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New evidence, however, supports the tales of ancient scribes and identifies brief but brutal times of worldwide ecological catastrophe. The evidence is in tree rings, which clearly show several years of cold weather that stunted growth beginning in A.D. 536 and especially after A.D. 540-541. The rings show similar events that began in 1628 B.C. and 1159 B.C., and rare written documents of those times seem also to describe cataclysmic social collapse. What weapon does nature wield that is powerful enough to alter the course of civilizations within a few years? The most likely explanation, the best fit with the evidence, is that described by both Chinese and Europeans as dragons in the sky: Pieces of comets (or perhaps of asteroids) crashed into Earth, spewing a veil of dust that encircled the world and dimmed the sun. A much larger and rarer bolide (an exploding meteoric fireball) is assumed to have ended the reign of the dinosaurs some 65 million years ago. A smaller and more common one exploded over the Tunguska River in the Siberian wilderness 91 years ago with 2,000 times the power of the bomb that devastated Hiroshima in 1945. And just five years ago, astronomers watched the fragmented comet Shoemaker-Levy 9 plow spectacularly into Jupiter.

I believe the association between the tree-ring data and historical documents and folktales is real: Earth faced catastrophic environmental dislocation at or around 1628 B.C., 1159 B.C., and A.D. 540 (and probably in 2354 B.C. and 208 B.C., as well) because of near-miss comets, either through dust-loading of the atmosphere as Earth passed through the comet's dusty tail or through direct bombardment by cometary fragments. (They must have been near misses, because if we had been hit by a full-blown comet in the past 10,000 years or so, we wouldn't be here today.) This hypothesis is not proven, but the circumstantial evidence is overwhelming. The strongest evidence comes from tree rings and the science of dendrochronology. Tree rings record the age of a tree, with a distinct ring of growth produced each year. The width of each ring depends on growing conditions, so each year's growth in a particular area leaves a unique signature (a reflection of fat, moderate, or lean growing conditions) in the tree-ring record.

By calibrating the rings through progressively older trees from a specific region, archaeologists can build millennia-long chronologies that allow them to date ancient wooden artifacts. (See Discovering Archaeology, May/June, page 45.) The pattern of tree rings in an artifact can be matched to the regional chronology to determine the year in which the tree died. A less-well-known consequence of these chronologies is that we can now identify periods in which trees grew very little or not at all. This is indicated by clusters of extremely narrow rings, which suggest extremely cold growing seasons. A band of these narrow rings occurred after A.D. 540 and lasted about six years in parts of Europe, Asia, and North America. Similar ring patterns are found around 1159 B.C. and 1628 B.C. These dates may coincide with the collapse of Bronze Age civilizations across Eurasia. They may also be recalled in the biblical book of Exodus and contemporary records from China.

The first inkling that tree rings might record catastrophic events came in the mid-1980s from dendrochronologist Val LaMarche and volcanologist Kathy Hirschboeck. In the extremely long-lived bristlecone pines of the western United States, they noted a frost-damage ring at 1627 B.C. and suggested it might reflect the massive eruption of the Santorini volcano in the Aegean Sea. Similar frost rings followed the eruptions of Krakatoa in Indonesia (1883) and Katmai in Alaska (1912). After a major volcanic eruption, Earth is veiled by a layer of fine debris circulating in the stratosphere. This layer reflects sunlight away from Earth, causing the surface to cool. As a result of their suggestion, I searched the ring patterns derived from oak logs that had been preserved in the peat bogs of Ireland. I found that many trees exhibited the worst growth - the narrowest rings - of their lifetimes starting in 1628 B.C. Only a few other such events were seen in the rings, but two others were at 1159 B.C. and A.D. 540. Those years are close to dates for acid-rich layers (attributed to volcanic eruptions) that had been identified in ice cores taken in Greenland. We seemed to be onto something.