Banner year for monster hurricanes stirs up debate
Researchers spar over the causes
As Wilma hurtles toward Florida, 2005 has already hit the history books as the year of the monster hurricane.
But another storm is gathering in the scientific world over exactly why. Most scientists agree that the United States is in the midst of a cycle of fierce hurricanes, but unknowns remain about the storms' intensity, forecasting, frequency, and duration. The lack of long-term hurricane data and the politics surrounding global warming has compounded the problem.
''This is a new field, it's relatively new research," said Isaac Ginis, a professor of oceanography at the University of Rhode Island. ''But as people become more creative on how to analyze the data, we will see significant improvements in our modeling and forecasting."
Hurricanes are fueled by warm ocean waters that allow heat and moisture to rise into the atmosphere. Winds near the ocean surface spiral air inward, gathering more moisture and energy. If conditions remain ideal, the telltale hurricane whorl forms and grows stronger as it picks up even more energy from the ocean surface.
Tropical sea surface temperatures have risen by 0.5 degrees Celsius in the past 35 years, and two papers in the journals Nature and Science in the past three months have linked that rise to an increase in hurricane intensity. Kerry Emanuel at the Massachusetts Institute of Technology concluded that today's North Atlantic and North Pacific human-induced global warming probably helped warm the seas.
The other paper, by Georgia Institute of Technology researchers, concluded that there was almost a doubling of the frequency of the most intense hurricanes -- categories 4 and 5 in oceans the world over.
But the papers drew deep criticism in part because accurate long-term hurricane data on which to base their results is not available -- and because some researchers don't believe global warming is intensifying storms.
''They weren't good papers," said William Gray, professor of atmospheric science at Colorado State University. Gray posted on his website yesterday letters he sent to both journals criticizing the researchers' methods and wrote that the authors have created a false belief that global hurricane intensity is rising and human-induced global warming may be a contributing factor.
The stakes are high because understanding what is going on with hurricanes could mean dramatic changes in how coastal regions are populated and ultimately how some greenhouse gases are regulated. Some scientists say sea surface temperatures are rising at least in part because of carbon dioxide released when fossil fuels are burned, trapping heat in the atmosphere.
''The key point is that global warming is not the whole story, but it's a big player," said Kevin Trenberth, head of the Climate Analysis Section at the National Center for Atmospheric Research. ''Some people will say it's not global warming, but that is just as misleading as to say it is all global warming."
Confusion abounds because hurricanes are influenced by many other factors.
After several quiet decades of hurricane activity, the number of the storms during the past decade has risen -- as anyone looking at this year's record-breaking 12 hurricanes knows. But to confuse the subject further, the overall number of hurricanes -- called typhoons and cyclones elsewhere in the world -- has remained at about 80 a year for decades.
Many scientists assert that the hurricane cycle is part of the natural variability of hurricanes.
Meanwhile, Katrina and Rita may also have been influenced by the weather system El Nino last winter, which tends to bring warmer waters to the Caribbean in subsequent years.
Ginis, however, thinks Katrina and Rita grew into such monsters because of a tongue of very deep warm water that escapes from Caribbean waters into the Gulf of Mexico.
Normally, Caribbean waters are warm as far down as 300 feet, while the Gulf's heat ends about 100 feet down. This year, the paths of Katrina and Rita passed over this area.
''That's why the hurricanes had much more heat energy to intensify," Ginis said. ''It was perfect storm conditions." 