Ayush Ish in her home with her son and grandson in the Gobi Desert in Mongolia. Extreme winter events killed off her livestock twice. CreditRachel Nuwer

ULAN BATOR, Mongolia — For all of his victories and skills, Genghis Khan always insisted that the god Tengri — the Eternal Blue Sky — deserved the credit for his triumphant success in uniting the vast Mongol Empire in the early 13th century.

Now 21st-century science may be proving him right. Not long ago, researchers studying ancient tree rings found evidence that the Great Khan rose to power during an exceptionally mild 15-year stretch.

Back-to-back years of plentiful rain and favorable temperatures — known as pluvials, the opposite of droughts — promoted vegetation growth, the researchers believe, and that in turn supported the livestock needed to power an army.

“The Mongol Empire pluvial was quite exceptional in its duration,” said Neil Pederson, an ecologist at Harvard Forest in central Massachusetts. “It was the only one in the past 1,000 years that lasted more than 10 years, so it’s really a singular event.”

These days, Mongolia’s climatic tides have been shifting toward another extreme. A 10-year drought and heat wave from 2000 to 2010, according to the tree ring data, was the most severe the country had had in a millennium.

“I’m more and more convinced that the only way we can understand this 21st-century event is within the context of climate change,” said Amy Hessl, a geographer at West Virginia University. “And the human side of that — combined with a constellation of other factors — is going to be incredible.”

Today, Mongolia is largely herders, not warriors. Sandwiched between Russia and China, it has almost three million people in a vast tract of desert and rolling steppe grassland, punctuated by mountains and forests. Climate continues to significantly shape the lives of Genghis Khan’s descendants, around one-third of whom still practice the seminomadic herding of their ancestors, moving their house — traditionally, a dome-shaped tent called a ger — with the seasons.

While televisions and solar panels are a common sight in modern gers, herders still rely on thousands of years of collective knowledge to thrive in the harsh Mongolian environment, where the temperature regularly dips below minus 15 degrees Fahrenheit in winter. Yet the predictable ebb and flow of warmth and cold, rain and snow, has begun to falter in recent years.

“I don’t know why the weather has become unusual, but I’m very worried about it,” said Urgamaltsetsg Suvita, 47, a herder in the Gobi Desert. Summer is hotter and drier and plagued by sandstorms, she said, and winter brings too much snow or too little.

In 2010, an extreme snowstorm killed her flock of livestock — nearly 1,000 animals, including horses, sheep and goats. “Winter is no longer winter,” she said.

Like much of the world, Mongolia is already experiencing the effects of climate change. The country’s average annual temperature has risen more than 3.8 degrees Fahrenheit since 1940; paradoxically, winter months have grown colder over the past 20 years. Streams and lakes have begun to dry up, and fires frequently blaze across millions of acres of steppe and forest.

“The steppe ecosystem is burning and burning and burning,” said Oyunsanaa Byambasuren, a lecturer in forestry at the National University of Mongolia. “But we really don’t have enough specialists or professionals dealing with those issues.”

Dzuds — extreme winter events that cause mass livestock die-offs — also seem to be increasing in frequency and intensity. From 1999 to 2002, a succession of winter dzuds followed by summer droughts killed 30 percentof all livestock in Mongolia, and a 2010 dzud claimed 8.8 million livestock — losses equivalent to 4.4 percent of the country’s economic output.

“Wealth in much of Mongolia is measured in animals,” said Nicole Davi, a research scientist at Lamont-Doherty Earth Observatory and at William Paterson University. “If you lose all of your animals, you lose everything.”

Ayush Ish, 69, has lived in the Gobi Desert all her life. She and her husband lost their flock of goats and sheep in the 2002 dzud and drought, then slowly rebuilt it with the help of 50 animals allocated by the government. But when the 2010 dzud struck, all but 20 died. Around the same time, her husband died as well.

“I don’t know if it will happen again,” she said, tears running down her cheeks. “I can only hope that we’re entering a good period now.”

Like many rural Mongolians who follow a shamanistic belief system, Ms. Ish says that mining — which has recently become widespread around the country — is to blame for the changing weather patterns. Troy Sternberg, a geographer at the University of Oxford, said that “under the Mongolian belief system, the earth and sky are connected, so if you take gold out of the ground, you’re disrupting the natural rhythms of weather and climate.”

Whether or not that is true, the rise of mining — along with overgrazing by herders chasing the cashmere market — has led to wide desertification. Some studies indicate that 70 percent of Mongolia’s grasslands are degraded.

Taken together, these patterns bode ominously for the herders’ way of life. A team of Mongolian and international experts warned in a 2009 report that such trends “may lead to the end of the Mongolian traditional way of animal husbandry as we know it, that at onetime was the very core of the entire nomadic civilization.”

Some herders have already reached that breaking point. After the 1999 to 2002 dzuds alone, 180,000 people moved to the capital, Ulan Bator, in search of a better life. “The movement from a rural, agrarian life to an urban industrial one is not necessarily a bad thing if people are interested in doing other things that are tied to a more diverse economy,” said Maria Fernandez-Gimenez, an ecologist at Colorado State University. “But opportunities and services have to exist to enable that.”

Those who reach the capital usually settle in the ger district, a sprawling, makeshift neighborhood that encircles the city and creeps into the surrounding valleys. Although the skyscrapers of downtown are in eyesight, basic services are luxuries. Families must trek up to a mile to collect water from communal wells, and in winter, they burn coal and garbage to keep warm, helping to make Ulan Bator one of the world’s most polluted cities.

Khishigee Shuurai, 36, moved from western Mongolia to the capital around 15 years ago. In 2002, after losing their flock, her parents joined her in the city. Then her father died of a heart attack. Before that, she said, he frequently expressed a longing to return to the countryside.

For many, however, life here is preferable to the uncertainty and harshness of nomadic herding. Ms. Shuurai, a school custodian and mother of four, does not share her father’s regrets.

She and her husband, a construction worker, have jobs, and they recently got electricity in their ger. Her 7-year-old daughter was honored as the top student in her class, and her 12-year-old son wants to become an engineer.
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