Did good weather lead to the greatest land empire of all time?

The Mongol Empire was the largest contiguous empire in human history. But its meteoric rise and remarkable run might not have happened if it weren't for an uncharacteristic change in weather that brought sustained rainfall and mild warmth for a short period of time.

At its peak, the Mongol Empire claimed an area that's now comprised of modern Korea, China, Russia, eastern Europe, southeast Asia, Persia, India and the Mideast. But prior to its ascension, during the late 1100s, the Mongol tribes were racked by disarray and internal warfare. This all came to an end with the rise of Genghis Kahn in the early 1200s. In a short amount of time, he united the tribes into an efficient horse-powered military state that rapidly conquered its neighbors and expanded outward in all directions.

Did good weather lead to the greatest land empire of all time?

There are a number of theories to explain why it happened at this particular point in history, including the suggestion that the Mongols expanded because they were fleeing harsh weather at home. But a new study of ancient tree rings in mountainous central Mongolia suggests the opposite — that a temporary run of nice weather allowed the Mongols to take their show on the road.

Indeed, as a band of nomadic warriors, the ability to sustain such a large fleet of horses was no small task. In fact, every Mongol warrior had five or more horses, while ever-moving herds of livestock provided nearly all the food and other resources. The ability to move them across hundreds and thousands of miles — especially across the normally arid Steppe — was contingent on an adequate supply of edible flora, namely grass.

"Before fossil fuels, grass and ingenuity were the fuels for the Mongols and the cultures around them," noted lead author Neil Pederson in a statement. "Energy flows from the bottom of an ecosystem, up the ladder to human society."

According to the researchers, the immediate years preceding Genghis Khan's rule, from around 1180 to 1190, were characterized by intense drought. But then, from 1211 to 1255 — precisely the years coinciding with the empire's meteoric rise — Mongolia experienced sustained rainfall and mild warmth never seen before or since.

"The transition from extreme drought to extreme moisture right then strongly suggests that climate played a role in human events," added co-author Amy Hessl. "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."

Read the entire study at Proceedings of the National Academy of Sciences: "Pluvials, Droughts, the Mongol Empire and Modern Mongolia."

Top image: Mongol (2007).