February 18, 2002

ureka! When the solution to a problem that has proved baffling for years suddenly becomes clear, it's a moment to savor. For

Harvey Weiss, a professor of Near Eastern archeology at Yale University, one such moment came in 1999.

Six years earlier, Weiss had been excavating the lower town area of Tell Leilan, the site of a major center of Mesopotamian civilization in present-day Syria. The remains he had unearthed were impressive. And yet they didn't make sense. Why would anyone build a massive stone foundation to support walls that were only one or two bricks high?

Years later, on a return visit to the site, he suddenly recognized the solution: the structure was never finished because the site was abandoned.

Changing Climate
During a lecture on "Abrupt Climate Change: The Rise and Fall - and Rise - of Mesopotamian Civilization" to a largely student audience at the Konover Auditorium Wednesday, Weiss described how his research has pieced together evidence from around the world. The lecture was the second in a new series on art and archeology, funded by a gift from philanthropists Raymond and Beverly Sackler.

An expert in climate change, Weiss explained that Mesopotamian civilization, one of the earliest in the world, was based on agriculture made possible by the local climate.

The region between the Tigris and Euphrates rivers, he said, is an area with one of the simplest climate patterns in the world. Westerly winds sweep eastward across the Mediterranean, picking up moisture along the way. When they hit the coastline of what are now known as Turkey, Syria, Israel and Lebanon, they are forced up, cool, and release all the precipitation that reaches western Asia.

In the northern part of the area, this pattern enabled agriculture fed by rainfall; in the southern part, agriculture was sustained by irrigation - man-made canals that redirected water to the land from the Tigris and Euphrates.

Scholars interested in climate history until recently had limited evidence to rely on, Weiss said. Recorded measurements of temperature and rainfall go back only a few centuries; the written historical record is patchy and also limited in reach.

During the past two decades, however, scientists have retrieved ice cores from two miles below the surface of Greenland, providing an annual record of the climate dating back 100,000 years. Even though there were no people living there and the site is thousands of miles away from the Near Eastern archeological site of Tell Leilan, "the Greenland record is taken as diagnostic for earth's history," Weiss said.

Analysis of one ice core has yielded the climatic equivalent of an electrocardiogram, he said, demonstrating a lengthy period marked by intense warming and cooling - the Pleistocene period - followed by a shift to warmer and more stable weather at the start of the Holocene period.

"Like an electrocardiogram, the Holocene is essentially stale and static, the long buzz after the machine stopped recording the pulsing heartbeat of the patient Earth," said Weiss.

But the record also shows that, soon after the warming began, there was a blip: an abrupt and very intense cooling that occurred within a decade and preceded subsequent warming.

Dryness and Dust
The picture obtained from Greenland is consistent with climate evidence closer to Tell Leilan, Weiss said. Records of water levels in the Dead Sea show that about 4,000 years ago, the level dropped 100 meters in a century; and lake sediments in Lake Van, the headwater of the Tigris and the Euphrates, reveal that around the same time, the amount of dust in the atmosphere doubled.

This suggests "a sudden onset of dry conditions," said Weiss, "which allowed the wind to rip off surface particles of earth and create dust."

A section in a test trench in the lower area of Tell Leilan tell the same story from a human standpoint. Successive layers of earth with man-made remains indicate 100 years of settlement from 2300 BC, when Mesopotamians moved northward from the region of irrigation agriculture to the region of rain-fed agriculture and established the first empire. Above is a meter of debris that accumulated at the site during a 300-year hiatus, prior to reoccupation around 1800 BC.

Both climatic and archeological evidence point to the abandonment of the entire region because of desertification, said Weiss. A 30 percent reduction in precipitation left the once rain-fed agricultural region a desert, and the population was forced southward into the region where irrigation agriculture remained possible because of the volume of water in the Tigris and Euphrates. The massive structure that was to have rested on the stone foundation constructed in Tell Leilan was left unfinished.

Future Prospects
"Although the geoscience and archeological communities don't understand why these incidents of climate change occurred, it is clear the impact of past climate changes was sensational," said Weiss. The abrupt change of 2200 BC was " surely a natural climate alteration and its effects were enormous," he said.

Current climate change, on the other hand, is being caused by humans, he said.

"We don't understand where this trajectory - global warming - is taking us at the moment," he noted. "Probably the moral for the future is we're playing with an instrument whose mechanism we do not understand."