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Empires in the dust
Discover, March, 1998 by Karen Wright
Instead of backing down in the face of such commentary, Weiss has continued to document his thesis. Echoing Mellaart, he points out that 2200 B.C. saw the nearly simultaneous collapses of half a dozen other city-based civilizations--in Egypt, in Palestine, on Crete and the Greek mainland, and in the Indus Valley. The collapses were caused by the same drought, says Weiss, for the same reasons. But because historians and archeologists look for internal rather than external forces to explain civilizations in crisis, they don't communicate among themselves, he says, and many aren't even aware of what's going on next door, as it were.
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"Very few people understand that there was a synchronous collapse and probably drought conditions in both Egypt and Mesopotamia," let alone the rest of the Old World, says Weiss.
It didn't help Weiss's extravagant claims for third-millennium cataclysm that his alleged drought didn't appear in the GISP2 oxygen-isotope record. The graph in deMenocal's office, for example, has no spikes or dips or swerves at 2200 B.C. just a nice flat plateau. That graph was drawn from an interpretation of the ice-core data. But according to Paul Mayewski of the University of New Hampshire in Durham, who is chief scientist of GISP2, there are plenty of reasons a drought in western Asia might not make it into the oxygenisotope record in the Greenland glacier. Greenland might be too far away to "feel" the regional event, or the drought may have left a different kind of chemical signature. Only a climatologist like Mayewski could explain these reasons, however. And no one asked him to.
"As a consequence, a lot of people called Harvey Weiss and said, `Well, the GISP2 record is the most highly resolved record of Holocene climate in the world. And if it's not in there, you're wrong, Harvey,"' says Mayewski. "I didn't realize that poor Harvey was being abused for not existing in our record."
Fortunately Mayewski, like deMenocal, is a curious sort with interests a bit broader than his own specialty. When he happened upon Weiss's 1993 paper, he'd already lent a hand on a few archeological projects, including one on the disappearance of Norse colonies from Greenland in the mid-1300s. But he figured other scientists had already looked for the Mesopotamian drought in the climate record. When he finally met Weiss in 1996, he learned otherwise. Mayewski began reanalyzing his core data with Weiss's theory in mind, and he uncovered a whole new Holocene.
"We can definitely show from our records that the 2200 B.C. event is unique," says Mayewski. "And what's much more exciting than that, we can show that most of the major turning points in civilization in western Asia also correlate with what we would say would be dry events. We think that we have found a proxy for aridity in western Asia."
Earlier interpretations of the GISP2 data had measured a variety of ions in ice cores that would reveal general information about climate variability. To look for the 2200 B.C. drought in particular, Mayewski used tests based on 2.5-year intervals in the climate record instead of 50- to 100-year intervals. He also collected a broader set of data that allowed him to reconstruct specific patterns of atmospheric circulation--not only over land but over land and oceans. When Mayewski focused on the movement of air masses over oceans, he found that air transport from south to north in the Atlantic--so-called meridional circulation--hit a significant winter low some 4,200 years ago. Mayewski and deMenocal are studying how this event relates to drought in western Asia.
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