Genghis Khan may owe his success to a period of excellent weather. A recent study of tree rings in Mongolia has shown that the rise of Khan’s empire in the early 13th Century coincided with the mildest, wettest weather in more than 1000 years.

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The Mongol empire was impressive by many standards. Covering modern-day Korea, China, Russia, Eastern Europe, India and south-east Asia, it was the largest continuous land empire the world has ever known.

Through studying the tree rings on a selection of ancient Siberian pine trees that date back nearly 2,000 years, American tree-ring scientists have discovered that the years from about 1180 to 1190 were ones of intense drought, and that they were followed by a period from 1211 to 1225 of sustained rainfall and mild weather.

This change in the weather would have led to unusual plant productivity, which was essential to Genghis Khan’s success. Each Mongolian horseman is said to have had up to five horses, and without the plant matter to feed them, expansion simply would not have been possible.

Amy Hessl, a tree ring expert at West Virginia University, said “The transition from extreme drought to extreme moisture right then strongly suggests that climate played a role in human events. It was not 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.”

The same tree ring studies have shown that, since the mid-20th Century, the region has warmed rapidly and that recent drought years have become more extreme than during any year these trees have experienced. This worrying trend has encouraged the Mongolian population away from the steppes and into Ulaanbataar, the country’s capital, where roughly half the population now lives.