Hurricane History: Natural Cycles or Global Warming?

Alex Chadwick talks with Kerry Emanuel, professor of earth, atmospheric and planetary science at the Massachusetts Institute of Technology, about the history of hurricanes, cycles of hurricane intensity and a possible link between massive hurricanes and global warming.

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ALEX CHADWICK, host:

This is DAY TO DAY. I'm Alex Chadwick.

There are many predictions that Hurricane Katrina will turn out to be the nation's most expensive natural disaster, but it's certainly not the first to leave an entire region reeling from its impact. Here to provide historical context is Kerry Emanuel. He's a professor of earth, atmospheric and planetary science at the Massachusetts Institute of Technology and the author of a new book, "Divine Wind: The History and Science of Hurricanes."

Professor Emanuel, welcome to DAY TO DAY.

Professor KERRY EMANUEL (Massachusetts Institute of Technology; "Divine Wind"): Glad to be here.

CHADWICK: Was Hurricane Katrina an especially powerful storm, one of the most powerful to ever hit the country?

Prof. EMANUEL: It certainly was one of the most powerful storms to hit the country. There have been storms whose wind speeds have been larger than Katrina. For example, the Labor Day storm of 1935 which devastated the central part of the Florida Keys was technically a more powerful storm, and even Camille had higher wind speeds. But Katrina also had the property of having an exceptionally large diameter, and the combination of that and quite high winds was certainly a contributor to the devastation.

CHADWICK: Or maybe it's just where it hit; it's so populous.

Prof. EMANUEL: Well, that's right.

CHADWICK: How does Katrina compared to the devastating hurricane that hit Galveston, Texas, in 1900?

Prof. EMANUEL: Well, I think when we get to looking at the records, Katrina will probably be rated as at least as powerful as that storm, but, of course, back in 1900, there was very little warning of such an event coming along. There were weather forecasters. They did suspect that there was a storm coming along, but they had no really good measurements in the Gulf. And in those days, it wasn't common to evacuate people in advance of an event like that.

CHADWICK: In your book, you say that recent changes in hurricane prediction technology have come along. How is that changing things?

Prof. EMANUEL: Well, I think now we have pretty good advance notice of a hurricane coming along. In the case of Katrina several days in advance it was fairly clear that that hurricane would strike fairly close to New Orleans and it allowed people the time, the option to evacuate. I might add, on the other hand, that two days ahead of time, it was not clear at all how strong Katrina would be when it did hit land. And this illustrates a real thorn in the side of hurricane prediction. We're still not very good at predicting the changes in the intensity of hurricanes. We're working on this, but it's obviously something we have a lot more work to do on.

CHADWICK: What about the speculation that global warming has something to do with these, I think, increased ferocity of hurricanes over the last 20 years?

Prof. EMANUEL: Well, it's a question of your horizons. If you're worried about global hurricane activity--remember that only about one in 10 storms occurs in the Atlantic; the rest occur elsewhere--and if you consider hurricanes over their entire life and not just when they make landfall, you really do see an upward trend in the power of hurricanes, not in their frequency but in the magnitude of the wind speed and also in their duration. But you really can't see such tendencies in landfalling storms in the US simply because the numbers are so small. You're just--it's impossible statistically to detect any kind of meaningful trend in that.

CHADWICK: So you're saying you can't really answer this kind of question about whether global warming had some role in this.

Prof. EMANUEL: That's right. The element of chance in a landfalling hurricane in the United States is so large that it swamps any effort to detect any sort of trend, but when you look at the global statistics of hurricanes all over the world, then you do see a trend which some of us believe is attributable to global warming.

CHADWICK: You believe that.

Prof. EMANUEL: I believe that. I didn't believe it seven or eight months ago before I actually did the work, but when you look at a particular measure of hurricane activity which is essentially a measure of how much energy the storms generate over their life, that's gone up so much and so beautifully in correlation with the rise in tropical ocean temperatures, which again globally is attributable, we think, to global warming, that the signal is pretty unmistakable.

CHADWICK: You're saying you were a doubter of this theory seven or eight months ago. You now believe it.

Prof. EMANUEL: That's right. I mean, actually I developed some of the theory that suggested that hurricane intensity should go up with tropical ocean temperature, but the predictions were very modest compared to what we actually have seen in the last 30 to 50 years.

CHADWICK: Kerry Emanuel is professor of earth, atmospheric and planetary science at MIT, and he's the author of the new book "Divine Wind: The History and Science of Hurricanes."

Professor Emanuel, thank you.

Prof. EMANUEL: You're quite welcome.

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