In Al Gore's Nobel-winning movie An Inconvenient Truth, hurricanes became symbols of the danger of global warming.
The reality is more complicated.
Scientists are locked in debate about whether global warming is spiking the intensity of hurricanes. Even those who agree that humans are causing global warming disagree about whether it is making hurricanes worse.
Leading experts are changing their findings. Climatologists desperate for clues are boring holes along Florida's coastline, trying to discern from grains of sand how many tropical storms pounded our shores in past centuries.
Amid the whirlwind of debate, most scientists agree on the most urgent hurricane threat. And it's not global warming.
Kerry Emanuel, an MIT professor of atmospheric science, was named by Time magazine in 2006 as one of "100 people who shape our world." The reason? Just before Hurricane Katrina smashed into New Orleans in 2005, he published a scientific paper in the journal Nature saying the power of hurricanes had nearly doubled in recent decades.
His findings were based on heat.
Hurricanes are born in tropical heat, beginning with seas that are at least 80 degrees. Warm, moist air rises from the sea surface and gets caught in converging winds, twisting upward. Moisture in the air condenses as it rises, giving off more heat. This provides the energy that pumps vast quantities of air from sea to sky and keeps storm winds whirling fiercely.
Emanuel charted the temperatures of the Atlantic's sea surface and hurricane power, and showed in his paper that the two rise and fall together. He found that hurricane power had increased, probably because of man-made global warming.
"While many researchers had been predicting an explosion of more powerful storms, Emanuel, 51, offered evidence that it was actually happening," Time wrote.
To test the theory, Emanuel and other scientists recently loaded tons of data into computer models, hoping to learn how bad it could get if global warming keeps pushing up sea temperatures.
The results were surprising: Hurricanes didn't increase dramatically in the projections, even after decades of simulated global warming.
Emanuel was not disappointed that the research seemed to undercut his old results. "One gets used to being mistaken, and we follow the evidence and sometimes the evidence is contradictory and then we have to sort it out."
He's uncertain whether the recent results are correct or the outcome of faulty models. "There is a real conundrum here."
Emanuel thinks a warming climate "will almost certainly have some palpable effect on hurricanes," and he expects more intense hurricanes. "But the jury is out on how it affects their frequency."
Emanuel is not the only top scientist to test the hurricane-global warming link. Thomas Knutson, who works for a National Oceanic and Atmospheric Administration laboratory in Princeton, N.J., also loaded tons of data into computer models.
The results were even more stunning: After several decades of simulated global warming, the number of Atlantic hurricanes slightly decreased.
"Our research has indicated that we don't think global warming is going to cause really large increases in tropical storm or hurricane numbers," Knutson said. "But it may cause the hurricanes that do occur to have greater intensity and higher rainfall rates."
Some studies say sea surface temperatures have increased about 1 degree Fahrenheit in the past 50 years. How could a barely perceptible change have such a dramatic effect?
Because it takes a huge amount of energy to heat a vast ocean, says Jay Gulledge, senior scientist with the Pew Center on Global Climate Change. That extra energy wouldn't create hurricanes, but it could make them more intense.
That's bad news, he says, because studies show that a slight increase in wind speed leads to an exponentially larger increase in a hurricane's destructive power.
No increase, they say
All of this has deepened confusion about a complex and controversial topic. But one hurricane guru who is not surprised is Chris Landsea, science and operations officer for the National Hurricane Center.
Landsea, like Emanuel, Knutson and most other hurricane researchers, thinks the Earth's climate is warming and that humans are contributing to it. But he doesn't see evidence that this warming has affected hurricanes.
As a researcher, Landsea has spent much of his career deep in the data of past hurricanes. To him, one thing is clear: That record is incomplete.
Tropical storms and hurricanes likely formed in the Atlantic during the first half of the 20th century, when there were no satellites or Hurricane Hunter aircraft to spot them. No wonder it looks like hurricanes increased in the latter 20th century.
That's the position of Stanley Goldenberg, a hurricane researcher at NOAA's Atlantic Oceanographic and Meteorological Laboratory in Miami.
"There's not a single scientist that I know at AOML or at the National Hurricane Center that feels that we have seen in any measurable increase in hurricane activity because of global warming," he said.
Goldenberg was the lead author of a highly cited article in the journal Science that said Atlantic hurricanes increase for periods of 10 to 40 years and then decrease for similar periods. That's a natural cycle related to the rise and fall of sea temperatures and to upper-level winds that can disrupt hurricanes, he said. The underlying cause may be something called "thermohaline circulation," a nearly global circulating stream that changes with the temperature and salinity of the ocean water.
The ongoing cycle of greater and lesser hurricane activity is not attributable to man-made global warming, Goldenberg says. In fact, he does not think humans are causing global warming.
So what's the basis for warning that hurricane dangers are increasing? Besides the possible global warming-hurricane link, there are at least two reasons.
As seas warm, they expand. Levels rise slightly. Over 100 years, that could add a foot or two to the storm surge of hurricanes, assuming the warming trend continues.
Of course, none of this would matter much if the world's coastlines were long undulating stretches of undeveloped sand.
Instead, coastal development has exploded. In Florida alone, the number of coastal residents increased 10-million between 1950 and 2000, according to historian Gary R. Mormino in the book Land of Sunshine, State of Dreams.
It's not just millionaires who are building along the sands, but increasing populations of the poor. That was the greatest source of devastation recently in the cyclone that struck the southeast Asian country of Myanmar, killing 130,000 people.
Leading and sometimes sparring researchers signed a statement in 2006 calling for governments to adopt policies to slow this overdevelopment and to address "the more urgent problem of our lemming-like march to the sea." The signers included Landsea, Emanuel, Knutson and former National Hurricane Center director Max Mayfield.
This is one point on which scientists agree.
Says Jeff Masters, director of meteorology at weatherunderground.com: "Pretty much every hurricane scientist that you'll talk to will say that coastal overdevelopment is probably the biggest issue affecting hurricane damages in the coming century."
Curtis Krueger can be reached at firstname.lastname@example.org or (727) 893-8232.