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Current Hurricane Amount at "Normal" Level, Study Says

John Roach
for National Geographic News
June 6, 2007
 
The number of hurricanes that swirl across the Atlantic Basin has shot up in the past decade, but the increase may just be a return to normal activity after a long lull, suggests a new study.

The findings throw additional fuel onto the debate over the effects of global warming on hurricane frequency and intensity. (Related: "Global Warming Link to Hurricane Intensity Questioned" [July 28, 2006].)

Some researchers have suggested the increase in hurricane activity since 1995 is a result of climate change, whereas others contend the trend is part of natural variability.

A problem plaguing researchers is the lack of a long-term hurricane record for comparison. Reliable observations stretch back only about 50 years.

Now a team of scientists led by Johan Nyberg at the Geological Survey of Sweden in Uppsala has reconstructed the hurricane record of the past 270 years by studying the growth patterns of coral skeletons and the abundance of tiny fossils in a marine sediment core.

The record shows that hurricane activity goes up and down in decadal cycles, with storms gradually decreasing overall from the 1760s until the early 1990s. An abnormal lull of just 1.5 major storms a year was reached between the 1970s and early 1990s.

A major Atlantic storm is defined as any Category 3 or higher hurricane, with sustained winds of at least 111 miles an hour (179 kilometers an hour). (See hurricane videos and get more information about the disasters.)

Since 1995, however, an average of 4.1 major hurricanes have formed, which is comparable to other periods of high hurricane activity in the 270-year long record, according to the researchers. (Related: "2007 Hurricane Season Begins, Will Be Busy, Forecasters Say" [June 1, 2007].)

Writing in this week's issue of the science journal Nature, Nyberg and colleagues say the current trend "thus appears to represent a return to normal hurricane activity."

New Normal?

While the hurricane activity of the past decade appears normal according to the new findings, global warming may still be influencing how many storms form each year, the researchers say.

For example, "the relatively low 'low level' phase from 1970 to 1990 could have been caused by a faster warming of the atmosphere than of the ocean," lead author Nyberg wrote by email.

This would have caused greater windiness in the atmosphere, which dampens hurricane formation.

Study co-author Terrence Quinn is a geoscientist who studies past climate changes at the University of Texas at Austin. In the past few decades, both the oceans and atmosphere have begun warming appreciably, he said.

This global warming effect likely adds to the natural variability in hurricane activity, he said, so researchers and forecasters will have to take both factors into account from now on.

The reconstructed 270-year hurricane record, Quinn added, should allow scientists to better model future climate change.

Proxy Record

To get such an extensive record, scientists need to examine things such as corals and marine sediments, since other data only goes so far, Quinn pointed out. (Related: "Over 200 Years of Hurricane Data Recorded in Trees, Study Says" [September 18, 2006].)

In the Caribbean major rainfall events wash large amounts of organic rich matter into the ocean, Quinn explained. Corals absorb these organic compounds.

When scientists shine a special light similar to a black light on a cross-section of these corals, banded layers rich in these organic compounds become luminescent.

"You can see them, you can count them, you can look at their thickness," Quinn said.

The researchers compared the bands to the record of known hurricanes over the past several decades and found they match up.

The researchers also looked at the abundance of a certain type of single-celled marine animal found in a sediment core from a region off the coast of Venezuela.

The abundance of the creatures is driven by trade winds. The stronger the trade winds, the more nutrient-rich waters from the bottom of the ocean rise to the surface. This allows animals to flourish but dampens hurricane activity.

This sediment core record also matches the observational record of hurricanes.

The luminescent banding in corals and changes in abundance of certain microfossils in marine sediment cores thus serve as "proxies" for hurricane activity, the scientists point out.

"The beauty of these proxies is that they can extend the record back to before we have instrumental records," Quinn said.

James Elsner is a professor of geography at Florida State University in Tallahassee who studies the connections between hurricanes and climate.

In a commentary in this week's Nature, Elsner said this type of research "is a valuable tool for answering questions on hurricane climatology."

However, the proxy data collected to date only come from a few points and may not capture shifts in hurricane storm tracks or accurately show when storms did and did not occur, he said.

"More records are needed," he wrote, "before localized prehistoric activity can be used to make sense of large-scale patterns of storminess."

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