"Usually if an El Niño is going to form, it's in the spring," Klotzbach said.
But El Niño had a different effect in the Pacific Ocean.
Xie said the El Niño increased water temperatures in the eastern Pacific and was responsible for a slightly above average hurricane season there, where 17 named storms formed. Ten of those storms became hurricanes, and six evolved into major hurricanes.
During an average season, the eastern Pacific has 16 named storms, 9 hurricanes, and 4 major hurricanes.
The recent spate of storms has sparked a lively debate among meteorologists about whether global warming is causing more hurricanes to form. (Related: "Is Global Warming Making Hurricanes Worse?" [August 4, 2005].)
The past three summers have been especially active, with an average of 19 tropical storms forming each season.
The 2005 season was unprecedented, with a record 27 tropical storms forming. Among those was Hurricane Katrina, which nearly destroyed New Orleans, killed more than 1,500 people, and became the most expensive hurricane on record in the U.S. (See "Hurricane Katrina: Complete Coverage.")
But Klotzbach and other researchers say the recent increase in tropical storms—an average of 15 per year since 1995—has been caused by a cyclical increase in the temperature of the Atlantic Ocean. They say the effect of global warming is minimal, if it exists at all.
That cycle of increased activity could continue for at least another 10 or 15 years, they add.
But others, including research meteorologist Kerry Emanuel at Massachusetts Institute of Technology in Cambridge, say the increase in hurricanes is directly linked to global warming caused by human activities such as burning fossil fuels.
Emanuel also says the contention that the increase in storms may just be part of a decades-long cycle is "nonsense."
"What we all agree on is that there's an upward trend in ocean temperatures over the last hundred years," Emanuel said. "But this is where the disagreement starts. One group is thinking it is caused by natural oscillations. Another is saying that this is all man-made. The truth may be somewhere in the middle."
N.C. State's Xie said global warming could eventually have some impact on hurricane seasons, but he doesn't think that's happened yet.
"I have no doubt that global warming will affect hurricane activity, but I'm a little skeptical about some of the large numbers that are being claimed, like the doubling of major hurricanes caused by global warming in the past 30 years," Xie said. "In my opinion, we don't have sufficient evidence to claim that."
More Than Meets the Eye
Meteorologist Joe Bastardi of AccuWeather doesn't buy the link between global warming and hurricanes. But he does think the press underplayed the effects of the 2006 hurricane season—especially the damage done by Ernesto, one of only two tropical storms to make landfall in the U.S. this year. (Read related story: "Ernesto Threatens Florida, Carolinas With Heavy Rains" [August 29, 2006].)
Ernesto came ashore in North Carolina during the Labor Day weekend and dissipated as it moved up the East Coast.
"Ernesto turned out to be a very bad storm for the mid-Atlantic," Bastardi said. "I'm puzzled at how the press ignored it. The damage to the [electrical] power grid by Ernesto was only exceeded by [Hurricane] Isabel [in 2003]."
The press also underplayed the hurricane season's effects on the Canadian Maritime Provinces, Bastardi said (Canada map).
Only one stormHurricane Isaac in late Septemberactually brushed the coastal province. But three others churned into Canadian waters and brought high winds and flooding, he said.
"They had a bad hurricane season in that part of the world."
Willie Drye is the author of Storm of the Century: The Labor Day Hurricane of 1935, published by National Geographic Books.
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