Surprisingly, a new study of tree rings preserved for over a thousand years has shown that warm wet weather in the late 12th century may be responsible for the rise of the Great Mongol Empire. Researchers studying the rings of ancient trees in mountainous central Mongolia found 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.
"The transition from extreme drought to extreme moisture right then strongly suggests that climate played a role in human events," said coauthor Amy Hessl, a tree-ring scientist at West Virginia University. "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."
In the late 1100s, the Mongol tribes were racked by disarray and internal warfare, but this ended with the sudden ascendance of Genghis (also known as Chinggis) 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 neighbors 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, and the Mideast. Several invasions against India were also launched by Genghis Khan and his successors. The empire eventually fragmented, but the Mongols' vast geographic reach and their ideas—an international postal system, organized agriculture research and meritocracy-based civil service among other things—shaped national borders, languages, cultures and human gene pools in ways that resound today. Genghis Khan's last ruling descendants ran parts of central Asia into the 1920s.
In a series of expeditions, Neil Pederson, a tree-ring scientist at Columbia University's Lamont-Doherty Earth Observatory, Hessl and colleagues sampled the pines' rings, sawing cross-sections from dead specimens, and removing harmless straw-like cores from living ones. 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 they found had rings going back 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 tree rings show that after the empire's initial expansion, Mongolia's weather turned back to its more normal dryness and cold, though with many ups and downs over the hundreds of years since.
The study appears in this week's early online edition of the Proceedings of the National Academy of Sciences.
Previous studies by others have advanced the idea that climate swings can change history. These include events such as the disappearance of the Maya, the expansion and fall of Roman imperial power, and, in a separate Lamont-led study, the 13th-century collapse of southeast Asia's Angkor civilization. A recent study showed that the Indus Valley civilization too collapsed because of abrupt changes in rainfall patters.
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