Unusual rain and mild weather likely spurred the rise of Genghis Khan's Mongolian empire, a new study has revealed.

Researchers had earlier hypothesized that the Mongols spread throughout Asia and Europe to escape extreme drought, but a new study examining pine tree rings in the area shows that the empire's rise was fueled by good weather, USA TODAY reported.

In the early 13th century, a substantial increase in rainfall and unusually mild temperatures in central Asia boosted the Mongols' campaign by providing grass to feed their horses.

"Where it's arid, unusual moisture creates unusual plant productivity, and that translates into horsepower," said physical geographer Amy Hessl of West Virginia University, who helped lead the study, as reported by USA TODAY. "Genghis was literally able to ride that wave."

The Mongols ruled the biggest land empire of all time, their territory including modern-day Russia, China, Korea, Southeast Asia, Persia, India, the Middle East and Eastern Europe, according to the USA TODAY report.

During 1211 to 1225 A.D., when Genghis Khan and his progeny rose to power, Mongolia had its most mild weather period, enjoying warmth and rainfall that hasn't been seen since.

To study the climate of that time period, scientists looked at "proxy" data, using the rings from pine trees. Published on Monday in the Proceedings of the National Academy of Sciences journal, the study examined both dead and living trees, according to The Times of India.

Hessl, Neil Pederson of Columbia University's Lamont-Doherty Earth Observatory and their research team analyzed sawed cross-sections from dead pine trees and "straw-like" core samples from living ones, The Times of India reported.

The trees' rings change each year due to temperature and rainfall level, so earlier climates could be analyzed when the researchers matched ring widths of living trees with weather data from 1959 to 2009, then compared the rings to older pines. 

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