Studies: A Little Warming Can Destroy Societies

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.

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