By studying the rings of ancient trees in mountainous central Mongolia, researchers think they may have unraveled 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 rise of the great leader Genghis Khan and the start of the largest contiguous empire in human history was propelled by a temporary run of nice weather.
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.
But the tree rings, spanning 1,112 years from 900 to 2011, also exhibit an ominous modern trend. Since the mid-20th century, the region has warmed rapidly, and the rings show that recent drought years were the most extreme in the record-possibly a side effect of global warming.
In a region already pressed for water, the droughts have already helped spark a new migration in a vast region where people until now have lived the same way for centuries, moving herds from place to place and living in tents.
Now, those herders are being driven rapidly into cities, and there could be greater future upheavals.
"Before fossil fuels, grass and ingenuity were the fuels for the Mongols and the cultures around them," lead author Neil Pederson, a tree-ring scientist at Columbia University's Lamont-Doherty Earth Observatory, said.
"Energy flows from the bottom of an ecosystem, up the ladder to human society. Even today, many people in Mongolia live just like their ancestors did. But in the future, they may face serious conditions," he said.
Some researchers have postulated that the Mongols expanded because they were fleeing harsh weather at home-but Pederson and colleagues found the opposite.
In 2010, Pederson and coauthor Amy Hessl, a tree-ring scientist at West Virginia University, were studying wildfires in Mongolia when they came across a stand of gnarled, stunted Siberian pines growing out of cracks in an old solid-rock lava flow in the Khangai Mountains. They knew that on such dry, nearly soil-less surfaces, trees grow very slowly, are exquisitely sensitive to yearly weather shifts, and may live to fantastic ages.
The study is published online in the Proceedings of the National Academy of Sciences.
