MONGOLIA: Did climate spur the Mongolian invasions of the 13th century? The accepted wisdom has been that an erratic, variable climate at home drove Genghis Khan’s expansionist desires, but according to early analysis of tree rings from Siberian pines, the opposite might have been true. Thick rings from around the time of the invasions suggest that it was a wet period, which would have fueled grasslands and provided fodder for thousands of Mongol horses. —Samir S. Patel