 By KEAY DAVIDSON San Francisco Chronicle October 03, 2005
They just can't agree on one thing: why it's happening. Some scientists say the wave of hurricanes is the product of global warming spurred by greenhouse gases from cars and industries. Others say it's the latest wave in a recurrent and completely natural series of cyclonic assaults on the United States' eastern and southeastern shores. Whichever theory is correct - and conceivably, both might be true - one thing seems sure: The expected hurricane assaults of the next decade or so could cost the nation plenty, both in lives and reconstruction costs. The "cycle" thesis received a boost from a high-placed figure recently. Max Mayfield, director of the Tropical Prediction Center at the National Hurricane Center, testified at a congressional hearing Sept. 20 that hurricane violence in the Atlantic ebbs and surges in cycles, each of which lasts several decades. "The 1940s through the 1960s experienced an above-average number of major hurricanes, while the 1970s into the mid-1990s averaged fewer hurricanes," Mayfield said. A new wave of hurricane activity resumed in the mid-1990s, and "the current period of heightened activity could last another 10 to 20 years." Mayfield added that in his opinion, the increased hurricane activity since 1995 is due to natural cycles of hurricane activity and is "not enhanced substantially by global warming." By contrast, another top expert thinks global warming, not natural cycles, is the main culprit behind the recent wave of super-storms battering our shores. The importance of natural influences in driving hurricane cycles is "way overblown by some hurricane guys," said Kevin Trenberth at the National Center for Atmospheric Research in Boulder, Colo. Rather, changes in hurricane incidence "in recent times are much more likely to be related to global warming," he said. According to some theorists, global warming of the atmosphere could breed hurricanes via several complex mechanisms. To give one example, global warming could directly warm Atlantic waters, thereby encouraging evaporation of tropical waters. Like a man throwing oil on a fire, evaporation would boost skyward the kind of warm, moist air parcels that fuel hurricanes. However, most weather scientists seem more comfortable with the cycles hypothesis, according to which the recent super-hurricanes aren't something new; rather, they're just the latest manifestation of a long-running hurricane cycle - a recurrent nuisance. Weather cycles have long been a controversial topic. Until recent decades, many experts regarded it as disreputable, little more credible than astrology. For example, in the 19th century some scientists speculated about a direct relationship between English rainfalls and cyclical variations in spots on the sun - relationships that later proved to be illusory. But today, scientists, aided by more thorough weather records and sophisticated computer models, acknowledge the existence of weather cycles and can better distinguish real ones from illusory ones. Hurricanes aren't the only weather phenomena that seem to come and go in a quasi-cyclical manner - that is, by reappearing with approximately regular timing. Decade-to-decade climate cycles of all kinds - technically known as "interdecadal climate variabilities" - are a very hot topic among researchers, says Prof. James O'Brien of Florida State University, who is also state climatologist for Florida. "For example," O'Brien said, "right now we're having a serious, multi-year drought in the United States. In 1950s, we also had a drought period, and one in the 1930s with the 'Dust Bowl.' " To complicate things, many authorities think there are cycles within cycles. Len Pietrafesa, a noted hurricane expert at North Carolina State University, pointed out that hurricane cycles tend to come in oscillations of various sizes. Exactly how each oscillation links to the others - in what Pietrafesa whimsically calls "the global wiring diagram" - is still unclear. "There are five fundamental modes of oscillation - three to five years, 10 to 12 years, about 20 years, about 35 years and 45 to 60 years," Pietrafesa said. "The (oscillation) modes superimpose, and we get what we get as a function of the year we are in."
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