More Hurricanes or Fewer in Warming World?

After Hurricane Katrina struck the Gulf Coast on Aug. 29, 2005, observers -- scientists and non-scientists alike -- began a lively debate over whether global warming is contributing to more or stronger hurricanes. Nearly two-and-a-half years later, that debate continues.

While we know it's the warm summertime waters of the Atlantic, Caribbean and Gulf of Mexico that help give birth to and fuel hurricanes, scientists are uncertain yet whether warmer waters brought by climate change mean the future will see a larger number of storms. They're also not sure if warmer temperatures on water and land will make storms stronger.

Hurricane forecasting, while it's improved dramatically in recent years, is still anything but a slam-dunk. That's because the factors leading to hurricane formation -- near-surface water temperatures, humidity and wind shear, to name a few -- aren't yet fully understood. Combine those uncertainties with the additional uncertainties raised by climate change, and the look of future hurricane seasons is blurry indeed.

For instance, new research to be published this week in Geophysical Research Letters indicates that warmer waters, particularly in the tropical Pacific and Indian oceans, might increase vertical wind shear over parts of the Atlantic where hurricanes are most likely to form. Greater wind shear could mean fewer, rather than more, hurricanes.

Chunzai Wang, an oceanographer and climate scientist with the National Oceanic and Atmospheric Administration (NOAA), and Sang-Ki Lee of the Cooperative Institute for Marine and Atmospheric Studies at the University of Miami, studied historical data dating back to the mid-nineteenth century to reach that conclusion.

Wang said, "we found a gentle decrease in the trend of U.S. landfalling hurricanes when the global ocean is warmed up. This trend coincides with an increase in vertical wind shear over the tropical North Atlantic and the Gulf of Mexico, which could result in fewer U.S. landfalling hurricanes."

However, Wang added, wind shear alone isn't the only factor affecting hurricane development in the Atlantic. Other factors could either spur on or depress storm formation.

Scientists attending this week's meeting of the American Meteorological Society in New Orleans are also engaging in animated discussions about how global warming might affect future hurricanes. There's no consensus yet, but hurricanes themselves are taking the stage front and center at the meeting, with a special forum on Hurricane Katrina, a two-day symposium on hurricanes, tours of storm-ravaged parts of the city and even opportunities to volunteer with Habitat for Humanity.