Throughout human history, civilizations have risen and collapsed as though their fates were written in the wind.
Now it turns out that maybe it wasn't the wind, but rain.
More than 11 centuries ago, the Mayan people of Central America built great cities with magnificent stone temples, developed hieroglyphic writing, and created beautiful works of art. And then their culture disintegrated.
To the north, the Anasazi Indians built multi-storied housing complexes and carefully structured communities in what is now the American Southwest before their civilization folded more than 700 years ago.
From the Middle East to Asia, other cultures rose and fell, and experts who have wondered why are finding a common thread in many of those amazing stories. It appears that drought has repeatedly played a major role in the collapse of societies that had, for a while at least, been at the apex of human civilization.
It's simplistic to say that rain — or the lack of it — has caused the demise of any complex society, because many forces were undoubtedly at work, but a growing body of scientific evidence now suggests that changes in the weather played a bigger role than had been thought.
Doesn’t Take Much
The findings are particularly significant during a time of great concern over evidence that the Earth's climate is changing once again, most likely because of human activities. Even subtle changes on a global scale — say a slight rise in temperature — are likely to cause dramatic changes in isolated areas, according to experts who have looked closely at the data.
For example, it may have taken nothing more than an almost immeasurable increase in solar radiation to bring down a sophisticated society, according to geologists at the University of Florida who have spent years studying the collapse of the Maya.
The researchers, led by geology professor David Hodell, found evidence several years ago that the Maya were hit by a prolonged and miserable drought about 750 A.D., just as their culture began to hit the skids. The researchers based that conclusion on core samples taken from the bottom of Lake Chichancanab on the north central Yucatan Peninsula in Mexico.
Those samples, which record weather conditions in layers of sediment laid down year after year, indicate the drought lasted for 150 years, according to the scientists, who reported their initial findings in 1995.
More recently, the scientists returned to Chichancanab and collected additional cores to document the amount of gypsum deposited annually on the lake floor over a 2,600-year period.
A reduction in rainfall causes more evaporation in the lake, leaving more gypsum on the bottom. Surprisingly, the scientists found that the region was hit by a drought every 208 years.
That pattern is strikingly similar to a "bicentennial oscillation" in solar activity that occurs about every 206 years. The sun was slightly warmer at the time of the droughts, the scientists report in the May 18 issue of the journal Science.
Mean Leaders Don’t Help Either
That sounds really tidy. A warmer sun caused drought in the Yucatan, wiping out the Mayan dynasty. The problem is the sun wasn't much warmer, and solar energy reaching the Earth probably rose by only about one-tenth of one percent — not enough according to most climate models to cause much of a drought.
So some mysterious mechanism that we don't yet understand "amplified" the effect in the Yucatan, the scientists suggest, wreaking havoc on local weather patterns.
Whatever the cause, the droughts appear to have been real, based on the core sediments from the bottom of the lake. But weather was not the only thing ripping at the Mayan culture.
Experts have spent decades deciphering the writings of the Maya, and the story that emerges is not entirely flattering. They tell of local rulers who waged war on other cities; of torture and mutilation and human sacrifice.
Like the Maya, the Anasazi built great edifices with as many as 1,000 rooms, which they abandoned around 1300 following a severe drought that lasted from 1276 to 1299. Those carefully engineered structures and other elaborate monuments were built on the backs of laborers, some of whom must have been unhappy with the process.
So it's hard to say how much of a role the drought had. Like the Mayans, many Anasazi probably found their culture increasingly difficult to bear and just drifted off. But the drought, no doubt, made everybody's life harder. By the time the Spaniards conquered Central America in the 16th century, the relics of the Mayan civilization had been buried beneath centuries of jungle growth.
Most likely, drought contributed to the demise of the Mayan culture by wiping out agricultural resources that had become vital to an urban population that some experts believe numbered 2 million souls. Whether it was the primary cause of the collapse, or just one of many factors, will be debated for decades by experts. But there is a lesson in it for the modern world.
Brace for Fast Change
If the global climate is changing, and many experts believe it is, then we could be in for some real surprises ahead. Donald Kennedy, former president of Stanford University and now editor of the journal Science, has warned that we are making a big mistake if we expect a gradual, steady warming trend because of changing global climatic conditions, like the greenhouse effect that many experts believe is resulting from the burning of fossil fuels. Many changes will be abrupt, Kennedy argues.
"We may be unwise to rest our economic projections on the assumption that the probable climate future is a slow, steady ramp of increasing average global temperature," he told a symposium last year at the State University of New York.
He cited El Niño as one example of just how vulnerable we are to changing climatic conditions. An El Niño is caused by warming of the waters in the southwest Pacific Ocean, yet in recent years El Niños have caused flooding in North America and so much loss in rainfall in Africa that corn yields in Zimbabwe dropped by 10 percent.
"I think our society ought to entertain, now, the prospect that a sudden shift in the climate regime is a highly possible consequence of the warming trend we are experiencing now," Kennedy told the New York symposium.
In other words, hold on to your rain bonnets. We may be in for quite a ride.
Lee Dye’s column appears weekly on ABCNEWS.com. A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.