Tuesday, January 23, 2007

Finding Hurricane History In The Trees

Long before weather satellites scanned the oceans looking for tropical systems, forecasters at the U.S. Weather Bureau had to find storms the old fashioned way: go out and look for them.

In the 1940’s and 1950’s, Air Force and Navy pilots would conduct daily patrols of pre-determined areas of the Gulf of Mexico and the Atlantic Ocean searching for a tropical storm or hurricane. A lot of airplane fuel and coffee would be consumed on these ten to twelve hour flights that were required to keep meteorologists updated on the latest in the tropics.

Obviously, the patrols would miss a few storms, especially the systems coming off the coast of Africa or developing in areas outside of where the planes flew. How many storms actually formed during that period remains a mystery. In fact, one could argue that our knowledge of hurricane activity is truly only accurate during the past 40 years or so, the period of time when satellites photographed every square inch of ocean waters.

Understanding past hurricane cycles is vital to determine if our current period of above-average activity is a natural pattern or linked to human-induced global warming.

Now, scientists may have unlocked hurricane history in tree rings.

University of Tennessee researchers Claudia Mora and Henri Grissino-Mayer have discovered that rain from a hurricane leaves a chemical mark in the tissue of old longleaf pines, a common tree found in the Southeast.

The information may provide “a high-resolution, precisely dated biological archive that could be extended back for centuries,” the scientists wrote in the Proceedings of Natural Academy of Sciences.

Using tree samples collected in southern Georgia, the scientists first compared tree-ring evidence against meteorological records for the past 50 years. Mora said the tree rings produced hurricane “fingerprints” in 18 of the 19 years in which storms had been documented.

Taking their investigation even further back in time, Mora and Grissino-Mayer plotted tree ring data against the less than reliable historical record of the 18th and 19th centuries. The team discovered “a lot of hurricanes for which there was no historical documentation that a hurricane had ever occurred in that area,” writes Mora. The team believes the 1820s through the 1840s may have been a very active time for land-falling hurricanes in the southeastern United States.

While long-leaf pine trees can only live to be 200 years old, Mora and Grissino-Mayer have discovered long-leaf stumps left over from timber harvesting in the region also hold the “biological fingerprint.”

“We know we can push the hurricane record back to the 1400s,” Grissino-Mayer said. “With the collection of more tree ring samples, we are certain we can push the record of hurricanes back to at least the year 1000.”

Admittedly, the duo’s work would only count tropical systems that actually made landfall in the southeastern United States. Still, it would still give scientists a more accurate record of past storms in the region and, perhaps down the road, a clearer picture of our current hurricane activity.

It is truly an amazing discovery and like those pilots who flew for hours and hours across the Gulf of Mexico and Atlantic Ocean, Mora and Grissino-Mayer found the hurricanes the old fashioned way: they went out and looked for them.

Posted at 9:06 AM