — about 60 million hectares. "We're talking about roughly the same size as France."

This growing forest would have sucked enormous amounts of carbon dioxide from the atmosphere, amounting to some 7 billion tons of carbon. This would have reduced global atmospheric carbon dioxide levels by about 5 parts per million, which corresponds to the drop seen in Arctic Ice core records dating back to the early 1600s, according to Koch.

We've done it before

This wouldn't have led to a large drop in temperatures. The global impact would have only been about 0.075 C, but that small drop would have been on top of the natural cooling trend known as the Little Ice Age, which has been held responsible for famines and conflict the world over.

As catastrophic as this episode in human and climate history was, it pales in comparison to the changes we're making today with fossil fuel emissions, said Koch. 

"We see the dip in carbon dioxide at a time but this dip alone if you could translate into in the present day relates to roughly three years of the growth rate of CO2 in the atmosphere."

This does, however, suggest that human activity has been affecting the climate for longer than we might have thought, so it's a misconception, said Koch, to think that this started with fossil fuels.

"It's much much earlier and there are people out there who believe that, really, we started impacting the climate when we settled down for agriculture."