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PRI's Environmental News Magazine

Hurricanes And Climate Change

Air Date: Week of September 2, 2005

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Kerry Emanuel. (Photo: Bruce Gellerman)

MIT Professor Kerry Emanuel talks about his book “Divine Wind: the History and Science of Hurricanes.” Emanuel’s latest research, published in Nature Magazine, shows a startling global increase in hurricane strength and duration, which he correlates to rising sea temperatures linked to global warming.


TV ANNOUNCER: And welcome back to our ongoing coverage of the aftermath of hurricane Katrina. I’m Kim Perez…

GELLERMAN: Kerry Emanuel doesn’t usually watch much tv, but as the devastation of hurricane Katrina unfolds he’s been glued to the Weather Channel on the little television he keeps in the kitchen of his home in Lexington, Massachusetts. These days, he’s resisting the temptation to say “I told you so.”

TV ANNOUNCER: Part of the Twin Span bridge on I-10 into New Orleans is gone…

GELLERMAN: Kerry Emanuel is a professor at MIT’s Earth, Atmospheric and Planetary Sciences. He has a new book called “Divine Wind: The History and Science of Hurricanes.” And his latest research, published in a recent issue of Nature Magazine, correlates the greater intensity and frequency of hurricanes with global warming. Even he was surprised by what he found.

Kerry Emanuel. (Photo: Bruce Gellerman)

EMANUEL: Well I looked at the record of hurricanes in the Atlantic and the western part of the North Pacific, and I looked at a measure of the production of energy by hurricanes over their entire life. When you look at this measure of energy consumption it’s gone up by about 70 or 80 percent since the 1970s. It’s a really big increase. It was startling. And we’re trying to understand why.

Part of it I think we do understand: that when we looked at the whole issue of wind speeds, we didn’t think about the issue of the duration of events. And this particular metric that’s gone up 70 or 80 percent is also depending on the duration of storms, and that duration has gone up quite a bit. It’s gone up about 50 percent in the last 30 years. And that explains part of the difference, but not all of it, to be sure.

GELLERMAN: Well, how close is the correlation between the surface sea temperature and hurricanes?

EMANUEL: Well, this particular measure of energy consumption is very closely tied to sea surface temperature.

GELLERMAN: So, the higher the temperature of the sea surface, the more intense and the greater the duration of hurricanes?

EMANUEL: That’s right. And we predicted that you should see about a ten percent increase in wind speed for every two degree sea change. That theoretical prediction has been backed up since then by lots of modeling that has been done elsewhere by other groups.

GELLERMAN: So what you expected to see was a nine percent increase and you’re actually getting 80 percent?

EMANUEL: Yeah, 70 to 80 percent. So the prediction was way off.

GELLERMAN: So you have scientists who are supplying you with the global warming data?

EMANUEL: That’s right.

GELLERMAN: And you feed that into your models.

EMANUEL: Mmm-hmm.

GELLERMAN: And, so right now you’ve gotten, what, about a five temperature degree increase in the surface temperature of the water?

EMANUEL: That’s right.

GELLERMAN: And it could go up one degree? Two degrees?

EMANUEL: That’s right. The predictions are one to two degrees. Now, ironically, one of the things that isn’t in the models is the feedback to the climate system from hurricanes themselves. We always talk about hurricanes responding to climate change as a one-way proposition--climate changes, the hurricanes respond.

It’s quite possible that it isn’t one-way. It may very well be two ways: that the hurricane activity feed back on the climate system. And one of the ways that that can happen is through the effects of hurricanes on the upper ocean, which is really what motivated my Nature study to begin with. They churn cold water to the surface, and they also export heat in the ocean to high latitudes.

So, one of the theories about this feedback would suggest that the effect of hurricanes on global warming itself would be to reduce that warming in the tropics, but to increase it at high latitudes. We think. And if that happens, then the projected warming at the latitudes of, say, New England or Europe, would be more than what is being forecast. But, the warming of the tropical regions would be somewhat less than is forecast.

GELLERMAN: When you look at the planet globally and you’re looking at hurricanes, what is the effect of man-made warming upon the intensity and duration of the hurricanes we have now, and may have in the future?

EMANUEL: We think we’re seeing a signal that the intensity of hurricanes is going up owing to global warming, and their duration is increasing, as well. And this has us worried. In terms of the influence of this on the rest of the world, I think it can’t be stressed enough that in the United States we have been enormously successful in reducing the loss of life. As horrible as Katrina has been – and it is horrible, sort of a worst-case scenario – so our problem is economic. That’s our big problem.

But in the rest of the world, in the developing world, the problem is loss of life. You know, tropical depression Jean last year – it was just a depression – killed almost 2,000 people in Haiti. Hurricane Mitch in 1999 killed 11,000 people in Central America. And a decade before that, a hurricane in Bangladesh killed 100,000 people. Now, this is really, really terrible, and our suffering ought to be weighed against that.

GELLERMAN: Professor, when you watch the Weather Channel and see the destruction from this Hurricane Katrina, do you say, “I told you so”?

EMANUEL: No, I think we’re not quite that heartless to think that. It’s a terrible, terrible scene of devastation. And I think I speak for my colleagues that, about the time Katrina came off the west coast of Florida and emerged into the Gulf, and the track forecasts were revised to bring it over or near New Orleans, we all felt this terrible sense of dread because people had talked about nightmare scenarios. We sort of played them through in our own minds and in our discussions, and this was certainly one of them.

And on top of that, Katrina’s track took it over a feature in the Gulf of Mexico called the Loop Current, where the warm water that you normally find in the Gulf in the summertime runs particularly deep. And we have known for some years that that’s very favorable for the rapid intensification of hurricanes.

GELLERMAN: So do you expect to see more hurricanes of the sort of Katrina coming our way in the next few years?

EMANUEL: Well, yes and no. Certainly we’re going to a lot of Cat 4 and 5 storms. Whether they hit land, and whether they hit a populated part of the land, is entirely a matter of chance and that makes it impossible to answer your question in a reasonable way.

Let’s just take Hurricane Brett. Now, not too many people, unless you live in Texas, remember Hurricane Brett. It was 1999. It was a Cat 4 hurricane when it went into the Texas coast. If it had gone into Houston or Galveston it would have been horribly devastating.

What really does make a difference, of course, is that the population of these coastal regions has exploded. And people are…their risk is being subsidized by federal policy, and state and local policy, and that is what the real problem is. It’s not meteorological, it’s societal. We’re putting all this wealth in harm’s way and we’re getting creamed.

GELLERMAN: Do you do anything different here, New England, to prepare your house, to prepare your family, for disasters?

EMANUEL: Massachusetts actually is an interesting place. It does have a decided hurricane risk. Right now, it is almost impossible to buy a private insurance policy, homeowner’s policy, in Cape Cod because almost all of them have pulled out. They’ve pulled out because new estimates of hurricane risk in Cape Cod suggested that they couldn’t charge a premium high enough to cover their exposure. The state won’t let them, and even if it did, people probably couldn’t afford it. And so we’re in kind of a crazy situation where very wealthy people on the beach in Cape Cod are being insured by a state insurance pool which was set up primarily to insure the uninsurable poor in inner cities. And guess who’s paying for that? The rest of us.

GELLERMAN: Well, meteorology’s about the prediction business. Looking forward, what do you see happening?

EMANUEL: Certainly I think most of my colleagues would join me in saying the next ten years are going to be a rough ride in the Atlantic, even forgetting about global warming because of these natural cycles. We’re in an upswing and this isn’t rocket science, we just look at the past record and extrapolate it into the future. We’re in an upswing, it’s bound to last another five or ten years. And if we have levels of hurricane activity that we had in the 1940s and 50s we’re in trouble.

Now, what has everybody in my profession so concerned – and we’ve been concerned for decades – is the confluence of a huge upsurge in the coastal population with a natural upswing in the number of storms in the Atlantic. And maybe global warming, you know, if you wait long enough will also start to show up in those statistics.

GELLERMAN: So global warming right now you don’t think is having an influence? Or it is having an influence?

EMANUEL: No, if you look at the global record of hurricane activity, you do see a pronounced upward trend that began in the 1970s, which is very highly correlated with an upward trend in the tropical ocean temperature. And the people who study tropical ocean temperatures believe that this recent upward trend is mostly a consequence of global warming, and that’s why we’re worried that we’re now seeing a global warming signal in hurricanes. But the big near-term problem is demographic and natural.

GELLERMAN: Professor, thank you very much.

EMANUEL: You’re quite welcome.

GELLERMAN: MIT Professor Kerry Emanuel’s new book is called “Divine Wind: The History and Science of Hurricanes.” His latest research appears in the magazine Nature. You can find the link on our website, www.loe.org.

[MUSIC: Arvo Part “Lamentate: Spietato” from LAMENTATE (ECM – 2005)]



Kerry Emanuel’s paper in Nature

“Divine Wind - The History and Science of Hurricanes” by Kerry Emanuel


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