Genghis Khan's secret weapon was rain

New research by tree-ring scientists from Columbia University's Lamont-Doherty Earth Observatory and West Virginia University may have uncovered the reason why an obscure band of nomadic Mongol horsemen were able to sweep through much of Asia in a few meteoric decades 800 years ago, conquering everything in their path: They enjoyed an unprecedented, and yet-to-be-repeated, 15-year run of bountiful rains and mild weather on the normally cold and arid steppes.

By sampling tree rings in the gnarled and twisted Siberian pines in the Hangay Mountains in central Mongolia, the team pieced together a remarkably precise chronology of local climatic conditions stretching from the year 900 A.D. to the present. The study, published in this week's Proceedings of the National Academy of Sciences, offers a new interpretation of why the Mongols suddenly went on the move....