A conventional historical narrative holds that the rise and expansion of the Mongol Empire -- first under Genghis Khan and, later, his progeny and successors -- were propelled by a deteriorating climate in the Mongolian steppe. Fleeing drought, the narrative runs, Ghengis Khan's Golden Horde pushed west, south and east in a bid for expansion that would someday form the world's largest contiguous empire.

Indeed, climate records indicate that the arid, landlocked steppe was seized by decades of drought in the late years of the 12th century, possibly exacerbating the violent conflicts that racked the region at the time. It was into such a world Genghis Khan was born and rose to power, crushing his rivals and uniting the fractured Mongolian tribes under his own horsehair banner.

But a new study of centuries-old tree core samples indicates an abrupt turn of the weather around the first decade of the 13th century. According to the findings of a team of scientists, published recently in the Proceedings of the National Academy of Sciences, the region appears to have entered into a period of uncharacteristically strong rainfall around the year 1211, ushering in a decadelong period of heavy rain the likes of which Mongolia has not seen since.

The vast surplus of livestock and crops brought on by such advantageous conditions may have played a critical role in supporting the Great Khan's centralized authority and military ventures, at least in the early years of global conquest, the researchers believe.

"This is a new kind of thought," said Nicola Di Cosmo, a historian at the Institute for Advanced Study in New Jersey and co-author of the report. "If we can prove that you need a certain amount of productivity of land to support an expanded political establishment, we can start to explain why it lasted so long."

The team's findings may also, in time, help scientists and historians understand how humans have responded to abrupt climate shocks in the past, and how they may do so again in the future.

History in the trees
In 2010, forest scientists Amy Hessl and Neil Pederson were driving along the Orkhon Valley, in central Mongolia, where eight centuries earlier Genghis Khan had established his capital of Karakorum. The scientists had received a National Science Foundation grant to study the possible future impacts of climate change on Mongolian wildfires, and were looking for tree core samples to read what they could of the past record.

"The kinds of trees that are good for establishing past climate tend to be in places that are extremely water-stressed," said Hessl, a tree-ring scientist at West Virginia University. Such trees tend to be porous and take up moisture readily, she said. They also grow slowly and are less susceptible to rot.

North of the ruined Karakorum, they found what they were looking for: gnarled Siberian pines jutting up from a 7,000-year-old lava field. Samples from their first foray dated over six centuries; later expeditions would yield tree rings more than a millennium old.

Unique among all climate records, tree rings can date events with almost calendar accuracy. A new ring grows on a tree every cycle of the seasons, meaning each ring stands for a particular year. In wet years, the rings are fat; in lean years, thin.

"No other proxy can tell you, with annual accuracy, when an event took place," said Hessl. The samples "allow us to tie in our data with human time scales, with what we believe to have been happening at that point in history."

The tree rings tell a story of severe drought in the 1180s, one of the worst seen in the central Mongolian climate record that the team would ultimately plot. That dryness persisted into the first decade of the next century. The year 1211 showed a jump in average rainfall, only to see levels drop again the next year.

Then, in 1214, rainfall appears to have risen above the mean and remained there, persistently, for the next 12 years. The effect on the landscape may well have been transformative: lusher, more abundant grasses capable of swelling the Mongols' horse and livestock populations to previously unseen numbers.

Forged in drought, fueled by rain
"It's the longest pluvial we've seen going back 800 years," said Pederson, a tree-ring scientist at Columbia University's Lamont-Doherty Earth Observatory. Still, he cautioned, with further findings, the exact shape of the phenomenon may yet change.

"The [pluvial] feature was present in the first two trees we sampled," he said. Adding in later samples, he said, "the shape and length has changed, but it remains a strong recognizable feature."

Much of what can be inferred about the relationship between climate and the Mongols' rise is still speculation, although the team is pursuing its theories with a range of ongoing research. However, many features of the two stories seem to fit well together, said Di Cosmo.

The late 12th century was a period of protracted conflict, internecine warfare and revenge killings between the large Mongolian clans, a time when previous political orders were shattered and a new militaristic, centralized order emerged under Genghis Khan.

That severe drought and resulting resource scarcity may have played a role in this upheaval is an idea historians need to take seriously, he said. "There are well-established correlations between climate degradation and conflict," he added.

The political order established by Genghis Khan was much different from the tribal equilibrium and conflict that had preceded it. Under him, former chieftains became subjects and family members courtiers. Within Karakorum, a small village of personal bodyguards, armed forces and servants encircled the Khan.

Supporting such a state apparatus would have required resources, Di Cosmo said. And with a turn of climate in 1214, those resources may have been in sudden abundance.

"If you have a valley that produces three, four, five times more nutrients for cattle, sheep and horses, this new political order has a chance to survive before collapsing, as so many nomadic states have collapsed, due to resource scarcity," he said.

When horses meant power
Agriculture, all but impossible in Mongolia in times of drought, could have resurfaced, as well, he said, lending stability to an economy previously dependent on livestock alone.

And an abundance of grass would likely have meant more horses. Horses were key to the Mongols' military tactics, so much so that each warrior was expected to have five mounts of his own.

"Energy flows from the bottom of an ecosystem, up the ladder of human society," said Pederson. "Even today, many people in Mongolia live just like their ancestors did."

The researchers aren't arguing that climate necessitated the Mongols' rise, of course. "Climate may have played a role, but it certainly wasn't the only thing shaping events," said Hessl. "Still, it's very interesting that our climate record appears to fit so well with the historical narrative."

The study also rings a more somber note. The tree rings verify what is already well-known: As in the late 12th century, Mongolia today is facing severe climatic conditions. Temperatures in parts of the country have risen by as much as 4.5 degrees Fahrenheit, far exceeding the global average. A series of droughts over the past two decades have killed millions of livestock and racked the country's still-substantial agricultural sector.

The tree ring study puts these changes in troubling perspective. Along the entire multicentury timeline they reveal, the last drought -- persisting from 2002 to 2009 -- is the hottest on record.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500