The Mongol hordes led by Genghis Khan carved out the largest contiguous land empire history has ever witnessed, reaching at its apex from Asia's Pacific coast to eastern Europe and down into Persia and southeastern Asia. Although conventional wisdom suggests drought may have pushed them across the steppe to conquer more bountiful lands, ancient, long-dead trees discovered in a forbidding lava field in Mongolia give evidence that unprecedented rains might actually have helped fuel their expansion.

The Mongols took the Old World by storm in the 13th century. Their invasions and expansion are often attributed to the unstable climate they experienced on the steppes, "with them preying on others because they did not have a constant set of resources," says geographer Amy Hessl at West Virginia University. "Now, we agree they experienced a variable climate. However, this idea of drought driving the Mongols to expand their territory isn't really based on any climate data from that time, but on inferences based on modern conditions there."

In 2010 Hessl was on a National Geographic–sponsored mission there with forest ecologist Neil Pederson at Columbia University's Lamont–Doherty Earth Observatory to look at the climate history of Mongolia and how climate change might affect the area's wildfire risks. Driving by the Orkhon Valley, the original seat of the Mongol empire, they saw a huge lava field that had been produced by a volcanic eruption 7,000 years ago and checked it for ancient wood. Tree rings can shed light on a region's history — fat rings suggest abundant water that promoted growth, thin ones mean years with less water and growth; the number of rings is linked with how many years a tree has lived — and the live Siberian pines they saw in the area can live about 700 years.  

The researchers also took samples from dead wood in the lava field because it can be much older than any living tree there, given how the cold, dry conditions can slow decay. Pinpointing a dead tree's age can be difficult, but a unique growth pattern of fat and thin rings in a living tree might act as a "bar code" to identify wood alive during a specific period such as the 15th century. Seeing the same pattern in living and dead wood of a certain species from the same area reveals that both were alive during the same historical period. Determining the dead wood's age then involves counting back all the rings before that span of time.

All in all, the research team of U.S. and Mongolian scientists sampled 17 trees. "We felt if we got records going back 500 years, that would be fantastic," Pederson recalls. Unexpectedly, they instead discovered two samples with tree rings dating back to A.D. 658, now the longest climate record for this part of the world— and with further research, "we might be able to find tree rings going back maybe 2,000 years," Pederson adds. "We collected these samples as an afterthought when we were exhausted and sick. To find they might go back that far is unbelievable."

Surprisingly, their preliminary findings based on the tree ring data suggest the Mongol empire actually rose during a time of abundant rain. These would have turned grasslands there extraordinarily lush, enabled the Mongols to raise vast numbers of horses and other livestock. "There are actually massive wetlands in the area, and during a warm, wet period, they might have been incredibly productive," Hessl says. "There's actually quite a lot of evidence that the Mongols were practicing agriculture around there in the early 1200s, contrary to this image of Mongols as only herders and these horseback hordes."

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."