Throughout human history, natural climate change has played a role in the rise and fall of civilizations around the world, from the Mayans to the Romans.
The empire of Genghis Khan of Mongolia, one of the most notorious characters in world history, was helped by a dramatic rise in rainfall and mild temperatures in central Asia in the early 13th century, according to a study released Monday in the Proceedings of the National Academy of Sciences.
Led by Genghis Khan and his sons and grandsons, the Mongols ruled most of modern-day Russia, China, Korea, southeast Asia, Persia, India, the Middle East and eastern Europe, forming the largest land empire in world history.
Previous research had speculated that the Mongols were escaping extreme drought, but this research takes the opposite tack.
Scientists led by physical geographer Amy Hessl of West Virginia University and Neil Pederson of Columbia University's Lamont-Doherty Earth Observatory believe that the rise of Genghis Khan and the start of his sprawling empire was propelled by a temporary run of nice weather.
How did the additional rain and unusually mild weather help Khan? Horsepower, Hessl said. The abundant grasses provided the fuel for the horses that helped the Mongols and their superb cavalry conquer vast lands. It also helped provide food in the way of livestock fed by the bounty.
"Where it's arid, unusual moisture creates unusual plant productivity, and that translates into horsepower," she said. "Genghis was literally able to ride that wave."
Khan's rise to power was in 1211 to 1225 A.D. During that time, scientists found that Mongolia saw sustained rainfall and mild warmth never seen before or since.

Khan and his crew didn't carry rain gauges or thermometers as they marauded over the steppes, so scientists used paleoclimatic "proxy" data -- in this case rings from dead pine trees -- to reconstruct the climate of that era.
The tree rings from those years were "persistently wide," said Pederson which he said "suggest that period, climatically-speaking, was persistently wet."
Since the mid-20th century, the region has warmed rapidly, and the rings show that recent drought years were the most extreme in more than 1,000-years — possibly a side effect of global warming.
"Future warming may overwhelm increases in precipitation, leading to similar heat droughts, with potentially severe consequences for modern Mongolia," the authors wrote.
"Though we cannot attribute a single event to climate change, warming temperatures have stacked the deck toward (more evaporation), so even if the amount of precipitation remains the same, high temperatures will generate a more intense drought," Hessl said.
"That's what we observed in the early 21st century, and based on past moisture variation in Mongolia and future predictions of warming, we would expect to see similar events in the future."