When European explorers and colonists first contacted Native American populations starting in 1492, it triggered a mass die-off of the native population as tens of millions of people succumbed to disease. Over the 16th century, the population in the Americas dropped by around 90 percent. New research finds that this rapid population decline was actually enough to cool the planet, triggering a period called the "Little Ice Age."

The researchers, from University College London, used estimates of population decline during the 16th century to estimate how much farmland was reclaimed by forests during that century. The 60 million people living in the pre-Columbian Americas needed a lot of farmland to grow the food they needed to eat, and when people started dying en masse they stopped needing so much land for crops.

That land went unused for so long that it slowly turned into forests. By those researchers’ estimates, around 200,000 square miles of new forest were created during the 1500s. For comparison, that’s a little less than the size of Texas, or about a tenth of the Amazon rainforest. All that new vegetation required a lot of carbon dioxide, and so much CO2 was pulled out of the atmosphere to fuel these growing plants that it ended up cooling the earth.

Starting in the late 1500s, the entire earth went through a minor cooling period called the Little Ice Age. Global temperatures dropped over a degree Fahrenheit and the climate was dramatically altered. Winters became more severe, crops were devastated, and famine was common throughout much of Europe. The immediate cause of the ice age was a drop in atmospheric carbon dioxide by around 8 parts per million.

According to the recent study, the additional forests in the Americas caused around half of that CO2 loss, around 5 parts per million. That’s enough to account for much of the colder temperatures of that era.

This study also reinforces just how severe our current climate crisis is, and how difficult it is going to be to solve it. Currently, we add around 5 parts per million of CO2 into the atmosphere every year. If we planted a brand new forest the size of Texas, we might be able to counteract a year or two of carbon pollution. Removing all the excess carbon from the air, as some people want to do, is going to be an extremely challenging task.

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