Image: Orkhon Valley
West Virginia University's Amy Hessl and a colleague walk through a grassy area in Mongolia's Orkhon Valley, which features monasteries and other historic sites.
By Senior writer
updated 7/20/2012 6:32:18 PM ET 2012-07-20T22:32:18

Beginning in the 13th century, the Mongol Empire spread across Asia and into the Middle East like wildfire, growing into the largest contiguous land empire the world has ever seen.

Historians have long speculated that periods of drought pushed the Mongol hordes to conquer their neighbors, but preliminary new findings suggest that theory may be exactly backward. Instead, consistent rain and warm temperatures may have given the Mongols the energy source they needed to conquer Eurasia: grass for their horses.

This idea, bolstered by the discovery of tree rings that preserve a climate history of Mongolia back to the year 657, is still in the preliminary stages of investigation. LiveScience spoke with Amy Hessl, the dendochronologist, or tree-ring researcher, who along with collaborators Neil Pederson and Baatarbileg Nachin first discovered the preserved trees hinting at the weather during the era of the Mongols.

LiveScience: How did you find the trees that held the Mongolian climate record?

Hessl: We were funded by the National Science Foundation and the National Geographic Society a few years ago to look at how climate change is impacting wildfire activity in Mongolia. So we had driven by this lava flow that looked reminiscent of other places I know of in the American West that have really long climate records from tree rings. The trees growing on these dry, exposed sites tend to grow until they're really old. And then once they die, the wood decays slowly. It allows you to reconstruct environmental conditions going back a very long time.

We drove by this lava flow, and I was like, "Whoa, that looks like an ideal place." So we went back, and even when we did sample, we didn't think we had anything that great. We were just throwing these pieces of wood back and forth to each other, like, "Oh, we'll make this one into coffee table art." We weren't taking it real seriously.

LiveScience: How did you realize that you'd found something important?

Hessl: I gave them to my colleague Neil Pederson [of Columbia University's Lamont-Doherty Earth Observatory]. He didn't look at them for months, until finally he didn't have anything else to do, so he started to date them. I started getting these texts from him on a Friday night and he was like, "I'm back to the 1200s."

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Finally, I get this text that just had three numbers on it, 657. I was like, "What was that, does he want me to call him at 6:57 in the morning?" It turned out to be the date of the oldest, most inner ring, 657 C.E.

There are certainly other tree-ring records that go back much farther, but this is special for Mongolia because it clearly covers the period of the rise of [Mongol Emperor] Genghis Khan. [Fight, Fight, Fight: The History of Human Aggression]

LiveScience: How can tree rings tell you what the past climate was like?

Hessl: These trees are growing on this lava, and there's very little soil development, so they're really, really water-stressed. When the tree rings are narrow, that tells you that during its growing season there was very little water available. The bigger the rings, the wetter it was.

LiveScience: What sort of climate patterns did you see as the Mongol Empire arose?

Hessl: It's very preliminary, but in the couple of trees we have in that time period we can see that the rings are not only wide, but they're consistently wide for the time that overlaps with the rise of Genghis Khan.

Our inference there is that this would have been an ideal time for high grassland productivity on the steppe, and that maybe translated into more livestock, especially horses for the Mongols.

To put it in perspective, each Mongol warrior had 10 horses at his disposal. Just right there, that's a huge amount of biomass that is required. In addition to that, when the Mongols expanded their range in their traveling and marauding, they brought with them large numbers of livestock that they used to feed themselves. Their whole military operation was basically predicated on the fact that they had large numbers of grazing animals. These climate conditions would have given them more energy to fuel their empires. [Top 10 Ways Weather Changed History]

LiveScience: What happened later on during the Mongol era?

Hessl: There's a well-known cold period that occurred after a volcanic eruption in 1258, and we can see this plunge into cold, dry conditions in Mongolia. At that same time, right around 1260, the Mongols moved their capital city out of the steppe and into Beijing, and we think it's possible that was related as well. We have a historian, Nicola DiCosmo of the Institute for Advanced Study, we're working with who is going to go back through all of the Chinese documents, Mongolian records and European accounts to try to see if there's information that would corroborate our findings or not.

LiveScience: Are you going back to Mongolia?

Hessl: I'm leaving in a week, actually! We're going to go back to the same lava flow and collect additional samples because we didn't really put our hearts into it the first time. We were only there for a few hours.

We've also identified some other lava fields in Mongolia that we think may have a similar ecological setting. We're also collaborating with some other people. One is Avery Cook Shinneman of the University of Washington, who studies lake sediments. She's going to be taking cores out of lakes in the Orkhon Valley, the seat of the Mongol Empire, looking for a little fungal spore that lives in livestock feces. What we're hoping is that we can get some general numbers and density of the livestock around these lakes going back in time.

LiveScience: What do you find interesting about linking past climate to history like this?

Hessl: It's fascinating to think about the energy sources that previous civilizations were dependent on, and when those energy sources were abundant how those societies responded, and when those energy sources evaporated, how did they adapt to that?

Society today is dealing with major threats to our primary energy source, so it's fascinating to me to look back on these earlier civilizations to see them going through the same transitions. It just puts our current situation in perspective.

Follow Stephanie Pappas on Twitter @sipappas  or LiveScience @livescience. We're also on Facebook  andGoogle+.

© 2012 All rights reserved.

Explainer: Seven deep mysteries of history

  • Image: Amelia Earhart
    FPG/Hulton Archive/Getty Images

    What happened to Amelia Earhart?

    Amelia Earhart raised the spirits of Depression-era America as she soared into the aviation record books with feats of altitude, distance and endurance. The mood took a gloomy turn, however, when she and her navigator Fred Noonan disappeared over the Pacific Ocean on July 2, 1937, during a much-heralded attempt to fly around the world. Their fate remains one of aviation's greatest unsolved mysteries.

    Theories abound: They ran out of fuel and crashed into the Pacific Ocean. They were captured by the Japanese and executed. They survived, and Earhart lived out her life as a housewife in New Jersey.

    A prominent theory with tantalizing clues holds that they survived the crash landing and but perished as castaways on Nikumaroro, an uninhabited island in the republic of Kiribati. An expedition to the island in 2010 recovered pieces of a pocket knife and a glass jar that may have belonged to the castaways. If DNA analyses on these and other items match Earhart's, the mystery may finally be resolved.

    Click ahead for six more stories of historical mysteries.

  • Where are Cleopatra and Mark Antony buried?

    Image: Kathleen Martinez, director of a Dominican-Egyptian archeological mission
    Orlando Barria  /  EPA

    Excavations underway at a temple near Alexandria, Egypt, may reveal the final resting place of the doomed lovers Cleopatra and Mark Antony. The Egyptian queen and Roman general committed suicide in 30 B.C. following their defeat in the battle of Actium for control of the Roman Empire. But where the lovers were buried is unknown.

    Zahi Hawass, Egypt's top archaeologist, believes the lovers were put to rest in the temple of Taposiris Magna and launched a dig with a Dominican-led team to locate the tomb. "It my opinion, if this tomb is found, it will be one of the most important discoveries of the 21st century because of the love between Cleopatra and Mark Antony, and because of the sad story of their death," he told reporters during a tour of the temple.

    Dominican archaeologist Kathleen Martinez is shown here with an alabaster bust of Cleopatra that was found at the excavation site near Alexandria.

  • Where is Genghis Khan buried?

    Image: The foundation of a Genghis Khan's mausoleum.
    Japan-Mongol Joint Research Team via AP

    Genghis Khan united warring tribes in 1206 and became the leader of the Mongols, creating an empire that eventually stretched from China to Hungary. The famed warrior's tomb, however, has remained a mystery ever since his death in 1227.

    According to legend, his burial party killed anyone who saw the procession. The slaves and soldiers who attended the funeral were also killed. Horses then trampled evidence of the burial, and a river was diverted to flow over the grave, which is thought to lie somewhere near Genghis Khan's birthplace in Khentii Aimag.

    Expeditions to locate the tomb have been aborted due to concerns that the excavations would disturb the site and destroy the soul that serves as its protector. In 2004, archaeologists uncovered Genghis Khan's palace, shown here, and they suspect the tomb lies nearby.

  • Did the Donner family resort to cannibalism?

    Image: James F. Reed and Margret W. Keyes Reed

    The legend is a harrowing tale of survival: A group of pioneers headed for California in 1846 got stuck on a mountain pass in the Sierra Nevada and resorted to cannibalism to survive the winter. But the claims that they feasted on human flesh may have been exaggerated, based on an analysis of bones found in a hearth along Alder Creek, where at least some of the Donner Party passed the time.

    The analysis shored up accounts that the family dog, Uno, was eaten, as well as a steady supply of cattle, deer and horse. No human bones were found at the site. While cannibalism may have occurred, if it did, the bones were treated in a different way. Perhaps the bones were buried. Or perhaps they were placed on the hearth last and have since eroded, according to project scientist Gwen Robbins, a professor of biological anthropology at Appalachian State University.

    Donner Party survivors James Reed and his wife Margaret Reed are shown in this photo from the 1850s.

  • Where is Billy the Kid buried?

    Image: William Bonney, aka Billy the Kid, circa 1880.
    Lincoln County Heritage Trust

    Legend holds that outlaw Billy the Kid was gunned down by Sheriff Pat Garrett in 1881 and buried in Fort Sumner, N.M. A headstone marks his grave, but a controversy has roiled since the 1930s when an Arizona man named John Miller claimed that he was the legendary outlaw. Garrett, he said, shot the wrong man and lied about it. Matters became even more confused a few decades later when a Texan named "Brushy" Bill Roberts came forth and said he was the real Billy the Kid.

    An investigation aims to resolve the case by exhuming the body of Billy the Kid's mother and comparing her mitochondrial DNA to genetic material from the three men. But the investigation is controversial on several fronts. For one, the graves have been moved over the decades and nobody is certain the bodies and headstones match up. In addition, if the real Billy the Kid turns out to be buried in Texas or Arizona, it would kill off a legend that helps draw tourists to the New Mexico gravesite.

  • Christopher Columbus' remains in Spain?

    Image: Alleged tomb of Christopher Columbus, Cathedral of Seville
    Cristina Quicler  /  AP file

    In 1492, Christopher Columbus sailed the ocean blue; after he died in 1509, his remains remained on the move. He was originally buried in the Spanish city of Valladolid, but his remains were shipped to the Caribbean island of Hispanola (modern-day Dominican Republic and Haiti) in 1537, in accordance with his will. When the Spanish lost the territory to France in 1795, they shipped Columbus's remains to Cuba, where they stayed until the Spanish-American War prompted their return to Seville in 1898. The tomb is shown here.

    The Dominican Republic, however, says Columbus' remains never left Hispanola. In 1877, a box was uncovered in a Santo Domingo cathedral with an inscription identifying the remains as belonging to the "illustrious and distinguished male Cristobal Colon (Spanish for Christopher Columbus)."

    DNA analysis of bone fragments from the Seville remains and those of Columbus' brother Diego, also buried in the city, are a perfect match. When researchers announced those findings in 2006, they declared that the century-old dispute was resolved. But DNA from the Dominican remains has yet to be studied, leaving the case not quite fully shut.

  • DNA seals fate of Russian czar's kids

    Image: Nicholas II, Prince Alexei
    AP file

    Bolsheviks gunned down Russian Czar Nicholas II, Empress Alexandra and their five children in 1918, but for 90 years the whereabouts of two of the children, Prince Alexei (heir to the Russian throne) and a daughter (Maria or Anastasia), remained unknown until 2008. That's when their bones were recovered from a grave near the rest of the Romanov family near Yekaterinburg in the Ural Mountains, about 900 miles east of Moscow.

    The bones from the second grave were burned and drenched in sulfuric acid, presumably to conceal the victims' identities or conditions at death. But scientists were able to examine mitochondrial DNA, which is passed down from mother to children. They also matched up Y chromosome markers from Crown Prince Alexei and Czar Nicholas II.

    Czar Nicholas II, left, and the Crown Prince Alexei, are shown cutting wood in this photo, taken at a Siberian prison months before their murder in 1918.


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