The meteoric rise of the great Genghis Khan and the start of the largest contiguous empire in human history was propelled by a long period of warm, wet weather spanning more than a decade, a new study suggests.
Researchers studying the rings of ancient trees in central Mongolia think they may have gotten at the mystery of how small bands of nomadic Mongol horsemen united to conquer much of the world within a span of decades, 800 years ago.
The rings show that exactly when the empire rose, the normally cold, arid steppes of central Asia saw their mildest, wettest weather in more than 1,000 years.
Grass production must have boomed, as did vast numbers of war horses and other livestock that gave the Mongols their power, researchers said.
"Before fossil fuels, grass and ingenuity were the fuels for the Mongols and the cultures around them," said lead author Neil Pederson, from the Columbia University.
In the late 1100s, the Mongol tribes were racked by disarray and internal warfare, but this ended with the sudden ascendance of Genghis Khan in the early 1200s.
In just a matter of years, he united the tribes into an efficient horse-borne military state that rapidly invaded its neighbours and expanded outward in all directions.
Genghis Khan died in 1227, but his sons and grandsons continued conquering and soon ruled most of what became modern Korea, China, Russia, eastern Europe, southeast Asia, Persia, India and the Mideast, researchers said.
In 2010, Pederson and coauthor Amy Hessl from West Virginia University came across a stand of gnarled, stunted Siberian pines growing out of cracks in an old solid-rock lava flow in the Khangai Mountains in Mongolia.
They found that some trees had lived for more than 1,100 years, and likely could survive another millennium; even dead trunks stayed largely intact for another 1,000 years before rotting. One piece of wood had rings dating to about 650 BC.
These yearly rings change with temperature and rainfall, so they could read past weather by calibrating ring widths of living trees with instrumental data from 1959-2009, then comparing these with the innards of much older trees. The trees had a clear and startling story to tell.
The turbulent years preceding Genghis Khan's rule were stoked by intense drought from 1180 to 1190. Then, from 1211 to 1225 - exactly coinciding with the empire's meteoric rise - Mongolia saw 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," said 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," Hessl said.
The study appears in the journal PNAS.