The Mother Nature Network has a good story on the research:
“Over the course of the century and a half run of the Mongol Empire, about 22 percent of the world's total land area had been conquered and an estimated 40 million people were slaughtered by the horse-driven, bow-wielding hordes. Depopulation over such a large swathe of land meant that countless numbers of cultivated fields eventually returned to forests," the website reports.
More trees mean fewer greenhouse gas emissions, because trees take in carbon dioxide and produce oxygen.
The study was conducted by researchers at the Carnegie Institute and the Max Planck Institute for Meteorology. Julia Pongratz, of the Carnegie Institute, told the Mother Nature Network that humans have been affecting the earth’s climate for centuries.
"It's a common misconception that the human impact on climate began with the large-scale burning of coal and oil in the industrial era,” Pongratz said, according to the Mother Nature Network. “Actually, humans started to influence the environment thousands of years ago by changing the vegetation cover of the Earth's landscapes when we cleared forests for agriculture."