Mystery of the 300-Year Drought

By Christine Gorman

Before the Han dynasty, before Alexander the Great, even before Ramses, the first empire the world ever knew was built by a Mesopotamian ruler named Sargon of Akkad. He conquered and subjugated dozens of cities and villages between the Tigris and Euphrates rivers more than 4,000 years ago, forcing them to pay tribute in wheat, barley and silver. For a century the regime flourished, first under Sargon and then under his grandson until suddenly, mysteriously, it collapsed. Neither the capital city of Akkad, famed for its harbor filled with vessels from distant shores, nor the imperial records, etched in cuneiform and possibly chronicling the empire's demise, have ever been found.

Now the mystery may have been solved by researchers from the U.S. and France. In last week's Science, they put forth evidence that the empire was undone by a combination of climatic catastrophes. First a volcanic eruption blanketed the region in ash. Then a drought, which eventually lasted 300 years, crippled the farming communities on which the cities depended, forcing urban dwellers to abandon their empty granaries and silent temples. Refugees migrated to southern Mesopotamia (now Iraq), which had escaped the disaster. But the unexpected influx of people from the north so strained the region's resources that the Akkadian empire fell to neighboring hordes.

According to legend, the Mesopotamians blamed their woes on Sargon's grandson, whose hubris had supposedly angered the gods. But the American and French researchers, led by Yale archaeologist Harvey Weiss, offer a more scientific, if no less surprising, explanation. They believe the drought was part of a major shift in weather patterns that affected the climate in many different areas of the globe 4,000 years ago. From Egypt to the Aegean to India, rainfall diminished and temperatures dropped. "This is opposite to what you might expect from global warming," explains George Kukla, senior research scientist at the Lamont-Doherty Earth Observatory in New York. "And it was an entirely natural change."

By excavating the ancient Akkadian city of Shekhna in what is now Syria, Weiss and his colleagues determined that the urban center, once a thriving home for 10,000 people, was deserted for three centuries. One layer, which they dated to 2200 B.C., revealed the crumbling walls of a ghost town. It also provided some important clues about the weather. "Ancient soils bear a climatic signature," Weiss explains. "In a dry climate, you see very little earthworm activity and lots of loose silt, for example."

The pieces of the puzzle started falling into place after Marie-Agnes Courty, a geologist with the National Center for Scientific Research in Paris, painstakingly examined and sorted the soil samples from the roofs of the abandoned buildings under a binocular microscope. She identified a thin veil of volcanic ash, one quarter of an inch thick, underneath 8 to 20 inches of silt. The layers showed no evidence of having been disturbed by earthworms and also showed patterns characteristic of soil that has settled after a dust storm. It looked like a volcano had erupted, perhaps in nearby Turkey, and a long drought had followed.

Wherever the researchers looked throughout the area, they found the same story: abandoned buildings and layers of volcanic ash and debris from fierce wind storms. After 300 years, when the rains returned, so did the people. The telltale scars of scarcity eventually were buried under 15 feet of new dirt. A new empire, whose capital was Babylon, arose and fell. Today the region is flush once again with wheat fields.

Did the volcano cause the ancient drought? "That's unlikely," Weiss says. "Volcanos are not known to generate climatic changes of this duration or intensity." So, with one mystery solved, researchers find themselves trying to explain how a drought can persist for three centuries. At least one thing seems certain. The ancient Mesopotamians did not cause the heavens to dry up. That raises the ominous possibility that it could happen again. And that modern humanity, by dumping pollutants into the atmosphere, is tinkering with a climatic system more complex and random than humans have realized.