Then, almost in an instant, a society of some 15 million people imploded, leaving deserted cities, trade routes, and immense pyramids in ruins. The sudden demise is one of the greatest archeological mysteries of our time. What caused the collapse of the great Maya civilization?

The answer, say researchers, is climate change. According to a new study published in the current issue of Science, a long period of dry climate, punctuated by three intense droughts, led to the end of the Maya society. "Climate change is to blame for one of the most catastrophic collapses in human history," said Gerald Haug, a professor of geology at the University of Potsdam, Germany, and one of the study's authors.

Identifying the Culprit

But the study of that lake also found man-made effects, such as deforestation and soil erosion, and therefore didn't reflect a "pure climate signal," according to Haug. For the new study, the scientists instead analyzed sediment core from the Cariaco Basin off northern Venezuela, where the record is cleaner.

Identifying annual titanium levels, which reflect the amount of rainfall each year, the Swiss and U.S. researchers found that the pristine sediment layers in the basin formed distinct bands that correspond to dry and wet seasons. According to the scientists, there were three large droughts occurring between 810 and 910 A.D., each lasting less than a decade.

The timing of the droughts matched periodic downturns in the Maya culture, as demonstrated by abandonment of cities or diminished stone carving and building activity.

Experts say the Maya were particularly susceptible to long droughts because about 95 percent of their population centers depended solely on lakes, ponds, and rivers containing on average an 18-month supply of water for drinking and agriculture.

Reading the Sun

The Maya were skilled astronomers who constantly followed the movements of the sun and the moon. They predicted eclipses, explained the movements of planets, and devised a sophisticated calendar of the solar year.

Scientists have found that the recurrence of the drought was remarkably cyclical, occurring every 208 years. That interval is almost identical to a known cycle in which the sun is at its most intense every 206 years. Nothing suggests the Maya knew anything about the sun's change in intensity.

The drought theory is still controversial among some archeologists who believe a combination of overpopulation, an internecine struggle for control among the nobles, a weak economic base, and a political system that didn't foster power-sharing led to the Maya's collapse. One hypothesis suggests the Maya people themselves were responsible for their downfall as a result of environmental degradation, including deforestation.

Defenders of the climate change theory, however, say the droughts sparked a chain of events that led to the demise of the Maya. "Sunny days, in and of themselves, don't kill people," said Richardson B. Gill, author of The Great Maya Droughts: Water, Life, and Death. "But when people run out of food and water, they die."

Living on the Edge

While the Maya had learned to live with shorter droughts, the study indicates that a more subtle, long-term drying trend was ongoing during the collapse. The three specific droughts may have been what pushed the Mayan society over the edge.

"Not only did the Maya have to face an intense climatic catastrophe, but the duration was something that they had never experienced before," said Haug. "If they had stayed for another two years, they may have survived. But how could they know that the drought would end?"

Learning from the Past

Other societies, however, have survived past climate changes by changing their behavior in response to environmental change. About 300 years after the Mayan collapse, the Chumash people on California's Channel Islands survived severe droughts by transforming themselves from hunter-gatherers into traders.

Experts say the Maya collapse could serve as a valuable lesson today to societies in Africa and elsewhere that are vulnerable to droughts. When droughts strike, they can trigger a chain reaction beginning with crop failures, leading to malnutrition, increased disease and competition for resources, and ultimately causing warfare between nations and sociopolitical upheaval.

"We can handle climate change if we're prepared for it," said Haug. "The Maya were not prepared."

March 13, 2003

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