Global Warming Link to Hurricane Intensity Questioned

John Roach
for National Geographic News

July 28, 2006

An expert with the U.S. National Oceanic and Atmospheric Administration (NOAA) is questioning the connection between climate change and the appearance of more intense hurricanes in recent years.

Historical data on hurricanes is too crude to determine long-term trends in intensity, says Christopher Landsea, a science and operations officer with NOAA's National Hurricane Center in Miami, Florida.

Extreme hurricanes like Katrina were likely as common around the world 30 years ago as they are today, Landsea says. But since satellite imagery was poorer, storm intensities were underreported.

Landsea is the lead author of a commentary in today's issue of the research journal Science.

The commentary rebuts a string of papers published in the last 12 months that link global warming with a surge in the number of extreme tropical cyclones over the past 30 to 40 years. (Related story: "Warming Oceans Are Fueling Stronger Hurricanes, Study Finds" [March 16, 2006].)

Better Resolution

Right or wrong, the flooded neighborhoods, floating corpses, and stranded survivors in the wake of Hurricane Katrina have come to symbolize the type of devastation the world can expect as global warming fuels bigger, more intense tropical storms.

(Get more information at the National Geographic magazine special feature on Hurricane Katrina.)

Satellite images of Katrina over the Gulf of Mexico in August 2005 feature prominently in former U.S. Vice President Al Gore's recent global warming documentary An Inconvenient Truth.

But is global warming really fueling a surge in extreme tropical storms?

In 1975, Landsea says, two geostationary satellites watched the weather with coarse, 5.6-mile (9-kilometer) resolution. Today eight satellites with 2.5-mile (4-kilometer) resolution have that task.

"Twenty, thirty years ago we didn't have either the number of or resolution in satellites or the variety of other ways to monitor [hurricanes] that we do now," Landsea said.

The better monitoring, he adds, allows researchers to identify more hurricanes as Category Four and Five—the strongest on the hurricane intensity scale. In the 1970s and 80s, he says, those storms would likely have been labeled Category Three.

In the commentary, he and colleagues write that firm conclusions on the link between global warming and hurricane intensity must wait until the historical hurricane record has been reanalyzed with modern methods.

But even then, Landsea said, he expects the increase in intensity to be small.

"Theoretical and numerical models do suggest [hurricanes] will become more intense due to global warming, but by a fairly tiny fraction, about 2 percent for every degree Fahrenheit [0.5 degree Celsius]."

Intensity Debate

The number of hurricanes that reach Category Four and Five has doubled since 1970, according to a report by Judith Curry, an earth and atmospheric scientist at the Georgia Institute of Technology in Atlanta.

Her team links the increasing storm intensity to a 1-degree Fahrenheit (0.5-degree Celsius) rise in global sea surface temperature in the same period.

Curry, the co-author of two of the papers linking global warming to increasing storm intensity, says that some of the storms in the database are likely misclassified.

But the difference between major storms like Katrina, which was a Category Five over the Gulf of Mexico, and Ophelia, which was a Category One over the Atlantic Ocean, is obvious.

"Our analysis separates the really big ones from the weaker ones," she said.

She adds that nobody has presented data to refute their findings.

"For the time being," she said, "this is what the data says and what analysis we have, and it says we are at elevated risk for increasing hurricanes and increasing intensity in hurricanes."

Curry said the reanalysis of the hurricane record by scientists at NOAA and the University of Wisconsin, Madison, will likely find storms misclassified as both too weak and too strong. But it is unlikely to find a trend much different than what it is today.

"Certainly [the trend] is there," she said. "We don't know exactly how big it is, but it's big. There's no evidence to suggest that it's zero."

But Landsea said he believes the historical databases "are so incomplete that this near doubling in Category Four and Five hurricanes may be an artifact. We are just monitoring things better."

Both Landsea and Curry agree the database reanalysis is critical to resolve this debate.

Bigger Problem

While the debate over global warming is important, Landsea and Curry have other priorities for the time being. They joined eight other climate experts in a joint statement to say the U.S. has a bigger hurricane problem to address.

The issue is an ever-growing concentration of people and wealth in hurricane-prone regions of the U.S. coast, the statement says.

"These demographic trends are setting us up for rapidly increasing human and economic losses from hurricane disasters, especially in this era of heightened activity," the statement said.

"Scores of scientists and engineers had warned of the threat to New Orleans long before climate change was seriously considered, and a Katrina-like storm or worse was (and is) inevitable even in a stable climate."

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