A Sept. 3 article incorrectly implied that there is debate over whether global warming is occurring. There is little dispute that atmospheric temperatures have risen in the past century, though debate continues about whether this is the result of human activity or natural processes. (Published 9/4/04)
Bad luck, not global warming, is the best explanation for the arrival of two severe hurricanes on the Florida peninsula in three weeks, several experts said yesterday.
The warming of the atmosphere -- if indeed that is happening -- may lead to hurricanes with higher winds and more rain than is generally seen now. But even if that happens, the change will take so long and be so subtle that the chance of detecting it in a single hurricane season is essentially zero.
"I wouldn't read too much into a couple of individual events like this," said Thomas R. Knutson, a research scientist at the National Oceanic and Atmospheric Administration, whose mathematical models predict that hurricanes are likely to be slightly more intense a century from now.
"The hurricane activity that we are having right now is quite adequately explained by [normal] variability and Atlantic sea-surface temperatures," said Stanley B. Goldenberg, of NOAA's hurricane prediction office in Miami.
"I think we should view this just as a natural statistical smoothing out of the records," said William M. Gray, of Colorado State University, whose expertise includes forecasting seasonal hurricane activity. History suggests the East Coast is overdue for several major storms, he thinks.
The "engine" driving hurricanes is the evaporation of water from the surface of the ocean. When water vapor in the air condenses back into liquid -- becoming droplets in clouds -- energy is released in the form of heat. The heat, in turn, increases the motion of the air and clouds. Under the right conditions, that motion takes the form of a cyclonic spiral and becomes a hurricane.
Some scientists believe that if atmosphere and ocean temperatures increase in the future, this system of evaporation-fueled storm creation will also heat up. The will result in hurricanes that have higher average wind speeds and produce more precipitation.
A mathematical model used by Knutson and Robert E. Tuleya, formerly of NOAA and now of Old Dominion University in Norfolk, predicts a 6 percent rise in hurricane intensity over the next 80 years.
"One may not expect to see a 'signal' for some time to come," Tuleya said yesterday. He added, "It's a fact that nobody so far has been able to show -- from the observed storms -- a tendency to have more intense storms."
Gray does not believe there is evidence of global warming caused by human activity. He sees in Florida's impending disaster only the play of chance.
Between 1966 and 2003, the Florida peninsula was struck by only one hurricane of Category 3, 4 or 5 intensity: Hurricane Andrew, in 1992. (On the conventional scale, that means a hurricane with wind speeds 111 mph or greater.) Between 1926 and 1965, however, the peninsula was hit by 14 such storms, he said.
"I've been saying this luck just can't go on," Gray said. "Florida people, rather than feeling they are unlucky for these two hits in the last three weeks, should realize how lucky they have been."
In general, the number of major hurricanes -- those Category 3 or higher -- that reached land in recent years is way below average, he said.
Since 1995, there have been 35 in the Atlantic basin, and only five have hit land (if Frances is included). In the 20th century, there were 218, and 73 came ashore -- a far higher proportion, he said.
Other researchers have cast doubt on the global-warming-and-hurricane-intensity hypothesis by looking at lake sediments and other geological evidence of ancient catastrophic hurricanes.
One study found evidence of 12 Category 4 or 5 hurricanes having touched one Florida site in the last 7,000 years. None occurred in the first half of that period, which was warmer than the second half.
Another study examined evidence from the last several centuries in Bermuda, Jamaica and Puerto Rico. The researchers found that major storms were nearly three times as frequent during a cool period called the "Little Ice Age" in the 1700s than in the much warmer last half of the 20th century.