The mass genocide of the Native American people by European colonizers during the 15th and 16th centuries—in which an estimated 56 million indigenous people, or 90 percent of the population, were wiped out by violence and disease—was so complete and devastating, new research shows, that it triggered a planetary cooling.

According to scientists at the University College London,the Europeans' mass killing of natives in the Caribbean and the Americas led to the populations' agricultural systems to go untended, leading to an overgrowth of vegetation all over the region. So many new trees and plants grew over a total area of about 55 million hectares, that the vegetation absorbed significant amounts of carbon from the atmosphere and caused the planet to cool down.

This period was marked by a drop in global temperatures by .15 degrees Celsius (or .27 degrees Fahrenheit), disrupting agriculture around the world and leading to several famines in Europe.

The genocide was so complete, the study says it led to what historians call the Little Ice Age in the late 16th and early 17th centuries.

Author and 350.org co-founder Bill McKibben pronounced the findings "beyond horrifying."

The report, published in Quarternary Science Reviews, "demonstrates that human activities affected the climate well before the industrial revolution began," Reading University climate science professor Ed Hawkins, who was not involved in the study, told the BBC.

The study also shows, researchers said, how much work is needed to reverse the current trajectory of deforestation and fossil fuel extraction—both of which cause carbon dioxide to build up in the atmosphere.

The genocide "resulted in an area the size of France being reforested and that gave us only a few parts per million" of carbon dioxide which was absorbed us from atmosphere, researcher Chris Brierley told the BBC.

That "shows us what reforestation can do," he added. "But at the same, that kind of reduction is worth perhaps just two years of fossil fuel emissions at the present rate."