According to this Slashdot article researchers from the Carnegie Institution credit Genghis Khan with eliminating 700 million tons of CO2 emissions from the atmosphere. He did this by slaughtering some 40 million people. The environmental researchers believe that by depopulating agrarian societies by massacring their inhabitants, he initiated a wave of reforestation. The resulting trees scrubbed the atmosphere of CO2 which they believe resulted in global cooling. The leader of the research project, Julia Pongratz, states here that:
Today about a quarter of the net primary production on the Earth’s land surface is used by humans in some way, mostly through agriculture. […]. In the past we have had a substantial impact on global climate and the carbon cycle, but it was all unintentional. Based on the knowledge we have gained from the past, we are now in a position to make land-use decisions that will diminish our impact on climate and the carbon cycle. We cannot ignore the knowledge we have gained.