Although this research is preliminary and needs more data before it can be properly assessed, "I'm not the slightest bit surprised that there are findings that contradict earlier studies," says archaeologist Brian Fagan, professor emeritus at the University of California, Santa Barbara, who did not take part in this study. "It is an area with wildly fluctuating climatic conditions."
This summer, the scientists plan on returning to the lava field to find more and older specimens. The lava field is large, about 50 square kilometers, "and it's very inhospitable — we saw horse skulls everywhere — meaning that people probably didn't venture in too much to gather firewood, and that there may be a lot of ancient wood there," Hessl says.
"We'd love to have at least 15 to 20 samples from the 1200s," Pederson notes. "Ideally we'd sample several species of trees from four to five lava fields to get a robust record of drought in central Mongolia for the past 1,000 years."
The researchers also hope to collaborate with ecosystem modeler Hanqin Tian at Auburn University in Alabama, who can use the tree ring climate data to estimate how many animals and other resources the Mongols could have secured from the landscape. In addition, historian Nicola Di Cosmo at the Institute for Advanced Study and his colleagues will comb old manuscripts from China to Europe for references to the climate at the time. Moreover, lake sediment expert Avery Cook Shinneman at the University of Washington also hopes to collect tubes of sediment from lake bottoms in the area for signs of how many livestock might have existed in the past, another sign of the region's past productivity. "Livestock would have disturbed the lake, affecting the sediment in ways we can see, and we can also look for Sporormiella, a tiny spore that thrives in livestock dung," Hessl says.
It may be that climatic changes may not only help explain the expansion of the empire, but its contraction as well. "It may be that a decline in moisture in the Orkhon Valley helped spur the Mongols to relocate their capital to Beijing," Hessl says.
The researchers caution that they are not arguing that climate was the sole or main driver of the rise and fall of the Mongol empire. "Genghis Khan was really key to uniting many tribes together and spurred them to expand in a way that's never been repeated — we just argue that it takes energy to create an empire, just as it does today, and rains may have helped provide the grass that 'powered' their horses," Hessl says. Likewise, "after Genghis Khan died, the empire became somewhat factionalized, with most historians arguing that it became too large to effectively administrate. We're saying maybe climate change may have made managing the empire difficult also."
The fate of the Mongol empire might have lessons for modern life , the researchers say. "The Mongols adapted to changing energy sources and to changing water quality, both of which climate change affected," Hessl says. "Exploring how they adapted might shed light on current challenges we face."