Published May 19, 2008
WASHINGTON – Global warming isn't to blame for the recent jump in the number of hurricanes in the Atlantic, concludes a study by a prominent federal scientist whose position has shifted on the subject.
Not only that, warmer temperatures will actually reduce the number of hurricanes in the Atlantic and those making landfall, research meteorologist Tom Knutson reported in a study released Sunday.
In the past, Knutson has raised concerns about the effects of climate change on storms. His new paper has the potential to heat up a simmering debate among meteorologists about current and future effects of global warming in the Atlantic.
Ever since Hurricane Katrina in 2005, hurricanes have often been seen as a symbol of global warming's wrath. Many climate change experts have tied the rise of hurricanes in recent years to global warming and hotter waters that fuel them.
Another group of experts, those who study hurricanes and who are more often skeptical about global warming, say there is no link. They attribute the recent increase to a natural multi-decade cycle.
What makes this study different is Knutson, a meteorologist with the National Oceanic and Atmospheric Administration's fluid dynamics lab in Princeton, N.J.
He has warned about the harmful effects of climate change and has even complained in the past about being censored by the Bush administration on past studies on the dangers of global warming.
He said his new study, based on a computer model, argues "against the notion that we've already seen a really dramatic increase in Atlantic hurricane activity resulting from greenhouse warming."
The study, published online Sunday in the journal Nature Geoscience, predicts that by the end of the century the number of hurricanes in the Atlantic will fall by 18 percent.
The number of hurricanes making landfall in the United States and its neighbors — anywhere west of Puerto Rico — will drop by 30 percent because of wind factors.
The biggest storms — those with winds of more than 110 mph — would only decrease in frequency by 8 percent. Tropical storms, those with winds between 39 and 73 mph, would decrease by 27 percent.
It's not all good news from Knutson's study, however. His computer model also forecasts that hurricanes and tropical storms will be wetter and fiercer.
Rainfall within 30 miles of a hurricane should jump by 37 percent and wind strength should increase by about 2 percent, Knutson's study says.
And Knutson said this study significantly underestimates the increase in wind strength. Some other scientists criticized his computer model.
MIT hurricane meteorologist Kerry Emanuel, while praising Knutson as a scientist, called his conclusion "demonstrably wrong" based on a computer model that doesn't look properly at storms.
Kevin Trenberth, a climate scientist, said Knutson's computer model is poor at assessing tropical weather and "fail to replicate storms with any kind of fidelity."
Trenberth, climate analysis chief at the National Center for Atmospheric Research in Boulder, Colo., said it is not just the number of hurricanes "that matter, it is also the intensity, duration and size, and this study falls short on these issues."
Knutson acknowledges weaknesses in his computer model and said it primarily gives a coarse overview, not an accurate picture on individual storms and storm strength. He said the latest model doesn't produce storms surpassing 112 mph.
But NOAA hurricane meteorologist Chris Landsea, who wasn't part of this study, praised Knutson's work as "very consistent with what's being said all along."
"I think global warming is a big concern, but when it comes to hurricanes the evidence for changes is pretty darn tiny," Landsea said.
Hurricane season starts June 1 in the Atlantic and a Colorado State University forecast predicts about a 50 percent more active than normal storm season this year.
NOAA puts out its own seasonal forecast on May 22.
In a normal year about 10 named storms form. Six become hurricanes and two become major hurricanes. On average, about five hurricanes hit the United States every three years.