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Hurricane Science

October 18, 2005 at 12:00 AM EDT
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JEFFREY BROWN: Arlene in June, Katrina and Rita in August and September, and now Wilma in October. It’s been a storm season for the record books, and one that has raised many questions about why so many hurricanes and, especially, why so many very powerful hurricanes?

We explore that now with two experts in the field: Christopher Landsea is a meteorologist with the hurricane research division at NOAA, the National Oceanic and Atmospheric Administration; Judith Curry is a climate scientist and chair of the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology. She recently co-authored a study on this subject in the journal Science. Welcome to both of you.

Starting with you, Christopher Landsea, are we in fact in a particularly intense period of hurricanes?

CHRISTOPHER LANDSEA: Oh, without a doubt. This last season has been extremely busy with 21 storms; 12 have become hurricanes already. But it’s not just been this year or even last year. It’s been over the last 11 years since 1995, we’ve seen a very busy hurricane era.

JEFFREY BROWN: And what explains it?

CHRISTOPHER LANDSEA: Well, two things: first we see changes in the ocean that gets slightly warmer waters by about a half degree Fahrenheit or so, both in the tropical Atlantic and the Caribbean, but also we see changes in the atmosphere that allow the hurricanes to become stronger, less wind sheer disrupting the storm circulation

JEFFREY BROWN: Ms. Curry, do you agree that we’re in a particularly intense period and, if so, what causes it?

JUDITH CURRY: Well, absolutely we’re in an intense period. And it’s not just the North Atlantic. It’s globally. I mean, that was the focus of our study, was to look at the trends in global hurricanes, and what we found was over the last 35 years that there was basically a doubling of the most intense hurricanes, the Category 4s and 5s, not just in the North Atlantic but in all of the ocean basins where we have hurricanes.

In terms of what’s causing it, it’s very complex. It’s associated with temperature increase, surface temperature increase in the tropical oceans, and that is — depends in complex ways on natural and forced variability. I don’t know how much you want me to talk about that right now.

JEFFREY BROWN: Well, you raised in your paper the question of global warming, and I know that’s been on a lot of people’s minds. To what extent is that a factor?

JUDITH CURRY: OK, first in our paper, we didn’t say anything at all about greenhouse warming. It’s a global increase in tropical sea surface temperatures. In the subsequent press releases and press conferences, everybody is asking us the question about is this greenhouse warming? And the answer is partially the warming is associated with greenhouse warming and the burning of fossil fuels. To what extent, you know, that’s debated by scientists. Nobody is arguing that increase is 100 percent due to greenhouse warming, but clearly a portion of it, very likely up to 50 percent or 60 percent, of that increase that we’ve seen over the period is associated with greenhouse warming.

JEFFREY BROWN: All right well let’s start with that question, Mr. Landsea. What’s your response on that?

CHRISTOPHER LANDSEA: Well, we certainly see substantial warming in the ocean and atmosphere over the last several decades on the order of a degree Fahrenheit, and I have no doubt a portion of that, at least, is due to greenhouse warming. The question is whether we’re seeing any real increases in the hurricane activity. And the study that the authors Judy Curry and Peter Webster and company have done, you know, they are very well renowned scientists in field, and anything that they’re putting together needs to be taken seriously.

But I think it’s a little more complex than what they’re suggesting. For example, for the Atlantic, this is the only part of the world we’ve been flying into hurricanes for 60 years. And so if you go back another 20 or 30 years we’d see for the Atlantic, instead of a trend up, we’ve seen instead a cycle of activity — the ’40s, ’50s, ’60s were busy, ’70s ’80s, and ’90s were fairly quiet, and then, the last 11 years, as I mentioned, are very been busy, so instead of a trend we’ve seen a cycle on the Atlantic.

JEFFREY BROWN: Ms. Curry, how do you address that? I mean, I noted — I’m sorry –

JUDITH CURRY: First, I agree with what Chris says. I mean, there have been ups and downs in the temperature record over the last 100 years. You’ve seen the hurricane activity correlate fairly well with the ups and downs in Atlantic Sea surface temperature. But what you’re seeing in the last 35 years, this is a period where we’ve had the — seen the biggest signal associated with greenhouse warming.

Over half of the addition of greenhouse gases in the post-industrial age has occurred since 1970. So this is a period where we’re seeing the greatest forcing associated with burning of fossil fuels. There’s a lot of cyclical variability in all the ocean basins associated with natural variability, El Ninos and some decadal and even longer variations.

But underlying that, you still do see this trend of warming that is global, and it appears to be associated, at least in part, with greenhouse gases.

JEFFREY BROWN: And, Mr. Landsea, you’re putting more emphasis on the natural cycles you referred to, but what causes those?

CHRISTOPHER LANDSEA: Well, with the Atlantic hurricanes in particular, they’re due to changes both in the ocean as well as the atmosphere. Just changing the ocean where it’s a little bit warmer isn’t sufficient. What you have to do is also get less wind sheer disrupting the hurricanes. And where we’ve identified this is a natural psyche they’ll goes back and forth, there’s kind of a feedback between the ocean and the atmosphere working together. And it may be different from the mechanisms of global warming.

When we look at the theory of what global warming suggests hurricanes will change is that in the order of about 80 years or so, near the end of the 21st Century even if we have say, a three or four degree Fahrenheit warming of the tropical oceans, we’re looking at about a 5 percent increase in winds and about a 5 percent overall increase in rainfall. That’s a fairly tiny change a long way in the future.

And what it implies for today, even if they’re off by a factor of two, is the hurricanes like Katrina and Rita may have been stronger due to global warming but maybe by one or two miles per hour. At least this is what the theory and the numerical modeling suggests to us today.

JEFFREY BROWN: But for the layman, when we see something like Katrina, should we think of it in terms of some larger pattern, or does each hurricane have certain factors that contribute to how large it’s going to be?

CHRISTOPHER LANDSEA: Well, that’s true in both aspects. The hurricane intensity depends on not only the warmth of the ocean but if it’s warm through a fairly deep layer, how much moisture it’s accessing in the lower levels of the atmosphere, and how much wind sheer is disrupting it. We know on these longer time scales, 25 to 40 years busy, 25 to 40 years quiet, we see changes in both the ocean and atmosphere that either promote more hurricanes in the busy period or diminish them in the quiet periods.

JEFFREY BROWN: And, Ms. Curry, another layman’s question just to help us understand this, how good is the data? We’re talking about going back 30, 50 years here. Has the technology advanced to help us understand these things? Do you have enough history there to understand it?

JUDITH CURRY: OK, you never have as much data as you would like, and it’s never as high a quality as you’d like. But this is a relatively clean data set compared to some of the other climate data records that have been in the news in recent years. Relative to the paleoclimate data record, the hockey stick, this is much cleaner, relative to atmospheric temperature trends, this is much cleaner and there certainly have been variations in terms of what different weather services, how they’ve classified the storms, but we’re looking at satellite data sets, the basic weather satellites. You don’t see any big jumps in the way it’s been processed.

So I think the data is pretty good. I hope that Chris and other hurricane research will collaborate with climate researchers to reprocess this data set and try to work out any inhomogenates in the data set, but since the signal was so large, especially of the strongest hurricanes, that I don’t think that our result is going to change in any big way as a result of reprocessing the data.

JEFFREY BROWN: You wanted to jump in on this data question?

CHRISTOPHER LANDSEA: Yeah. The methodology of the study, I have no problems with that. It’s the data that I am concerned may be biasing the results. At the start of the study in 1970, while there were satellites peering down on the earth, there was no objective way to take a satellite picture of a hurricane and say what the winds were.

It wasn’t until the mid-’70s that is a technique was even invented, and it wasn’t until the mid-’80s before it was used globally and used all hours of the day and night. So I’m afraid that the changes in the satellites themselves, they’re more numerous, they’re giving more information. The techniques have changed. I’m concerned that that’s giving the big jump in the number of Category 4s and 5s that may be artificial.

JEFFREY BROWN: Either way, very briefly, do you see this period continuing?

CHRISTOPHER LANDSEA: Unfortunately, yes, it’s probably going to continue another 10 to 20 years of busy hurricane activity.

JEFFREY BROWN: OK, Christopher Landsea and Judith Curry, thank you both very much.