Clues from hurricane 'fingerprints'
Scientists decode hurricane 'records' left in trees and rocks to try to predict the strength of future storms.
from the August 16, 2007 edition
Page 2 of 3
'Footprints' in the sand
As if to underscore the rising interest in what paleotempestology has to offer, the Brazil-based Inter-American Institute for Global Change Research (IAI) last year funded a six-year project to apply these proxy approaches to the entire Caribbean basin. The effort involves a dozen scientists from four countries, including the US.
"We have a fairly limited set of records at any one location, especially for the strongest storms," explains James Elsner, a professor at Florida State University specializing in hurricane climatology, distribution, and risk. The biggest concern revolves around recurrence rates for the most powerful tropical cyclones. The first step involves identifying these in the prehistoric record; the next step is to expand the network of observing points. "Once you have those, you can start to see what kind of patterns there were in these prehistoric events," he says.
To gather that information, scientists first turned to sediment cores taken from marshes, lagoons, and lakes behind barrier beaches. This "is the most useful and proven technique to date," says Kam-biu Liu, a Louisiana State University paleoclimatologist who first applied the approach to the hurricane problem in 1989. As a tropical cyclone makes landfall, the storm surge washes beach sand into these bodies of water. The sand settles to the bottom, then gets covered with organic material that forms most of the muck at the bottom. The sand layers show up in the cores. Scientists can find out when the storm or storms struck by using radiocarbon dating techniques on the organic material above and below the sandy layer. Dr. Liu and Jeffrey Donnelly of the Woods Hole (Mass.) Oceanographic Institution have pioneered this approach along the Gulf Coast and in the Northeast and Puerto Rico.
Scientists have been able to trace storm histories back at least 5,000 years. After looking at data from four sites along the Gulf Coast, for example, Liu notes that hurricane activity in the region was fairly low during the first 1,200 years in the samples and the past 1,000 years or so. But for 2,800 years in between, activity was relatively high; a major hurricane struck each site as often as once every 200 years.
Researchers are now trying to figure out the atmospheric and oceanic drivers for such long-term swings.
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