Scientists estimate that about 700 million acres—roughly the size of Argentina—returned to forest and grassland during and after the Mongol conquests. That's not a small garden project. That's an area larger than the entire Amazon rainforest reforestation efforts of the past 50 years combined.
When forests regrow, they pull carbon dioxide out of the atmosphere and store it in trees, soil, and vegetation. The reforestation following Mongol conquests removed approximately 700 million tons of carbon from the atmosphere over the course of a century or so.
To put that in modern terms: that's equivalent to the annual carbon emissions of about 100 coal-fired power plants running for a decade. Or every car on Earth stopping operation for a full year. Or roughly the amount of carbon that modern climate change initiatives are desperately trying to capture and store using billions of dollars in technology.
Genghis Khan achieved it through the considerably less expensive (though morally horrifying) method of conquest and demographic collapse.
Ice core data from Antarctica and Greenland show measurable dips in atmospheric CO2 levels during this period. Tree ring data confirms the timing. Medieval climate records describe a cooling period that correlates with the Mongol conquests. Scientists can literally see the environmental impact of the Mongol Empire in geological records.