DAWN TO DUSK NEWS: Genghis Khan's Secret Weapon Was Rain

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Tree-ring studies suggest that a long wet spell gave him the resources to invade and conquer.</h2>
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Some
 800 years ago, ancestors of modern Mongolians conquered the world on 
horseback. A period of unusually mild weather may have helped propel 
them by making them rich in livestock.</div>
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<br /><strong>New research by  tree-ring scientists from Columbia University's Lamont-Doherty Earth Observatory
 and West Virginia University may have uncovered the reason why an 
obscure band of nomadic Mongol horsemen were able to sweep through much 
of Asia in a few meteoric decades 800 years ago, conquering everything 
in their path: They enjoyed an unprecedented, and yet-to-be-repeated, 
15-year run of bountiful rains and mild weather on the normally cold and
 arid steppes.</strong><br />

By sampling tree rings in the gnarled and twisted Siberian pines in 
the Hangay Mountains in central Mongolia, the team pieced together a 
remarkably precise chronology of local climatic conditions stretching 
from the year 900 A.D. to the present. The study, published in this week's <em>Proceedings of the National Academy of Sciences</em>, offers a new interpretation of why the Mongols suddenly went on the move.<br />

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<img alt="A photo of a gnarled tree in a lava field in Mongolia." src="http://images.nationalgeographic.com/wpf/media-content/photos/000/775/cache/77517_990x742-cb1394478325.jpg" width="100%" />
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To create the climatic chronology, the researchers 
sampled tree rings in ancient Siberian pines (like the one above) in the
 Hangay Mountains in central Mongolia.</div>
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PHOTOGRAPH BY KEVIN KRAJICK/THE EARTH INSTITUTE, COLUMBIA UNIVERSITY</div>
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The traditional view has been that the Mongols were desperately 
fleeing harsh conditions in their craggy, mountainous homeland. The 
Lamont-Doherty team, however, found just the opposite: Between 1211 and 
1225—a period that neatly coincides with the rise of Genghis Khan and 
the Mongol empire—central Mongolia enjoyed a spell of sustained benign 
weather unlike anything the region has experienced during at least the 
past 1,100 years and probably much longer.<br />

"What makes our new record distinctive is that we can see 15 straight
 years of above-average moisture," says the study's lead author, Neil Pedersen,
 a tree-ring scientist with the Lamond-Doherty Earth Observatory. "It 
falls during an important period in Mongol history and is singular in 
terms of persistently wet conditions."<br />

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<img alt="A photo of a gnarled tree in a lava field in Mongolia." src="http://images.nationalgeographic.com/wpf/media-content/photos/000/775/custom/77531_990x769-cb1394489068.jpg" width="100%" /></div>
<strong>More Grass, More Horses, More Power</strong><br />

The long run of unusually good conditions meant abundant grasses and a
 huge increase in herds of livestock and war horses that became the 
basis of Mongol power—a marked contrast to the long and exceptionally 
severe droughts that gripped the region during the 1180s and 1190s, 
causing unrest and division.<br />

The Mongols saw their opportunity and seized it—and were fortunate 
enough that this great tide in their affairs happened to coincide with 
the rise to power of a vigorous chieftain who would go on to unite them:
 Genghis Khan.<br />

"The transition from extreme drought to extreme moisture strongly 
suggests that climate played a role in human events," said tree-ring 
scientist Amy Hessl
 of West Virginia University in Morgantown, a co-author of the study, 
whose research was funded in part by the National Geographic Society's 
Committee for Research and Exploration. "It wasn't the only thing, but 
it must have created the ideal conditions for a charismatic leader to 
emerge out of the chaos, develop an army, and concentrate power. Where 
it's arid, unusual moisture creates unusual plant productivity, and that
 translates into horsepower—literally. Genghis was able ride that wave."<br />

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<img alt="A photo of the rings of a tree." src="http://images.nationalgeographic.com/wpf/media-content/photos/000/775/cache/77513_990x742-cb1394477286.jpg" width="100%" />
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Tree rings, which can be dated year by year, are clearly visible in well-preserved specimens.</div>
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PHOTOGRAPH BY KEVIN KRAJICK/THE EARTH INSTITUTE, COLUMBIA UNIVERSITY</div>
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And ride it he did. By the time he died in 1227, he and his followers
 had founded an empire that would ultimately cover Korea, China, Russia,
 the Middle East, Eastern Europe, Persia, India, and Southeast Asia. 
Although the Mongol empire would break up over the centuries, some of 
Genghis Khan's direct descendants were still ruling pockets of central 
Asia into the 1920s.<br />

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The timely change in climatic conditions that helped to 
launch the Mongol empire "doesn't appear to have been associated with 
any change in volcanic eruptions or solar irradiance," says Kevin Anchukaitis,
 a paleo-climatologist at the Woods Hole Oceanographic Institute. The 
task that lies ahead: determining if weather influencers like El Niño 
and the North Atlantic Oscillation might have brought the beneficial 
rains in Central Asia.</div>
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<img alt="A photo of a person standing on a shrunken lake." src="http://images.nationalgeographic.com/wpf/media-content/photos/000/775/cache/77511_990x742-cb1394477753.jpg" width="100%" />
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High on a landlocked plateau, Mongolia is generally
 cold and arid, with tenuous supplies of grass and water. This huge lake
 is salty and shrunken from drought.</div>
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<strong>Mongol Invasion: Modern Version</strong></div>
The climate chronology compiled by the Lamont-Doherty team reveals 
that the dawn of the 21st century has seen some of the hottest weather 
and most severe droughts in centuries—hotter and drier than even the 
severe drought years of the 1180s and 1190s. It also suggests that 
climate may play a role in a modern Mongol invasion—from the arid 
steppes to the crowded cities.<br />

The recent drought years have followed a string of fairly wet years 
toward the end of the 20th century. That's a double whammy, says 
Pedersen, since the relative abundance of rainfall in the 1990s could 
have made herdsmen less prepared for the harsh conditions of a severe 
drought.<br />

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The hard years, coupled with sweeping changes in Mongolia's
 post-communist society, have sparked a mass exodus from the steppes 
into the country's capital, Ulaanbataar, where roughly half of 
Mongolia's three-million-strong population now lives.</div>
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Genghis Khan's Secret Weapon Was Rain

Tree-ring studies suggest that a long wet spell gave him the resources to...