The most damaging types of hurricane are getting more frequent

AMID the handwringing that has followed the devastation of New Orleans by Hurricane Katrina, a persistent question whispered in the background has been whether hurricanes are getting worse. A paper in this week's Science, by Peter Webster of the Georgia Institute of Technology, in Atlanta, and his colleagues suggests that they are, but only in one, specific way.

Hurricanes can form only over oceans that have a surface temperature above 26°C. That is well known. What is debatable is what effect, if any, raising the temperature beyond that has. It might increase the number of storms, the length they last, their maximum strength or the proportion that are strong. Or it might have no effect. Since average ocean-surface temperatures have risen by about half a degree since 1970, this is not an idle question, and it has, indeed, been asked in the past. But it has been asked largely of the North Atlantic and North Pacific, because they are fringed by countries that can afford to do the asking. Dr Webster, by contrast, has looked at the whole planet—or, rather, the six ocean basins on its surface that act as hurricane nurseries.