Genghis Khan had a key ally you might not have expected: a stretch of nice weather. That, at least, is what a group of scientists is saying is written in the trees of Mongolia.

Human records of climate conditions only go back so far, but thankfully, the history of weather can inscribe itself in other deeply rooted sources like tree rings. A tree lays down a new layer of wood twice a year, creating recognizable rings. These rings tend to be narrower in times of drought and wider in good growing conditions. The science of scrutinizing tree rings is called “dendrochronology”; with it, researchers can read temperature and moisture trends stretching back hundreds or even thousands of years.

Based on a study of 107 living and dead Siberian pines, with a tree ring history stretching over the past 1,100 years, a team of scientists from the U.S. and Mongolia found that the climate during Genghis Khan’s conquests of 1211 to 1225 was unusually warm and wet.

“These climate conditions promoted high grassland productivity and favored the formation of Mongol political and military power,” the authors wrote in a paper published in the Proceedings of the National Academy of Sciences on Monday.

Some researchers had thought the Mongol conquests were driven in part by drought in Mongolia – the invaders might have been looking to grab more resources. And, indeed, the researchers found that Mongolia was experiencing intense drought conditions before Genghis Khan ascended to his leadership, between 1180 and 1190. That time also coincided with a lot of internal strife among the Mongol tribes. But as the weather turned, a new leader managed to unite the clans and expand outward toward all points of the compass.

"The transition from extreme drought to extreme moisture right then strongly suggests that climate played a role in human events," study author and West Virginia University researcher Amy Hessl said in a statement. "It wasn't the only thing, but it must have created the ideal conditions for a charismatic leader to emerge out of the chaos, develop an army and concentrate power. Where it's arid, unusual moisture creates unusual plant productivity, and that translates into horsepower. Genghis was literally able to ride that wave."

With each Mongol warrior commanding five or more horses, and with vast livestock herds supplying the nomadic tribes, grass was an essential resource for the great khan’s troops.

"Before fossil fuels, grass and ingenuity were the fuels for the Mongols and the cultures around them," the study’s lead author, Columbia University tree-ring researcher Neil Pederson, said in a statement Monday.

It’s a hot scientific trend of late to try and match up climate history to political history. The rise and fall of the ancient Maya civilization appears to track the ups and downs of rainfall trends, another team of researchers hypothesized in a 2012 paper published in Science.

The researchers also looked at recent drought conditions in Mongolia. Current climate models have pointed to a troubling scenario in Central Asia, which is expected to warm faster than the average temperature rise across the globe. In landlocked Mongolia, which is largely desert already, the strain could be particularly hard felt.

"Energy flows from the bottom of an ecosystem, up the ladder to human society,” Pederson says. “Even today, many people in Mongolia live just like their ancestors did. But in the future, they may face serious conditions."

SOURCE: Pederson et al. “Pluvials, droughts, the Mongol Empire and modern Mongolia.” Proceedings of the National Academy of Sciences, 10 March 2014.