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It's no accident that the documentary An Inconvenient Truth opens with a satellite image of Hurricane Katrina bearing inexorably down on a helpless New Orleans.

Since hurricanes draw their destructive power from heat in seawater, you would expect that global warming would intensify these terrifying storms and multiply their number, leading to increased devastation on land. All other things being equal, that's probably what would happen.

But all other things are not equal, which is why the relationship between climate change and hurricanes is anything but settled. Two new studies released this week have moved the ball significantly forward, however.

The first, appearing in Science, says the frequency of Atlantic hurricanes will actually decrease during this century but that the most powerful Category 4 and 5 storms will likely double in number.

The second study, presented at a conference of the American Meteorological Association in Atlanta, says that whether or not the nature of hurricanes changes, the property damage they wreak in the U.S. will rise an average 20% over the next two decades because of the rising sea level caused by global warming.

The idea that there might be fewer Atlantic hurricanes in a warming world is not a new one. Earlier studies led by Thomas Knutson of the National Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics Laboratory (GFDL), a co-author of this week's Science paper, had reached the same conclusion.

The explanation for this seemingly paradoxical finding: hurricane activity is governed not only by ocean temperatures, but also by factors such as ocean currents and the speed and direction of wind in different layers of atmosphere.

It turns out, says Knutson, that the key to hurricane frequency is not simply how much the Atlantic warms, but how it warms in relation to the rest of the tropical oceans. If it warms more than average, he says, you have an increase in storms. If it warms less, you have a decrease.

The rise in Atlantic hurricanes that we have seen since 1980 or so, he says, is probably the result of exactly that kind of differential warming, not so much global warming overall.

The significance of the current paper, says GFDL's Morris Bender, the lead author, is that it simulates the statistics of the most severe future hurricanes — something previous studies couldn't do because of an inability to accurately reproduce a hurricane's structure.

A new modelling approach used in this week's study remedies that problem, he says, and suggests that category 4 and 5 storms will become relatively more common by 2100 — with the important caveat that the change will not become clearly detectable until the second half of the century.

The locus of the biggest increase, continues Bender, is projected to be in the western Atlantic, north of about 20 degrees latitude — about where the southern edge of Cuba lies.

And because these stronger storms are so damaging, the increase in their number would likely outweigh the decrease in overall numbers, in terms of destruction.

The authors warn that their findings, while important, are not the final word and must be validated by others. Bender Knutson and their colleagues ran the simulation using an average of 18 climate models at once, then reran it with four models individually.

One of the models, Bender says, showed a decrease in the most powerful storms, demonstrating once again that the effects of climate change are complex and not easily predicted.

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