As Genghis Khan built his Great Mongol Empire at the start of the 13th Century, he really did have the winds at his back -- the winds of a milder climate, that is.

A new report co-authored by Amy Hessl, an associate professor of geography at West Virginia University, asserts that -- the period's political realities notwithstanding -- the regional climate at the time appeared to promote Khan's conquests.

The weather veered away from little if any precipitation when Khan came into power into the wettest period for the area during the last millennium, resulting in far more abundance across the countryside.

That bounty literally provided the fuel, that is to say the food for his armies and horses, for Khan's relentless push forward, the study said.

"Such a strong and unified center [under Khan] would have required a concentration of resources that only higher productivity could have sustained in a land in which extensive pastoral production does not normally provide surplus resources," the report states.

But, how did scientists figure all that out?

The answers were in the trees, according to lead researcher Amy Hessl, an associate professor of geography at West Virginia University and one of the authors of "Pluvials, Droughts, the Mongol Empire, and Modern Mongolia," which details the Khan study and will appear in the latest issue of the Proceedings of the National Academy of Sciences.

This first-of-its-kind research hinges on the team's discovery of old trees and the evidence of growth periods, and thereby climatic conditions, recorded in their rings.

While understanding the climate's impact on Khan's empire is important, Hessl said, people also need to recognize how human-caused warming has "exacerbated" drought scorching central Mongolia today.

"If future warming overwhelms increased precipitation, episodic 'heat droughts' and their social, economic, and political consequences will likely become more common in Mongolia and Inner Asia," the report warns.