Study: Hurricanes Getting Stronger

In the 1970s there was an average of about 11 storms of the powerful category 4 and 5 range. Since 1990 that has climbed to an average of 18 per year worldwide, researchers led by Peter J. Webster at the Georgia Institute of Technology report in Friday's issue of the journal Science.

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Were New Orleans and coastal Mississippi victims of global warming? Greenhouse alarmists and the tabloids say yes, but until recently, most scientists would have answered no way. There was no evidence that global warming has had any effect on the planet's most powerful storms--dubbed hurricanes, typhoons, or cyclones depending on the ocean that spawns them.

Now, however, a connection is emerging between warming oceans and severe tropical cyclones. On page 1844, meteorologists report a striking 80% increase worldwide in the abundance of the most powerful tropical cyclones during the past 35 years. The study lends support to another, independent study published just last month that found a similar intensification in the Atlantic and western North Pacific. At the same time, the tropical oceans have been warming, driven, most researchers agree, by rising greenhouse gases. "There's a strong suggestion of a link" between the growing greenhouse and intensifying tropical cyclones, says meteorologist Kerry Emanuel of the Massachusetts Institute of Technology, sole author of the earlier paper.

But you still can't blame Katrina's damage on global warming, says Emanuel. There have been too few powerful storms striking densely populated coasts to declare with any confidence that intensifying storms are increasing the damage. And vulnerable coastal populations have swollen so much in recent decades that the increase in damage due to demographics is swamping any sign of increased damage due to storm intensification. But just wait until the second half of the century, he says.

Global warming and tropical cyclones are naturally linked by the storms' appetite for heat. Tropical storms are heat engines that draw their energy upward from warm ocean water to drive their winds before expelling waste heat to the upper atmosphere. So warming the tropical oceans--in effect throwing more wood on the fire--might be expected to spawn more frequent or more intense tropical cyclones. To find out whether warming has done that, meteorologist Peter Webster of the Georgia Institute of Technology in Atlanta and his colleagues examined satellite records of storms around the tropics, a history now 35 years long. The temperature contrast between a storm's eye and the adjacent cloud tops provides a gauge of maximum wind speed, as calibrated in the Atlantic and western North Pacific against direct measurements of wind speeds by storm-penetrating aircraft.

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http://www.sciencemag.org/cgi/content/full/309/5742/1807