Why the Maya Fell: Climate Change, Conflict

But first came the boom years, roughly A.D. 300 to 660. At the beginning of the so-called Classic Maya period, some 60 Maya cities—each home to between 60,000 and 70,000 people—sprang up across much of modern-day Guatemala,Belize, and Mexico's Yucatán Peninsula. (Explore an interactive map of key Maya sites.)

Surrounded by pyramids, plazas, ball courts, and government buildings, the urban Maya discussed philosophy, developed an accurate solar-year calendar, and relished a thick, bitter beverage made from cacao beans: the world's first hot chocolate.

Farmers, too, were riding high, turning hillsides into terraced fields to feed the burgeoning population.

Then came the bust, a decline that lasted at least two centuries. By 1100 the residents of once thriving Maya cities seem to have just up and left. But where did they flee to, and why?

In the 19th century, when explorers began discovering the overgrown ruins of "lost cities," theorists imagined an immense volcanic eruption or earthquake or superstorm—or maybe an empire-wide pandemic. (Related: "Maya Mystery Solved by 'Important' Volcanic Discovery?")

But today scientists generally agree that the Maya collapse has many roots, all intertwined—overpopulation, warfare, famine, drought. At the moment, the hottest field of inquiry centers on climate change, perhaps of the Maya's own doing.

(Also see "Climate Change May Have Killed Off Maya Civilization, Study Says.")

The latest Maya climate-change study, published Friday in the journal Science,analyzes a Belizean cavern's stalagmites—those lumpy, rocky spires on cave floors—to link climate swings to both the rise and fall of the empire.

Formed by water and minerals dripping from above, stalagmites grow quicker in rainier years, giving scientists a reliable record of historical precipitation trends. One sample used in the new study, for example, documents fluctuations as far back as 2,000 years ago.

Among the trends revealed by the Belizean stalagmites: "The early Classic Maya period was unusually wet, wetter than the previous thousand years," according to study leader Douglas Kennett, an environmental anthropologist at Pennsylvania State University. "During this time, the population proliferated," aided by a surge in agriculture.

During the wettest decades, from 440 to 660, cities sprouted. All the hallmarks of Maya civilization—sophisticated political systems, monumental architecture, complex religion—came into full flower during this era.

(Read about the rise and fall of the Maya in National Geographic magazine.)

But the 200-year-long wet spell turned out to be an anomaly. When the climate pendulum swung back, hard times followed.

"Mayan systems were founded on those [high] rainfall patterns," Kennett said. "They could not support themselves when patterns changed."

The following centuries, from about 660 to 1000, were characterized by repeated and, at times extreme, drought. Agriculture declined and—not coincidentally—social conflict rose, Kennet says.

The Maya religious and political system was based on the belief that rulers were in direct communication with the gods. When these divine connections failed to produce rainfall and good harvests, tensions likely developed.

Within the scant 25 years between 750 and 775, for example, 39 embattled rulers commissioned the same number of stone monuments—evidence of "rivalry, war, and strategic alliances," according to Kennett's study.

But times would get even harder.

The stalagmite record suggests that between 1020 and 1100 the region suffered its longest dry spell of the last 2,000 years. With it, the study suggests, came Maya crop failure, famine, mass migration, and death.

By the time Spanish conquistadors arrived in the 16th century, inland Maya populations had decreased by 90 percent, and urban centers had been largely abandoned. Farms had become overgrown and cities reclaimed by forest.

(Take a Maya quiz.)

The collapse, though, wasn't exactly all natural. To some extent, the Maya may have designed their own decline.

"There were tens of millions of people in the area, and they were building cities and farms at the expense of the forest," climate scientist Benjamin I. Cook said.

Widespread deforestation reduced the flow of moisture from the ground to the atmosphere, interrupting the natural rain cycle and in turn reducing precipitation, says Cook, of NASA's Goddard Institute for Space Studies and the Lamont-Doherty Earth Observatory.

According to computer simulations Cook ran for a study published in Geophysical Research Letters this past August, the localized drying decreased atmospheric moisture by 5 to 15 percent annually. Even a 10 percent decrease is considered an environmental catastrophe, he says.

Add this to the broader drying trend and the situation becomes dire—a cautionary tale for modern society, according to Cook. Today, as more and more forestland is turned into farms and cities, and as global temperatures continue to rise, we may risk the same fate that befell the Maya, he says.

But, according to Arizona State University professor of environment and societyB.L. Turner, "that's the kind of oversimplification we're trying to get away from. The Mayan situation is not applicable today—our society is just so radically different now."

In a study published in August by the journal Proceedings of the National Academy of Sciences, Turner attempts to correct some common misconceptions, beginning with the idea that Maya civilization vanished after the conquistadores arrived.

"It didn't cease to exist; there are still today Mayan people in the area. The culture, the traditions have been maintained," he said. But the cities, historically, have not—and that's odd.

Throughout global history, he said, "rarely can you find a large sustained population that just left and never came back," Turner said. The closest analogue he can think of is the sudden, and final, abandonment of Cambodia's Angkor Wat complex in the 15th century.

Turner's study concludes that the natural environment recovered rather quickly after the dry centuries. Why, then, didn't the Maya reclaim their glorious cities?

Turner points to the coasts. Fleeing starving, warring inland cities, many Maya made a beeline for the shore. Trade also shifted, from overland paths to coastal routes, he suggests.

With life relatively comfortable on the coast, the inland Maya cities may have simply been forgotten, Turner says. No catastrophic earthquake, no plague, no curse, but rather a gradual migration to the beach, where life was a bit mellower.

That is, until the Spanish arrived.