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."