The Why Files The Why Files --

Hurricanes: The heat is ON

Simmering simulation
One way to anticipate how global warming may affect future hurricanes is to simulate climate with computer models, and Tom Knutson of the Geophysical Fluid Dynamics Laboratory is a master of the method. For a 2004 study (see "Impact of CO2-Induced Warming..." in the bibliography), 100-foot ship resting entirely on the beach.Knutson and Robert Tuleya used nine different climate models to project climate change over the next 70 years. Each year, they jacked up the level of carbon dioxide 1 percent compounded (as a simplified experiment to mimic the expected net effect of various emissions caused human activity over the next 70 years or so.)

In Plaquemines Parish, La., Katrina showed her strength by shoving this ship ashore. Photo: NOAA

As sea surface temperature and other factors that affect hurricanes changed, the researchers "made hurricanes" in the computer, and found that they indeed gained intensity as the globe warmed:

Atmospheric pressure fall: increased 14 percent at the center of the storm.

Peak winds: Up 6 percent.

Rainfall: up 18 percent within 100 kilometers of the storm center.

Storm intensity: Up one-half category on the Saffir-Simpson scale.

A half notch on the Saffir-Simpson scale, notes McKinley, "is a significant increase," since it could add several feet to the destructive storm surge. In low-lying places like New Orleans, every foot matters in the battle between levee and lake or river.

While each computer model spat out a slightly different set of conditions, they all pointed in the same direction: stronger storms. "There are many different climate models, run at high CO2 levels, that uniformly, even with different options for key processes, show an increase in hurricane intensity," says McKinley. "Predictions of increased storm intensity are more in agreement than the studies of what has happened in the recent past. Drawing a signal from the past data is harder at this point."

Bands of clouds alternate dark and white.
Aug. 28, 2005: Katrina's eyewall was seen from a hurricane-hunter plane. Photo: NOAA

Warming warning
Yet even skeptics who don't think global warming has -- or even will -- produce stronger hurricanes say we are in a for a bad string of storms. Hurricanes in any particular region seem to wax and wane over a scale of decades. In 1995, a bad string of hurricanes started in the Atlantic that is still lambasting the Caribbean and United States. "... the shift since 1995 to an environment generally conducive to hurricane formation-warmer North Atlantic SSTs [sea-surface temperatures] and reduced vertical wind shear-is not likely to change back soon," wrote Stanley Goldenberg of the National Oceanic and Atmospheric Administration (NOAA), and colleagues. "This means that during the next 10 to 40 years or so, most of the Atlantic hurricane seasons are likely to have above average activity, with many hyperactive, some around average, and only a few below average" (see "The Recent Increase..." in the bibliography).

Colored images show water encroaching on land after the hurricane.
Images from the Terra satellite show Hurricane Katrina flooding rivers feeding the Gulf of Mexico, in Mississippi and Alabama. The powerful storm surge and heavy rains caused massive flooding along the coast. In these false-color images, water is black and dark blue; vegetation is green; clouds are pale blue and white; flooded river banks are dark green. Photos: NASA

We tried to talk with Goldenberg, but got intercepted by a helpful government PR minder, who directed us to Chris Landsea, another NOAA scientist, to speak about the subject. But Landsea apparently had better things to do; we never heard from him.

Here's one final thought. Hurricanes are not predictable. The Gulf Coast may not see another category 4 hurricane for 10 years. Or it could get hit again in a week. NOAA just predicted that the bulk of the storms are still ahead; the hurricane season runs until Nov. 30.

The hurricane prediction experts at Colorado State University are no more comforting: in 2005, "we will witness seasonal tropical cyclone activity at near record levels."

Breeze on over to our warming warning bibliography.


Megan Anderson, project assistant; Terry Devitt, editor; S.V. Medaris, designer/illustrator; David Tenenbaum, feature writer; Amy Toburen, content development executive

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