Scientists track hurricane activity through tree rings

Published 12:21 am Sunday, October 1, 2006

Within the annual growth rings of old longleaf pines, scientists are discovering a previously unknown record of hurricane activity in the Southeast.

A University of Tennessee-led team has found that hurricane rain can leave a chemical mark in the woody tissue of these shallow-rooted trees that can date when storms occurred.

That may provide “a high-resolution, precisely dated biological archive of climate that could be extended back for centuries, and even millennia,” the scientists write in a new study published in the Proceedings of the National Academy of Sciences.

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Understanding how often hurricanes occurred in the past could help predict how often they will occur in the future.

“This is an exciting new development,” said Chris Landsea with the National Hurricane Center. “That they can see historic hurricanes in trees is just an amazing discovery.”

A noted increase in hurricane activity in the Southeast from the Atlantic Ocean and Gulf of Mexico since the mid-1990s has scientists looking for historical trends and an explanation of whether this is a natural pattern or linked to human-induced global warming.

“The question of how global warming is impacting hurricanes is crucially important,” Landsea said. “I don’t think it is a yes or a no question. I think the real question is how much of a change is going on today and in the future.

“And it may be that studies like this can help answer some of those real important questions,” he said.

Precise meteorological records go back only about a half century. From the 1860s to the 1940s, newspaper accounts of big storms and ships’ logs are the best source for data. For the 1700s, scientists must rely on plantation accounts, and virtually nothing else before then.

So researchers look for signs of hurricane activity — or “proxies” — such as the unique absence of certain oxygen molecules in hurricane rain water. They have looked for signs in coastal cave deposits and coral, but neither is as exact as the natural growth rings of a tree.

The University of Tennessee’s Henri Grissino-Mayer, a dendrochronologist, or tree-ring expert, and Claudia Mora, a geologist, have pursued this arboreal record of hurricanes for more than two years. They were aided by doctoral student Dana Miller, lead author of the latest study, and others.

Using tree samples collected in south Georgia near Valdosta State University, where Grissino-Mayer previously taught, the team first plotted tree-ring evidence against the 50-year meteorological record.

The site is within 200 kilometers — 124 miles — of Florida’s Gulf Coast and Georgia’s Atlantic Coast, so hurricanes from both could be detected. The tree rings produced hurricane isotope “fingerprints” in 18 of the 19 years in which the storms were documented, Mora said.

The latest study, funded in part by the National Science Foundation, looked back 220 years against a far sketchier record. Again, the tree-ring evidence corresponded with available accounts.

In the earliest years, particularly the 1820s and 1840s, the researchers found evidence of “a lot of hurricanes for which there was no historical documentation that a hurricane had ever occurred in that area,” Mora said.

With their methods proven, Mora and Grissino-Mayer hope to secure funding to go back even further in time — before the polluting advent of automobiles in the 20th century and the dawn of the industrial revolution in the 19th century — and study a larger geographic area, from the Carolinas to Texas.

Longleaf pines can live to be 200 years old. Grissino-Mayer has found stumps left from long-ago timber harvesting at Hope Mills, N.C.; Sandy Island, S.C.; and Eglin Air Force Base at Pensacola, Fla.; and preserved wood in swampy Lake Louise, Ga., that are much older.

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

“And imagine that,” Grissino-Mayer said. “Imagine being able to see if there have been changes in the frequencies of hurricanes on time scales of decades or even centuries. That is what the tree-ring isotope record can provide.”