Fewer Hurricanes in the Greenhouse World

There's a reason Americans suddenly started to believe in the reality of climate change starting in late 2005 and early 2006, and it wasn't (only) Al Gore: they saw the devastation that hurricane Katrina wreaked, listened to scientists saying global warming made such storms more likely, and said "oh %^$#!."

So it will be interesting to see how the public reacts to new research saying that hurricanes may not be more frequent after all (though they will be more destructive).

Since at least 1950 scientists have known a warmer tropical Atlantic will increase hurricane activity there—other things being equal. As a result, the sea-surface warming caused by climate change has made some scientists, not to mention regular people, expect that the east and Gulf coasts of the United States will see more intense and more frequent hurricanes. (Those concerns have also raised home-insurance rates.) But while evidence from climate models as well as observations keeps accumulating for the “more intense” part, the “more frequent” bit is decidedly dicier.

So expect a paper published online this afternoon by Nature Geoscience to cause a bit of a storm, so to speak. In it, scientists describe the results of a climate model of the Atlantic basin which simulates hurricanes. The model retro-casts (that is, reproduces past events) the increase in the number of hurricanes that occurred between 1980 and 2006,. That gives the scientists confidence in what the model forecasts: by the end of the 21st century, they report, “Atlantic hurricane and tropical storm frequencies are reduced,” although the amount of rain each storm brings, as well as storm intensity, will “increase substantially.”

Since the mere mention of climate change scrambles some people’s brains, let’s take a moment to briefly note what’s known and indisputable. First, sea surface temperatures in the tropical Atlantic have risen over the past century. Human activities—the release of greenhouse gases, which warm the atmosphere—are partly responsible, though natural climate variation has also probably contributed. Second, the number and destructive potential of Atlantic storms have increased markedly since 1980, along with those sea-surface temperatures.

Now for the disagreements. Some scientists, looking at the observational record, conclude that greenhouse warming caused a substantial rise in Atlantic tropical storms during the 20th century. Others do not.

The new simulation isn’t all that surprising, given that even the scientist most strongly associated with the prediction that hurricanes will get fiercer—Kerry Emanuel of MIT—has already said that “global warming should cause hurricane frequency to fall.”Now, scientists led by Tom Knutson of the NOAA Geophysical Fluid Dynamics Laboratory calculate that there will be 27 percent fewer tropical storms and 18 percent fewer hurricanes by century’s end. The number of major hurricanes should fall by 8 percent.

Although the precise reasons for the reduced storm frequency haven’t been nailed down—the scientists basically built their computer and pressed “go,” but have not scrutinized what physical processes produced their results—one factor is straightforward physics. A warmer world is bringing enhanced vertical wind shear (wind shear means the different speeds of winds at different heights). As a result of greater shear, such as that expected in some regions more than others, a greater number of nascent storms will get ripped apart before they can get organized into a hurricane. But Knutson and his colleagues also suspect that “changes in circulation and/or moisture are more likely the dominant factors.”

So, fewer storms, but those that come will not be pretty. “More than twice as many hurricanes occur with wind speeds exceeding [100 miles an hour],” the new model predicts. Those storms will bring 37 percent more rain within 30 miles of their center.

Silver lining: the number of storms that make landfall should decrease even more than the number of hurricanes—30 percent vs. 18 percent. Oh, except for one little thing: because of uncertainties in the model, “it is plausible that the model’s quantitative projections of increased intensity and increased numbers of the most intense storms . . . are underestimates.” That is, there could be more hurricanes and more-powerful ones after all. Don’t count on home-insurance rates going down anytime soon.