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 size and 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 are so desperate for clues they 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 one premise:

The most urgent hurricane threat has nothing to do with global warming.

'A real conundrum'

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 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.  Read rest...