Until now, it was believed that humanity's massive impact on the Earth's climate and atmosphere had coincided only with the industrial revolution. Our species in fact contributed to major climate changes long before that, though in the past, rather than causing global warming, it triggered a cooling. Over the course of roughly a century, between the late 1500s and the 1600s, the planet's average temperature underwent a sharp drop, culminating in a sort of little ice age, and this too was the result of human impact. That time, however, we did not fell thousands of hectares of forests, nor did we burn so many fossil fuels as to saturate the air; instead, we killed, directly or indirectly, approximately 56 million people.

A history written in blood

The European colonisation of the Americas, set in motion by Christopher Columbus's famous landing in 1492, gave rise to an enormous genocide — the largest perpetrated by Westerners — that led to the extermination of indigenous peoples from North America to Central America and all the way to South America. The colonisers killed an incredible number of people in conflicts, but condemned even more through the diseases they brought with them, giving rise to terrible epidemics that killed around 90 per cent of native populations in the following century.

The abandonment of cultivated land

According to the study Earth system impacts of the European arrival and Great Dying in the Americas after 1492, conducted by a group of researchers at the University College London, this sudden and widespread depopulation meant that vast tracts of cultivated land were left untended. Agriculture had been deeply rooted in the Americas long before the arrival of Europeans, particularly in Mexico, Central America, Bolivia, the Andes and throughout Amazonia, where terracing and irrigated farming were practised. The forced abandonment of cultivated fields led to the spontaneous return of forests, which once again began storing large quantities of CO2.

The little ice age

The researchers believe that the reforestation that followed the genocide of indigenous peoples was so vigorous that it may have driven sufficient CO2 absorption to cool the planet's average temperature by 0.15 degrees between the late 1500s and the early 1600s. This drop in temperature had repercussions across the entire planet and is known as the "little ice age". Europe in particular experienced major disruptions: in Portugal, for example, snowstorms became frequent, and in many European countries frost damaged harvests, causing severe famines.

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The return of the forests

To carry out the study, the researchers attempted to reconstruct the size of the pre-Columbian population of the Americas, per capita land use, population loss from 1492 onwards, and the extent of subsequent carbon absorption following the abandonment of rural activities. According to their calculations, in the wake of the colonisers' arrival, around 55 million hectares of cultivated land — an area comparable in size to France — were abandoned and "returned" to nature, thereby considerably reducing the amount of carbon dioxide present in the atmosphere.

The power of reforestation

"The results of the study show us what reforestation can achieve," said study co-author Chris Brierley. Given the current rate of emissions and the fact that approximately 409.44 ppm (parts per million) of CO2 are floating in the atmosphere, a far more extensive reforestation effort would be needed to counter the rise in global temperatures that threatens the survival of our species. This research nonetheless demonstrates, should there be any remaining need to do so, that forests are indispensable allies in the fight against climate change and that reforestation and rewilding initiatives are both necessary and urgent.