Five Thousand Years of Cyclones

SAN FRANCISCO--Researchers digging into coastal mud along the Gulf of Mexico have compiled the first long-term hurricane record. This diary of destruction shows that far fewer intense cyclones have pummeled the Gulf coast in the last millennium than in previous times, according to a report presented here yesterday at a meeting of the American Geophysical Union. The opposite seems true on the Atlantic seaboard, hinting at a curious climate switch that steers the most powerful storms.

Hurricanes wreak havoc when they strike the shore, especially those with winds exceeding 200 kilometers per hour. These monster storms drive lethal surges of seawater far inland, smashing buildings and depositing thick blankets of sand and debris. About 8 years ago, geographer Kam-biu Liu of Louisiana State University in Baton Rouge realized that finding such layers of storm-driven sand in old sediments offered a way to track hurricanes that came ashore thousands of years ago.

Liu's latest efforts in "paleotempestology" focused on several lakes and marshes in coastal Louisiana, Alabama, and the Florida panhandle. His team drilled sediment cores up to 10 meters deep. When the scientists found the telltale bands of sand, they measured the ages of organic matter in the layers with carbon-14 dating. Most sites had been hit by a powerful hurricane every 200 to 600 years, on average. However, a startling pattern emerged: While about 10 giant storms ravaged each site from 3500 to 1000 years ago, no site had more than one such impact in the last millennium.

"This is the first set of numbers to show what the landfall probabilities are for catastrophic hurricanes," Liu says. "Before now, we only had 120 years of historic records, not nearly enough to see these patterns." He believes that a shift in the "Bermuda High," a zone of strong atmospheric pressure in the mid-Atlantic Ocean, may propel the great majority of intense storms toward either the Gulf coast or the Atlantic coast, but not both, for centuries at a time.

Preliminary research in South Carolina supports that notion, says coastal geologist David Scott of Dalhousie University in Halifax, Nova Scotia. Ribbons of sand and saltwater microorganisms in the sediments of an inlet point to six strong hurricanes in the last 1800 years, but none for 3000 years further back. "The big storms come in clumps," Scott says. "Clearly, the probability is higher on the Atlantic coast right now."