9 October 2005Sandi DoughtonSeattle Times staff reporter
With nearly two months to go, the 2005 hurricane season is already one of the most active — and deadly — on record.
Christopher Landsea, science and operations officer for the National Hurricane Center, attributes the spike to a natural cycle. Things started picking up in 1995, he said. If the historical pattern holds, the number and ferocity of hurricanes will remain high for the next 10 to 20 years.
But a growing number of scientists suspect nature may be getting a nudge from global warming.
Two studies this summer found the destructive power of hurricanes has been increasing worldwide, in parallel with a rise in ocean and air temperatures.
"The fact that this seems to be associated with more-intense storms isn't surprising," said Judy Curry, co-author of one of the studies and chairwoman of earth and atmospheric sciences at the Georgia Institute of Technology.
Warm water is the fuel that powers hurricanes. Called typhoons or cyclones in other parts of the world, the swirling storms form only where there's a deep layer of water at 80 degrees or hotter.
Theory and computer models have long predicted global warming will boost the storms' intensity — as measured by wind speed and rainfall — but not their frequency.
Tropical ocean temperatures worldwide have increased about 1 degree since 1970, and up to 4 more degrees of warming is predicted over the next century.
Hurricane Katrina's deadly punch grew as it passed over the Gulf of Mexico, which was up to 5 degrees warmer than normal in August. Hurricane Rita also got a boost as it passed over a meandering Gulf current of unusually warm water.
"It went: Boom. From Category 2 to Category 5 in one afternoon," Curry said.
She and her colleagues examined worldwide records and found no change in the number of hurricanes over the past 35 years. But the biggest storms — categories 4 and 5 — nearly doubled, from about 10 per year in the 1970s to about 18 per year during the past decade. The shift toward stronger storms occurred in every ocean basin.
Atmospheric scientist Kerry Emanuel of the Massachusetts Institute of Technology also calculated a power index for hurricanes in the Atlantic and North Pacific back to 1949. Beginning in the 1970s, he found, destructive power began climbing and is now nearly double what it was four decades ago.
"The large upswing in the last decade is unprecedented and probably reflects the effect of global warming," Emanuel concluded.
On the Web log RealClimate.org, a group of top climate scientists pose the question: "Could New Orleans be the first major U.S. city ravaged by human-caused climate change?"
Their answer: It's impossible to tell.
Any complex storm is the result of so many factors — some of them random — that no one can point to a single cause. That's why scientists look beyond local weather to broader, statistical patterns when trying to sort out global warming's impacts.
Like rolling loaded dice, global climate change should increase the odds of more-intense hurricanes. But just as it takes repeated tosses to realize a pair of dice is loaded, the effects of global warming will show up only over time.
Landsea of the National Hurricane Center isn't convinced by the evidence so far, which he says is based on shaky records.
Early hurricane data often came from ships' captains, who probably overstated wind speeds. Emanuel corrected for this in his analysis but may have gone too far — with the result that past storms appear weaker than they really were, Landsea said.
Curry and her colleagues didn't look at storms before 1970, so they didn't take into account the fact that the current upswing in hurricane intensity in the Atlantic fits a natural cycle that has alternated between quiet and active periods back to the mid-1800s. The cycle is driven by shifts in currents and trade winds, Landsea said.
But even if a natural trend is at work in the Atlantic, that doesn't explain why hurricanes are getting stronger around the world, said Greg Holland of the National Center for Atmospheric Research in Boulder, Colo. A 35-year veteran of hurricane research, Holland said he was skeptical of global warming's impact until recently.
"If you look around the globe, this is a consistent change that's happening everywhere, which leads me to say what's going on in the Atlantic may not be a [natural] oscillation. It may be a trend related to global climate change."
Most computer models say global warming should boost the power of hurricanes by only about 5 to 8 percent over the next century.
"That sounds pretty tiny to me," Landsea said.
But destructiveness rises exponentially with wind speed, Holland said. At the same time, global warming is expected to raise sea levels up to 6 feet in the next century, which would make storm surges more deadly.
"This is very serious stuff," Holland said.
It's also possible computer models have underestimated global warming's effects on hurricanes.
The increases in storm power reported in the two studies this summer are far larger than any models predict, said modeler Thomas Knutson of the National Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics Laboratory in Princeton, N.J.
"If you take these studies at face value, I find them alarming," Knutson said.
The destruction wrought by modern hurricanes is also strongly influenced by a factor that has nothing to do with science or climatology: The growth of coastal communities in harm's way.
For America's hurricane-prone regions, it really doesn't matter in the short term if man or nature is driving a trend toward more-powerful storms, Curry said.
"We have to figure out how to make ourselves more resilient, rather than using the argument about global warming to do nothing."
Sandi Doughton: 206-464-2491 or [email protected]
