Eight hundred years ago, relatively small armies of mounted warriors suddenly exploded outward from the cold, arid high-elevation grasslands of Mongolia and reshaped world geography, culture and history in ways that still resound today. How did they do it?
Tree-ring scientists at Columbia University’s Lamont-Doherty Earth Observatory have worked in Mongolia since 1995. In 2010, Lamont researcher Neil Pederson and Amy Hessl of West Virginia University were seeking old trees for a study of wildfire history. High in the Khangai Mountains, north of the steppe where the long-disappeared Mongol capital of Karakorum once lay, they explored a nearly solid-rock plain of hardened lava left by a volcanic eruption some 8,000 years ago. Growing out of fissures and thin soils were thousands of gnarled, stunted larches and Siberian pines–a tree-ring scientist’s treasure. Annual rings of many species reflect rainfall or temperature in predictable ways. These can be read like books; and trees in the driest, harshest sites like this are exquisitely sensitive to rain, live to extraordinary ages, and leave trunks that may stand for centuries after they die. They are truly ancient manuscripts, writ with a fine hand.
Pederson and Hessl analyzed 17 trees to chart a yearly record of rainfall back to 658 AD. They saw that from 1211-1230—the exact time of the Mongols’ rise—central Mongolia saw one of its wettest periods ever. That time also was unusually warm, as shown by a