| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
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horizons Too Warm For The Maya On a hilltop within the ruined city of Chichen Itza rest the round stone walls of The Giant Conch Shell observatory, or El Caracol. Its sky windows, designed more than a thousand years ago with a sure knowledge of the heavens, still mark precisely the positions of the stars and planets wheeling high above the Yucatan. Today, El Caracol stands as a testament to the power and learning of the people who built it, the classic Maya of Central America. In 800 A.D., an estimated 15 million people lived within an empire that stretched from the white sand beaches of Mexico’s Yucatan Peninsula into what is now Belize and the lowlands of Guatemala. The Maya had flourished for more than 2,000 years when disaster struck out of the clear blue Caribbean sky. Between 800 and 910 A.D., more than 90 percent of the Maya simply disappeared. Construction on the pyramids ground to a halt, the elaborate temple complexes were abandoned, and the most advanced cities in the Western Hemisphere melted back into the jade green jungle from which they had been carved. By the time the conquistadores landed on the shores of the New World, only a few hundred thousand of the Maya’s descendants remained. The sudden collapse of the Maya has long posed a tantalizing mystery to students of archeology. Prevailing theory ascribes the decline of major civilizations to human errors, which domino into the crash of an empire. Accordingly, archeologists have blamed the decline of the Maya on the people themselves, targeting everything from their slash-and-burn agricultural practices to their religious customs, their wars and administrative practices. What little they know is that the far-flung Maya empire probably succumbed sequentially: the last dates carved into the limestone palaces correspond to 810 in the west, 860 in the southeast, and 900-910 in the north. Now, a marriage between archeology and climatology are adding chapters to the saga of the Maya. In December, the fruitful results of this union were presented at the American Geophysical Union meeting in San Francisco. Independent archeologist Richardson Gill says drought alone could have been sufficient to topple the Maya. “Being a Texan, I’m very aware of drought. It’s something we deal with on an annual basis; we never know from one year or the next if we’ll have enough rain.” When Gill first presented his theory, there were few ways to confirm it. Archeologists initially sniffed at the idea that a single natural cause could be responsible for the fall of a civilization as advanced as the Maya. So Gill spent ten years amassing data to make his case. He learned that most Mayan cities relied on surface reservoirs that had to be replenished each year; the cisterns of Tikal, one of the largest, accommodated an 18-month water supply. And the few Mayan cities that did survive-a paltry five percent-were those with access to long-term water supplies such as rivers and lakes. What Gill needed was a meteorological time machine, a way to reconstruct the climate of the Yucatan 1,200 years ago. The Maya weren’t in the habit of recording exact rainfall and temperature, so Gill turned to clues written by one of the few reliable observers of the age: trees. Warm, wet weather allows trees to add on extra bulk, while colder, more arid weather can virtually arrest trunk growth to fractions of a millimeter. By measuring the width of each ring in a tree trunk, a dendrochronologist can assemble a relative gauge of precipitation and climate during the life of the tree. Climate record searches eventually paid off. Central American tree records don’t go back that far, because dead trees soon decay in the tropical conditions, but a tree ring record from arctic Sweden showed growth patterns that seemed to match the historical record of disaster in Mayan lands. “I almost fell out of my chair,” Gill said. “Every time there was severe cold in this record, there was demographic disaster in the Maya lowlands.” A weather system called the North Atlantic High, Gill says, might be the meteorological connection between snowy Scandinavia and tropical Yucatan. This area of high pressure tends to travel diagonally across the Atlantic, from the Caribbean to northern Europe. While in the northeast, it brings balmy temperatures to Europe and ample rainfall to the Yucatan, but during its stays in the northwest, it pours bitter cold over Europe and leaves the Yucatan thirsty. Weather records from 1902 support Gill’s meteorological model. The North Atlantic High lingered in the northwest corner of its channel that year, bringing both the coldest year of the century to the Arctic and devastating drought to the Yucatan from 1902-1904. “Just because (the phenomenon) happened in 1902 does not mean it happened in the 9th century. But it demolishes the idea that it could never happen,” Gill says. Climatologists have since corroborated the pattern with computer models and are trying to find the force controlling the High’s swings. Recent research on lakes within Maya territory lend further support to the drought hypothesis. Geologists Mark Brenner and David Hodell of the University of Florida studied cores taken from lakes in the Maya lowlands to learn about ancient climate conditions. Water entering the lake typically contains equal amounts of two isotopes of oxygen. Isotopes are atoms which have the same number of protons but different numbers of neutrons. In dry years, more of the lighter form, O16, evaporates from the lake than the O18. In those years, animals incorporate perceptibly more O18 into their shells. The scientists found that the shells of microscopic lake crustaceans called ostracods roughly dating back to the same dates of the three periods of the Maya collapse contained unusually high amounts of O18. Mineral records in the core samples echoed these results perfectly. The lakes’ waters are always saturated with a soft mineral called gypsum. When water levels drop during drought, more gypsum precipitates to the bottom and comprises a proportionately larger percentage of the sediment deposits. Brenner and Hodell say the Maya of the late classic period probably faced a dry spell the likes of which their culture had never encountered. In the previous 2,500 years, the Yucatan had never suffered a century of drought. In the face of such long-term devastation, few civilizations could survive unscathed. “The Maya may have organized around two to three year droughts, but there was really nothing they could do about this one. There were no societal, organizational, or cultural practices that would have changed the outcome,” Gill says. Gill believes the driving force behind these Caribbean droughts may have been volcanic eruptions. He found that eleven of the thirteen drought-related famines recorded in the Yucatan between 1440 and 1840 followed the eruptions of large tropical volcanoes within two years. The massive quantities of sulfur particles belched into the atmosphere by volcanoes can often deflect enough sunlight to cast a sooty pall over the atmosphere and disrupt global weather patterns. Hodell disagrees. In a paper published this spring in the journal Science. Hodell argues that the wax and wane of the sun’s life-giving rays is the dynamo behind drought in the Yucatan. Hodell and Brenner’s lake core research shows that over the past 9,000 years, drought came to the Yucatan in roughly 208- year cycles, including smaller 50-year cycles that seem to correspond to the Maya collapse. Other researchers have noted 208-year fluctuations in the amounts of certain isotopes formed when cosmic rays hit atoms in the upper atmosphere. When the sun’s furnaces burn stronger, the solar wind emanating from its surface blasts more cosmic radiation from the earth than usual, drastically reducing the amount of such isotopes, known as cosmogenic nuclides, that form and fall to Earth. Hodell says the coincidence between the drought and sun cycles is just too perfect to ignore, but neither he nor other climatologists know if or how these effects translate into drought over the Caribbean. The thick green tangle of vegetation that covers the Yucatan actually cloaks a land with poor soils only marginally suited to agriculture. The land’s deficiencies forced the Maya to practice slash and burn agriculture to grow their maize and beans-a risky practice in a region that often receives no rainfall for five months on end. “To be able to know when to cut and do your burn, you must know when it will start raining, to plant seed before it rains,” Brenner says. Dependence on rainfall, he adds, is one of the reasons the Maya became such skilled astronomers and developed a calendar. How ironic that a culture so obsessed with astronomical cycles may have met its demise from a prime source of their fascination: the sun. Kathleen M. Wong is Senior Editor of California Wild. |
Spring 2001
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