Inner workings: Story of an empire encased in tree rings.

A centuries old lava flow, spreading from the base of the now dormant Khorgo volcano in Central Mongolia, seems an unlikely place to find history lessons. However, on spotting it, Amy Hessl wanted to explore. In this lava flow, Hessl supposed, could be clues to how, many centuries ago, a small band of nomadic horsemen took overmost of the easternworld. “We were driving by on our way to a different site and I said, ‘Look at this classic lava flow! Imagine what we could find here,’” says Hessl, a tree ring scientist at West Virginia University. Initially, her colleagues didn’t share Hessl’s excitement. One had visited the flow years before and failed to find interesting trees. Locals had collected much of the wood for fuel. “Then we’ll have to come back and walk in farther than most people are willing to walk,” Hessl said. Upon returning, they spotted treasure about two hours in: a collection of Siberian pines. These long-lived trees are usually found at high altitude, yet here was a large collection, some dead, some living, eking out a barebones living on a giant evaporative pan. Under extreme water stress the trees grew very slowly, with rainfall as essentially their only water source. Hessl says they were thrilled with the find, but “we didn’t recognize how good the record was until we got it back to the lab.” In their samples they had a record of moisture conditions for the past 1,112 years. Hessl and her colleague Neil Pederson, of Columbia University’s Lamont-Doherty Earth Observatory, are piecing together past climate conditions on the steppe (1). What they’ve found defies conventional wisdom: rather than leaving Mongolia to escape drought conditions, Genghis Khan likely soared to power during a time of unprecedented warm, wet, mild weather. Lush grasslands during this time fueled the horse herds on which the empire’s power literally rode. However, the rings also carry a lesson for modern Mongols, Hessl said. The rings indicate that the devastating droughts of recent decades have been worsened by human-caused climate change. In the past, during times of drought, the empire contracted and ancient Mongols survived by moving and adapting. Today Mongolians are increasingly opting for permanent dwellings and jobs. Retaining an ability to move and adapt as the climate becomes more unpredictable might be key to the survival of their ways.