03/11/2014 02:34 EDT | Updated 05/11/2014 05:59 EDT

Climate likely fuelled Genghis Khan's conquests

Scientists think they have figured out a key factor that allowed Genghis Khan's Mongol empire to take over huge swaths of Europe and Asia during the 13th century.

The Mongols under Khan, his sons and his grandsons took over most of modern-day Russia, China, Korea, southeast Asia, Iran, India, the Middle East and eastern Europe during their conquest. But in some ways, they seemed unlikely conquerors. Mongolia has a very harsh climate – cold and dry, with few resources, said researcher Neil Pedersen in a video interview posted by Columbia University's Earth Institute.

"How did these people that were perceived as being primitive, how did they rise up and just start dominating Eurasia?"

Pedersen and Amy Hessl of West Virginia University think they have figured out a big part of the answer, thanks to their analysis of rings from Mongolian trees that are more than 1,000 years old. The stunted larches and Siberian pines were growing out of cracks in hardened volcanic lava north of the former Mongol capital of Karakorum.

Because trees grow faster when it is warmer and wetter than when it is cooler and drier, their rings can provide a record of past climate.

While Mongolia is normally cold and dry, the tree rings showed that the regional climate during the rise of Khan's empire was "warm and persistently wet," the researchers reported in a paper in Proceedings of the National Academy of Sciences.

In fact, there was an "unprecedented" 15 straight years of above average rainfall right coinciding with Khan's rise, the researchers wrote.

Pedersen noted that major sources of the Mongols' military strength were their horses and advanced cavalry tactics.

"Grass is the fossil fuel, the oil for the Mongol empire," he said.

Hessel said in a video interview that she thinks of it as "nature set the table and Genghis Khan came to eat… It was a matter of whether or not the culture and the people capitalized on that."

The study didn't just look at climate during Khan's rule, but over the entire 1,112 years visible in the tree rings. One of its findings was that the recent drought in central Mongolia was the hottest in the entire period of more than a millennium. Similar droughts due to future warming may have "potentially severe consequences for modern Mongolia," the researchers wrote.