AMY GOODMAN: This is New Orleans Mayor, Ray Nagin.

MAYOR RAY NAGIN: Ladies and gentlemen, I wish I had better news for you, but we are facing a storm that most of us have feared. I do not want to create panic, but I do want the citizens to understand that this is very serious, and it’s of the highest nature, and that’s why we are taking this unprecedented move. Every person is hereby ordered to immediately evacuate the City of New Orleans, or if no other alternative is available, to immediately move to one of the facilities within the city that will be designated as a refuge of last resort.

AMY GOODMAN: The City of New Orleans is below sea level and faces major flooding from Hurricane Katrina. More than half a million people have been ordered to evacuate the city, and the storm could leave up to one million people homeless. President Bush declared a state of emergency for Louisiana and Mississippi over the weekend. Federal emergency workers are preparing staging centers to deal with the fallout of the storm. As Katrina pounds New Orleans, we’re going to look at how this storm compares to others that have hit the United States and at the link between climate change and hurricane strength.

Tropical storms may be growing in overall intensity due to human induced global warming, this according to a new study by leading hurricane researcher, Kerry Emmanuel. His report, which has generated controversy among climate specialists, was published earlier this month in the journal, Nature. Emmanuel looked comprehensively at storm data since the mid-1970s and concluded the destructive power of hurricanes has nearly doubled over the past 30 years, at least partially because of global warming. Critics of the study say hurricanes are not intensifying and that the cause of the rising ocean temperature is natural, not man-made.

We’re a joined on the phone right now by Kerry Emmanuel from Massachusetts, Professor of Meteorology at the Massachusetts Institute of Technology. His new book is called Divine Wind: The History and Science of Hurricanes. Welcome to Democracy Now!

KERRY EMMANUEL: Glad to be here.

AMY GOODMAN: It’s good to have you with us. Well, Professor Emmanuel, can you simply start off by explaining what we’re seeing in New Orleans right now?

KERRY EMMANUEL: Well, this is one of the scenarios that have given weather forecasters nightmares for many decades. We have a very strong hurricane. It’s fortunately a little bit weaker than it was yesterday, category four, but it’s still about as strong as Andrew was when it went into Florida back in 1992. And it’s also going a little bit east of New Orleans, but that means that New Orleans right now is experiencing very strong winds from the northeast. It’s piling up water in the lake, and if history is any guide, there will be a lot of flooding in the city. So, it’s a very good thing, I think, that the mayor ordered an evacuation yesterday.

AMY GOODMAN: Professor Emmanuel is author of Divine Wind. Now, these two words that we almost never hear in weather coverage in this country, global warming, can you talk more about this phenomenon and whether it relates to what we’re seeing right now in New Orleans?

KERRY EMMANUEL: Well, it’s very hard to say that, and the problem is purely a statistical one. What my study did, the one that was alluded to earlier, is to look at hurricanes worldwide and not just at the time that they make landfall, but during their entire lifetimes over the ocean, and when you do that and you look at their intensity and you look at how long the hurricanes lasted and you measure the total amount of energy released by the hurricanes, that is going up decidedly in most of the world’s oceans, and we have tried very hard to see whether this might be an artifact of the way hurricanes are measured or the data, but no matter what you do, you get this signal. And that signal lies on top of regional phenomena.

So if you look at just the Atlantic, and everyone’s always focused on the Atlantic, which has only 11% of the total number of storms in the world, what you see is a dominance of perfectly natural cycles that tend to last a few decades. So, unfortunately in the 1970s and 1980s, we were in a lull, and during that lull, the population of the coastline in the U.S. increased quite a bit, and a lot of construction went on very close to the coastline. Those natural signals very much dominate any signal you would see from global warming. So, of course, it’s tempting. We have had this very active last ten years, along the U.S. and gulf coasts to blame that on global warming, but looking at it statistically, that’s a very difficult connection to make. I think what you are seeing mostly is a natural cycle in this case.

AMY GOODMAN: Professor Emmanuel, speaking to us from Massachusetts Institute of Technology. Can you talk about the significance of what it means to be at or below sea level?

KERRY EMMANUEL: Yes. I mean, New Orleans is in that situation. It’s — the Mississippi River is held back by a long series of levees, and this has been a big source of concern. The Army Corps of Engineers over the years have done a great job trying to build up those levees and to put in massive amounts — massive system of pumps to try to keep the water out of the city. And I’m not intimately familiar with the system, but I understand that it’s been designed to handle as much as a category three hurricane. But most of the people down there yesterday were fearful that it would not be able to handle a storm of this magnitude. And what happens is that the combination of the storm surge, which is created by the winds of the hurricane piling up the gulf waters into the lake in this case and the enormous amount of fresh water starting to come down the Mississippi River because of the torrential rains would put the city in danger of having those levees over-topped, and water then would rush into the city. This has happened before. It happened in Hurricane Betsy in 1965, and a lot of the city went underwater.

AMY GOODMAN: Can you talk overall about your new study, talking about the destructive power of hurricanes in the North Atlantic and the North Pacific in the last 30 years?

KERRY EMMANUEL: Sure. It took us a little bit by surprise for two reasons. There is a body of theory that’s been developed over the last 20 years that suggests that the intensity, as measured by, say, the maximum wind speeds in the hurricane, should go up if you warm the tropical oceans. And the calculations we did suggested that for every degree centigrade of warming of the tropical oceans, you might get about a 5% increase in the peak winds. The data, when people started to look at the historical hurricane data, the first thing you look at is the frequency of events. That is, how many hurricanes occur every year around the world, and if you look at that, you don’t see any trend. It’s a fairly constant number. There are about 90 storms worldwide per year, and that’s not changing very much. So, what we thought a few years ago was, well, global warming should have an effect on hurricanes, but it’s small enough that you won’t really see it for many decades to come.

When we looked at this other measure of total energy consumption by hurricanes, we were a bit surprised to see a much bigger trend than one had expected. And in hindsight, part of the reason for that is that these storms seem to be lasting longer. That is, they’re maintaining high intensity for a longer period of time, and this is certainly contributing to this. But I think it’s fair to say that we don’t fully understand it. The intensity seems to have gone up a little bit more than we would have expected it to, based on theory and models of global warming. And so, we’re in the process now of trying to reconcile the theory with the observations.

AMY GOODMAN: And for those who criticize you, those scientists like William Gray of Colorado State, saying that hurricanes are not intensifying and that the cause of rising ocean temperature is natural, not man-made?

KERRY EMMANUEL: Well, Bill has spent his career looking at the Atlantic hurricanes. And I want to emphasize again that the Atlantic is only 11% of the total. If you look at the Atlantic, it’s perfectly fair to say that both the increase in ocean temperature in the last couple of decades and the upswing in hurricane activity is mostly natural. If there’s a global warming signal in that, it’s very hard to see. And that natural cycle, we don’t fully understand it, by the way, I don’t think anyone pretends that we do, but there have been in history, you know, periods of 20 or 30 years of inactivity followed by 20 or 30 years of activity. It’s nothing new, in fact. Before the 1990s, a lot of hurricane specialists had forecasts that we were going to go back to an active period in the Atlantic, and again, this has nothing to do with global warming.

So, when you focus on the Atlantic, it’s the natural cycles that are the big thing, at least up until now. And I don’t think anybody, certainly I wouldn’t look the at the Atlantic record all by itself and say, oh, there’s global warming. It’s when you look at the global record of storms that this signal really starts to stand out. And we’re interested in that, of course, because we’re interested in how the climate responds to global warming. But there is the other question of what does this mean for actual damage done by hurricanes? And the surprising answer is that in the last — next 20 years or so, probably not much, simply because of statistics. I mean, big hurricanes hitting land are a comparatively unusual phenomenon, only a few per year globally. And you just don’t expect to see a trend like this in the land-falling statistics, at least not for another 50 years or so.

AMY GOODMAN: Professor Emmanuel, I want to thank you for being with us, professor of meteorology at the Massachusetts Institute of Technology, author of the book, Divine Wind.