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Roots of Empire on October 9, 2012 A climate history project in is charting the unexpected conditions that may have propelled the rise of Genghis

KHORGO, MONGOLIA—As the sun descends The odds that he is right are vanishingly Swiss Federal Research Institute for Forest, over the ancient lava fi eld, John Burkhart— small. But this chunk of pine could help illu- Snow, and Landscape in Zurich, who is not www.sciencemag.org clad in a fluorescent orange field vest, leg minate the rise of one of history’s great lead- affi liated with the project. guards, and ear covers—sinks a chainsaw into ers. Beginning in the 12th century, Genghis Samples collected here last year hint at an a dead Siberian pine. The saw’s earsplitting Khan—known in Mongolia as Chinggis intriguing history that Pederson says could whine drowns out the hum of insects, until one Khaan—rose from obscurity to conquer overturn “the prevailing wisdom on the Mon- end of the misshapen log drops to the ground a territory that covered as much as 31 mil- gol empire.” Some 8000 to 9000 years ago, with a soft thud. lion square kilometers, extending from an eruption of the now-dormant Khorgo Vol-

As the geography graduate student slices Korea to the Balkans—the largest contigu- cano covered a vast expanse with lava. Today, Downloaded from a second time, dendrochronologists Amy ous land empire in the history of the world. the landscape is dotted with hawthorn, rose Hessl of West Virginia University in Morgan- And yet little is known about how a man of hips, and buckwheat. Siberian pine and larch town and Neil Pederson of Columbia Univer- humble origins managed to lead an army that have a precarious foothold: Some trees pro- sity’s Lamont-Doherty Earth Observatory in claimed more territory in 25 years than the trude awkwardly from cracks in the volcanic Palisades, New York, watch expectantly from Romans conquered in 400. rocks, while others are twisted and stunted a few meters away. The wood is blackened To uncover clues to this puzzle, like bonsai. “This is the edge of what’s pos- on one side, probably by a forest fi re. That Pederson and Hessl are spearheading a sible for forests to charring has likely protected it from micro- multidisciplinary project bridging climate grow,” Hessl says. organisms and decay, meaning it could have science, energetics, and history—part of a Online That makes the site fallen into this crevice between two wedges growing body of work connecting climate sciencemag.org an ideal barometer for Podcast interview of volcanic rock hundreds of years ago. The change to centuries-old societal and politi- with Mara climate conditions on trunk’s exposed interior appears free of the cal shifts. A $1.4 million grant from the U.S. Hvistendahl (http://scim. the surrounding grass- rot that would render it scientifi cally useless. National Science Foundation, announced ag/pod_6102). lands. Throughout But only after Burkhart extracts a disk of last week, will fund fi eldwork at Khorgo and the centuries, the lava wood a few centimeters thick do the tree-ring other sites over the next 3 years. That research fi eld’s stressed trees have been acutely sensi- scientists celebrate. “That’s just an excellent could expand the climate record for a poorly tive to changes in moisture levels. Their rings remnant,” Hessl says. Pederson grabs a black understood region and “develop long, robust bear the imprint of those fl uctuations. marker and scribbles a standard fi eld label chronologies” that “would fi ll a gap” in cli- In 2010, Pederson and Hessl surveyed on the sample, then proclaims, jokingly: matologists’ understanding of medieval Asia, the area for a project exploring the link

“Chinggis looked upon this tree!” says Ulf Büntgen, a paleoclimatologist at the between climate change and wildfi re risk MARA HVISTENDAHL CREDIT:

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Window on the past. Ancient trees that found a “captured water and used it to make energy”— lone relic anchoring a grassy knoll. The loca- foothold in this lava-covered landscape are yielding for agriculture, for irrigation, for war. tion of the tomb of , who died insights into centuries-past rainfall patterns. That’s a diffi cult task, notes Büntgen, who in 1227, is unknown. led an earlier study that gauged the effect of Much of what historians know about the in Mongolia. On their last day of fi eldwork, climate on the rise of the Roman Empire ascent of the is based on a they took samples from 10 living Siberian (Science, 4 February 2011, p. 578). “With single source: The Secret History of the Mon- pines and seven downed logs. They aimed collapse, you have a more precise date,” he gols, an account of Genghis Khan’s life writ- to strengthen a 600-year-long precipita- says, “whereas the rise is vague and less well- ten by an anonymous author shortly after the tion record derived from lava-fi eld wood by documented.” But it’s one worth tackling, ruler’s death. The document disappeared for Lamont-Doherty’s Gordon Jacoby and col- Hessl says: Energy is “a more fundamental centuries, only to reemerge in Beijing in the leagues. “We were just hoping for 500 or linkage and one we can’t deny, even in our 19th century. Printed in a bedeviling script 600 years” of history, Pederson recalls. In modern world.” using Chinese characters to represent Mongol the 13 years he’d been working in Mongolia, sounds, the text was not fully deciphered until the 1500s were as far back as his group of Cryptic conquest the 1980s. But while rich with details about dendrochronologists had gotten. In the mid-1100s, Mongolia’s grasslands were Genghis Khan’s life, the Secret History is also Their last batch of samples turned back an unlikely wellspring for a world conqueror. heavy on folklore, and it says nothing about the clock to much earlier. Several trees pre- Nomadic tribes roamed the sparsely popu- resource use or climate during the period of dated 1300; one was from the mid-600s C.E. lated steppe, frequently waging war with each the leader’s rise. A preliminary climate record that Hessl other. Nor was Genghis Khan an anointed Based on the shreds of information that and Pederson constructed from the 17 trees leader. Annals say he was born to a concubine exist, some historians have explained the suggests that in the period from 1211 to his father had kidnapped. As a young boy, ’ achievements as a product of supe- 1230 C.E., when Genghis Khan was in his Genghis was banished with his mother and rior military tactics, proposing that because heyday, Mongolia enjoyed abundant rainfall: siblings to a bleak corner of the steppe, where Genghis Khan’s troops grew up hunting and apparently more than in any other 20-year he spent much of his childhood scavenging riding horses they had an advantage over sed-

stretch over the past 900 years. The steppe’s for food. Yet within a few decades, the outcast entary armies. Or, in exploring why the Mon- on October 9, 2012 grasses and other vegetation would have would unite Mongolia’s tribes, and then van- gols managed to rule over a large empire, flourished, allowing the Mongols to raise quish peoples across Asia and Europe. scholars have pointed to an innovative leader- more livestock and giving them what Hessl The Mongol conquest “has always been ship style. Genghis Khan favored meritocracy calls “more horsepower” for conquests. a bit of a mystery,” says Nicola Di Cosmo, a over cronyism and embraced conquered sub- The tree-ring scientists hope to round out historian at the Institute for Advanced Study jects who pledged loyalty. the climate record by uncovering more wood in Princeton, New Jersey. Chinese troops In 1934, British historian Arnold J. Toyn- from the time of Genghis Khan and before. during the Ming Dynasty sacked Karako- bee may have been the fi rst to point to a possi- They are helped by the fact that many Mon- rum, the Mongol capital, in 1388, and today ble climate connection, but he apparently got golians venerate the land—some worship only a few artifacts remain. A stone turtle it wrong. He proposed that a “push exerted www.sciencemag.org trees—and have pointed the scientists to sites that once marked the city’s boundary is now a by the climate of the steppes”—implicitly an strewn with old wood. “Mongolians have really good knowledge about the environment because of our lifestyle of moving around,” says project collaborator Baatarbileg Nachin, a dendrochronologist at the National Univer-

sity of Mongolia in Ulaanbaatar. And through Downloaded from sediment analysis and energy modeling, they will attempt to confi rm that livestock popula- tions grew as rainfall increased. For Hessl, an easygoing California native, the project offers a tantalizing opportunity. Research into the relationship between cli- mate and the decline or collapse of civili- zations is well-established—from Angkor in Cambodia (Science, 20 February 2009, p. 999) to the Maya in Mesoamerica and South America (Science, 14 March 2003, p. 1731). But much of that research has focused on water. “When we look back on ancient societies and say lack of water caused an empire to collapse, it’s pretty easy to think that won’t happen to us. We have reservoirs and other ways of managing water,” Hessl says. In Mongolia, her team will look at energy use and availability over the entire arc Tree-ring detectives. Neil Pederson (left) and Amy Hessl are using long-dead Siberian pine and larch (lower

CREDITS (CLOCKWISE FROM LEFT): KEVIN KRAJICK/EARTH INSTITUTE, COLUMBIA UNIVERSITY; MARA HVISTENDAHL (2) INSTITUTE, COLUMBIA UNIVERSITY; CREDITS(CLOCKWISELEFT):KEVIN KRAJICK/EARTH FROM of an empire, by examining how the Mongols right) to reconstruct 13th century climate records.

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Where Asia’s Monsoons Go to Die and long-lived trees. The annual growth of Siberian pine and larch is lim- ited by the availability of moisture; thicker tree rings are the product of KHORGO, MONGOLIA—He has scaled a Bhutanese mountaintop and ven- wetter years. tured into Vietnamese highlands, a Costa Rican cloud forest, and the Alas- Anchukaitis’s adventure here is part of an ambitious effort to gather tree- kan tundra in search of ancient tree-ring samples. Few paleoclimatologists ring samples from across Asia as proxies for past climate conditions. Records are worldlier than Kevin Anchukaitis of the Woods Hole Oceanographic Insti- from this site and more than a dozen other Asian countries—including Bhu- tution in Massachusetts, yet his fi rst foray into a Mongolian lava fi eld left tan, Cambodia, China, India, Japan, and Vietnam—populate a database him awestruck. “This site is pretty exceptional,” he says, bounding over an known as the Monsoon Asia Drought Atlas, developed at Columbia Univer- “ocean of rocks” toward one of the few standing trees on the arid and desolate sity’s Lamont-Doherty Earth Observatory in Palisades, New York. Rolled out landscape. After rapping his knuckles on a gnarled Siberian larch—by the 2 years ago (Science, 23 April 2010, p. 486), the database is “a living atlas,” sound, he could tell that it wasn’t rotten and hollow—Anchukaitis unsheathes Anchukaitis says. a T-shaped tree corer, screws it into the trunk, and extracts a cylindrical sample about the length and diam- eter of a pencil. “Sites in Southeast Asia tend to be wetter and not as moisture- stressed,” he explains. “It’s hard to fi nd any sites like this.” As he eyes the sample’s undulations of dark and light hues, he estimates the tree is about 500 years old. The lava Rain man. Kevin Anchukaitis and colleagues are using fi eld’s harsh conditions make it a tree-ring cores (above) to build an atlas that will help

prime location to fi nd slow-growing scientists model shifts in the Asian monsoon. on October 9, 2012

inhospitable climate—may have propelled ographic Institution in Massachusetts who is change that was sustained for a few decades, nomadic armies like the Mongols to venture assisting on the project. and then that was it,” Hessl says. Around out for resources. “That was not a picture sup- The tree-ring data collected by Hessl and 1258, after an unidentifi ed volcano unleashed ported by any data,” Di Cosmo cautions. But Pederson show that in the late 12th century, a massive eruption that spewed sulfur and the idea persisted because of historical records around the time Mongolia was wracked by ash into the stratosphere, a cool, dry climate showing a 12th- and 13th century drought in intertribal warfare, the area did experience a returned. moved the Mongol neighboring China and Tibet, and because cold, dry period. But a few decades later, as capital to Dadu, now Beijing. Thereafter, he www.sciencemag.org history was largely written by the people the Genghis Khan began consolidating power, ruled as emperor of China, founding the Yuan Mongols conquered. Such an inference is weather conditions appear to have substan- Dynasty. But outside the region, the Mon- akin to “talking about what happens in Ari- tially improved—and to nomads who rely gols’ power had begun to wane. zona based on what happens in Mississippi on access to lakes for watering animals, that and New York,” says Kevin Anchukaitis, a would have made all the difference. In times of Depths of time paleoclimatologist at the Woods Hole Ocean- abundant rain, pastoralists thrive, Hessl says: After Burkhart sheathes his chainsaw, the

Very little human effort is needed to “create researchers hike back to camp with samples Downloaded from large amounts of meat that is mobile, that can from some 30 trees, including larch. Multi- be used for war, and that can be used to trans- ple species will allow them to “better capture port things.” Whole herds can be tended by the climate signal,” Pederson says. They drop children—leaving the men free to fi ght. their gear in their gers, or yurts. Before feast- If more rainfall boosted grassland pro- ing on lamb goulash and fried fi sh washed ductivity and overall energy output, that down with salty milk tea, they change into could help explain why the Mongols were swimsuits as dusk falls and dive into frigid able to transition from a “chieftain society, Lake Terkhiin Tsagaan to cool off. where positions are hereditary” to manag- The new samples should help sharpen ing a complex state covering a vast empire, the climate record, but another piece of the Di Cosmo says: “A centralized state requires puzzle lies at the bottom of the lake. Start- more resources.” The horses and food accu- ing next year, Avery Shinneman, a postdoc- mulated on the steppe would have enabled toral scholar in biology at the University of the Mongols to set out for China in pursuit Washington, Seattle, will paddle out into Ter- of gold and silk—and from there on to more khiin Tsagaan and other lakes to take cores of distant lands. sediment. The sediment accumulates at the For its scale and grandeur, the expanded lake bottom like a “layered history book,” Unlikely ruler. Raised in poverty, Genghis Khan Mongol Empire was remarkably short-lived. she says. Using dating techniques based on conquered more territory in 25 years than the Climactic shifts may help explain why. The the radioactive decay of the isotopes 210Pb 14 Romans did in 400. preliminary tree-ring record suggests “a rapid and C, Shinneman will look for fossil dia- CREDITS TO BOTTOM): MARA (TOP HVISTENDAHL (2); WIKIMEDIA COMMONS

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The Asian monsoon systems shape rainfall patterns over a region home to verges and rises upward—will shift, and how the role of black carbons in the roughly half the world’s population, alternating between relatively dry winters atmosphere above Asia may impact warming. Nicolas Jourdain, a modeler and relatively wet summers. But modeling how a warming climate will impact at the Climate Change Research Centre in Sydney, Australia, has analyzed the monsoons has proved challenging. “When it comes down to creating 60 climate models from leading international institutions in a paper under

actual computer models for explaining and predicting monsoon behavior, by review at Climate Dynamics. Assuming a scenario in which atmospheric CO2 and large the models don’t yet work that well,” says Lamont-Doherty dendro- continues to rise through 2100, Jourdain found that nearly all models that chronologist Edward Cook, who dreamed up the drought atlas. But modelers are accurate throughout recent history predict that monsoon rainfall will seeking to forecast future monsoon rhythms can compare their algorithms with increase in South Asia during this century, with upticks ranging from 5% to the past: “It gives climate modelers grist for the model mill,” Cook says. 20%. Yet, he says, there’s “no consensus among the models about rainfall” One driver of the Asian monsoon cycle is the difference between land in countries like Indonesia, the Philippines, and Papua New Guinea. “Some and ocean temperatures. In spring and summer, land warms more quickly models project a rise and others a dip in rains.” Varied topography, he adds, than seawater, causing the winds to reverse direction and blow inland, bring- makes it tricky “to reproduce the physics of this region.” ing rain. As the climate warms, this temperature difference will increase, India’s recent experience illustrates just how tricky modeling can be. explains Caroline Ummenhofer, a Woods Hole climate modeler who has used In recent years, monsoon rains on the subcontinent have weakened rather records from the drought atlas in her work. “In a warming climate, we would than strengthened, confusing modelers. This year, precipitation is down expect enhancement of the land-sea temperature contrast and therefore a 12% across India. One possible explanation, put forward online on 26 June strengthening of the monsoon,” she says. Another key variable, says David in Nature Climate Change, is that black carbon in the atmosphere may be M. Anderson, director of the U.S. National Oceanic and Atmospheric Admin- refl ecting solar radiation back into space and thus reducing warming. istration’s World Data Center for Paleoclimatology in Boulder, Colorado, is Mongolia is at the outer edge of the monsoon rain belt, or as Lamont- the fact that “a parcel of air can hold more water if the air is warmer.” Doherty dendrochronologist Neil Pederson puts it, “where climate systems With climate change, Anderson says, “we expect that monsoon rains will be go to die.” But in mapping the complex climate systems of Asia, past and greater; wet areas will get wetter.” future, this otherworldly lava fi eld is a vital piece of the puzzle. Two puzzling questions are how the location of the intertropical con- –CHRISTINA LARSON

vergence zone—the band of rainfall created as a ring of atmosphere con- Christina Larson is a writer in Beijing. on October 9, 2012

toms, or small algae, that can indicate high phosphorus levels in lake water, as well as spores of Sporormiella, a fungus that lives in the dung of grazing animals. The idea is to gauge the number of livestock that have frequented the lakes over time. Mongolian lakes are like “gas stations” for thirsty ani- www.sciencemag.org mals, Hessl says—and thus a good way to measure population fl uctuations that might be compared with the climate record. The researchers will use such data to draw conclusions about total energy fl ows at the time of Genghis Khan. Hanqin Tian, an

ecologist at Auburn University in Alabama, Downloaded from will apply a model he developed to simu- Ghost town. Little remains of the Mongol capital . late how climate change affects grassland productivity, water availability, and energy- Much of the data on the period comes from yet, Hessl points out, in the face of appar- use effi ciency across Asia. Already, Tian has Europe, leaving open the question of how ently severe climate conditions, the Mongol found that precipitation was the key factor Asia and other regions fared. Empire did not collapse. Instead, she says, in grassland productivity in Mongolia over Büntgen hopes the endeavor may even “they restructured” and diversifi ed energy the past 100 years. He intends to extend that help elucidate the epidemiology of the Black sources. A revamped Mongol army no lon- model back another 900 years. Death, which some scientists believe origi- ger depended on the health of the grassland. The team’s fi ndings will help fi ll a “void nated in central Asia. “The extension of the The tree-ring research is poised to reveal in the human and environmental history of Mongol Empire was probably one factor that much more about that enigmatic empire. But central Asia,” says David Stahle, a dendro- allowed the virus to spread,” he says. the team’s insights should not detract from the chronologist at the University of Arkansas, The research could also provide lessons individual pluck behind one of history’s great Fayetteville, who is not involved with the for coping with modern-day climate change, military triumphs, Pederson cautions: Ulti- project. From the 11th to the 14th centu- Hessl says. A decade ago, drought devas- mately, “Chinggis did it, and his army did it.” ries, much of the world was in the grip of the tated the steppe, prompting a mass migration Still, he adds, a favorable climate and abun- Medieval Climate Anomaly. Europe entered to Ulaanbaatar. Meanwhile, the burgeon- dant energy would have helped the ruler con- a warm spell, while episodic droughts hit ing mining industry is adding to stresses on solidate his power. That intriguing possibility the southwestern United States. Sometime the water table. Mongol history may prove could reshape our understanding of the seeds between 1250 and 1350, the global climate a stark reminder that while resources can of empire—in Mongolia and beyond.

CREDIT: MARA HVISTENDAHL CREDIT: turned cooler, ushering in the Little Ice Age. prop up an empire, they are also fi nite. And –MARA HVISTENDAHL

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