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.

He and his team used satellite data to obtain consistent observations from around the world. (This was the reason they were able to go back only as far as 1970; before that, there were not enough observations.) Analysing the sea-surface temperatures in the six basins (the North Atlantic, the West Pacific, the East Pacific, the Southwest Pacific, the North Indian Ocean and the South Indian Ocean), they found statistically significant temperature rises in all but the Southwest Pacific.

Looking at the hurricanes themselves, though, they found no long-term trends in the number of storms per ocean basin or the length a storm lasts, except in the North Atlantic, where both increased. That is unfortunate news for Caribbean countries and the United States, which bear the brunt of those storms. But it suggests that whatever is increasing hurricane incidence it is not—or, at least not solely—to do with ocean warming. If it were, such increases would have shown up in other places where the sea is getting warmer.

Nor was there any increase in the maximum windspeed that storms attained anywhere. What there was, however, was a doubling around the world of the proportion of storms in the most destructive categories (4 and 5 on the Saffir-Simpson scale usually employed by meteorologists). And, although the exact rise in that proportion varied from basin to basin, all of them saw a significant increase.

What caused that increase is, of course, debatable—and since the second-largest percentage increase was in the Southwest Pacific, where no significant temperature rise was observed, leaping on changes in sea-surface temperature as the sole cause might be premature. But what Dr Webster and his colleagues have shown beyond much doubt is that something rather nasty has been happening. Time, perhaps, to batten down the hatches.