After Christopher Columbus discovered the Americas in 1492, 90% of the indigenous population – nearly 55 million people – were killed by violence and disease. Diseases such as smallpox, measles and influenza that European colonists brought to the Americas were responsible for many millions of deaths. Now, a new study published in the journal Quaternary Science Reviews shows that this rapid population decline and subsequent reduction in land use led to a "little ice age": an era of global cooling of the planet between the 16th and mid-19th centuries 19th century.
Before Columbus' arrival in 1492, the American continent was a prosperous country with more than 60 million inhabitants. Just over a century later, that number had fallen to almost 6 million.
European colonization brought not only war and great famine, but also diseases such as smallpox, which were fatal to much of the population. Researchers at University College London found that large areas of vegetation and farmland were abandoned following rapid population decline. The trees and plants that repopulated the uncultivated farmland absorbed more carbon dioxide and kept it locked in the soil. This removed so much greenhouse gas from the atmosphere that the planet's average temperature fell by 0.15°C.
As a rule, experts consider the industrial revolution to be the cause of thehuman-driven climate impacts. However, this study shows that the effects began 250 years earlier. “Humans were already changing the climate before we started burning fossil fuels,” said the study’s lead author Alexander Koch.
By 1600, more than 50 million died in America
Experts have long had difficulty quantifying the extent of the killing of Native Americans in North, Central and South America. This is primarily because there are no census data or population size records to determine how many people lived in these areas before 1492.
To approximate population numbers, researchers often rely on a combination of European eyewitness accounts and records of tribute payments (encomienda) during colonial rule. But neither metric is accurate—the first tends to overestimate population size, as early colonizers wanted to advertise the wealth of newly discovered lands to European backers. The latter reflects a payment system that was introduced after many disease epidemics had already taken hold, the authors of the new study noted.
So the new study offers a different method: the researchers sharedNorth and South Americain 119 regions and examined all published estimates of the pre-Columbian population in each region. The authors calculated that before European colonization, around 60.5 million people lived in America.
After Koch and his colleagues compiled the before-and-after numbers, the conclusion was clear. Between 1492 and 1600, 90% of Native Americans had died. That means about 55 million people died from violence and never-before-seen pathogens like smallpox, measles and influenza.
According to these new calculations, the death toll at that time represented about 10% of the Earth's total population. There are more people than the residents of New York City, London, Paris, Tokyo and Beijing combined.
The deaths of so many people meant less farming
Using these population numbers and estimates of how much land people use per capita, the study's authors calculated that indigenous peoples farmed about 62 million hectares of land before European colonization.
This number also fell by around 90% around 1600 to just 6 million hectares. Over time, trees and vegetation took over the previously cultivated land and absorbed more carbon dioxide from the atmosphere.
Carbon dioxide traps heat in the planet's atmosphere, but plants and trees absorb this gas as part of photosynthesis. So when the previously cultivated land in North and South America - the equivalent of almost 650,000 km² - was reforested with trees and plants, the carbon dioxide levels in the atmosphere fell.
Ice cores in Antarctica from the late 1500s and 1600s confirm the decline in carbon dioxide.
This drop in CO2 was enough to reduce global temperatures by 0.15 degrees Celsius and contribute to the mysterious global cooling trend called the “Little Ice Age,” during which glaciers expanded.
Not all scientists are convinced by Koch's explanation about the cooling of the planet
“The researchers are probably overestimating their case,” said Jörg Schäfer of Columbia University’s Lamont-Doherty Earth Observatory. “I am absolutely certain that this paper does not explain the cause of the carbon dioxide change and the temperature change during this time.”
Koch said some of the carbon dioxide drop may have been caused by other natural factors such as volcanic eruptions or changes in solar activity. However, he and his colleagues concluded that the deaths of 55 million indigenous Americans accounted for approximately 50% of the total reduction in atmospheric carbon dioxide.
“So you need to consider all natural and human forces to explain global cooling,” he said.
Koch said the results would revise our current understanding of how long human activities have influenced Earth's climate.
“Human actions at this time caused a cooling of the planet long before human civilization had grappled with the idea of climate change,” he and his co-authors wrote.
However, they warned that a similar reforestation event today would not do much to stop the Earth's current warming. The decline in atmospheric carbon dioxide concentrations that occurred in the 1600s represents only about three years of fossil fuel emissions today.
“There is no other way but to reduce fossil fuel emissions,” he said, adding that forest restoration and afforestation are also crucial.
The study was carried out inQuarternary Science Reviewspublished.
