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Over 200 Years of Hurricane Data Recorded in Trees, Study Says

John Roach
for National Geographic News
September 18, 2006
 
A chemical signature of just about every hurricane to roar across southern Georgia during the past 220 years is preserved in the region's longleaf pine trees, according to a new study.

Further detective work should extend the record for the southeastern United States back another 400 years, says Claudia Mora, study co-author and a geochemist at the University of Tennessee in Knoxville.

Such an extension would be welcome to researchers trying to understand how hurricane activity varies over long periods of time—and could help settle the debate about whether human-driven global warming is affecting hurricanes.

"There are a lot of questions right now about what the root causes of changes in hurricane frequency and intensity are," Mora said.

(Related news: "Global Warming Link to Hurricane Intensity Questioned" [July 2006].)

The current instrumental record is reliable for only about 60 years, which is too short to determine, for example, whether the observed increase in hurricane activity since the mid-1990s is part of a natural cycle or due to human activity.

The new technique, which is based on analysis of the oxygen isotope content in tree rings, "will contribute to our understanding of what controls [hurricane] frequency," Mora said.

She and colleagues report their findings today in the early online edition of the journal Proceedings of the National Academy of Sciences.

But the technique can't tell much about hurricane intensity, only frequency, Mora says.

Hurricanes are dynamic, moving systems, so the oxygen isotope signature in the trees, which are stationary, would be an unreliable gauge.

Hurricane Rain

The oxygen isotope content of rain that falls during a hurricane is different than in rain that falls during a normal thunderstorm, Mora says.

"Hurricanes are very organized weather systems that are very large, very high in the atmosphere, and long lived," she said.

Water vapor is cycled through hurricanes with greater efficiency than in a typical thunderstorm.

This allows some of the heaviest oxygen atoms, an isotope called oxygen-18, to fall out of the system early in the storm's cycle.

Hurricanes usually make landfall later during this cycle (interactive feature: how hurricanes form), so the rain that falls over land contains fewer heavy oxygen atoms than the rain from normal storms.

Conifers, such as longleaf pines, have shallow roots and rely mainly on rainwater in the surface soil layers to grow.

During and after a hurricane, the trees suck up the molecularly distinct rainwater, which they use to grow new cellulose, a vital building block in all plant tissues.

The depleted oxygen-18 signal is thus incorporated into the trees' rings.

"The signal is distinct enough to pick up in the cellulose itself, and that's what we're measuring," Mora said.

Christopher Landsea is a science and operations officer at the National Oceanic and Atmospheric Administration's National Hurricane Center in Miami, Florida.

He said "the whole concept" that oxygen isotopes in tree rings can be used to record individual hurricanes "is just fascinating."

"It does have potential for telling us more about long-term changes in hurricane activity," he added.

Proof of Concept

To test their storm-record theory, Mora and colleagues created a tree-ring record of hurricanes from a research site in Valdosta, Georgia.

Over the past 60 years scientists have used weather satellites and other high-tech tools to compile a complete and accurate record of hurricane activity.

The team first compared their data against this known information on local hurricanes over the past half-century. Only one "false storm" showed up in the tree rings.

"The proxy does appear to work very well," Mora said.

The best available record of hurricanes that struck further back in the past comes from old newspapers, ships' logs, and other historical documents.

(Related news: "Magnetic Field Weakening in Stages, Old Ships' Logs Suggest" [May 2006].)

Such sources have long been decried by scientists for carrying less useful data, given a lack of sophisticated instruments and a higher chance for human error.

The researchers were therefore unsurprised to find that their tree-ring record shows more hurricanes striking the region than reported in historical documents.

However, the tree rings failed to detect several known hurricanes in the late 1800s that coincided with a period of drought in the study area, Mora and colleagues note.

Droughts also create an isotope signal that appears to cancel out the hurricane signal, Mora says.

NOAA's Landsea said that "despite these shortcomings, it's still amazing to see how strong a signal there is preserved [in tree rings]."

The team plans to study tree rings of ancient stumps and logs preserved in swamps to further extend the southeastern U.S.'s hurricane record.

Such a record should allow researchers to compare hurricane trends before and after humans began pumping greenhouse gases into the atmosphere.

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