For this episode of Eco Minutes, Dr. Enid Sisskin interviews Dr. Frank Gilliam, a professor in the Department of Earth and Environmental Sciences at the University of West Florida. Gilliam has researched how numbers and intensities of tropical cyclones has changed since records have been kept by the National Hurricane Center starting at about 1850. He is an ecosystem ecologist with an interest in forest ecosystems, especially longleaf pine. He received a B.S. degree from Vanderbilt University and a master's and Ph.D. from Duke University in Plant ecology. He has authored or co-authored more than 170 publications, including seven books and nearly 120 peer-reviewed articles.
Dr. Enid Sisskin: Frank, thank you for coming in today. What started your research of hurricanes?
Dr. Frank Gilliam: Going back even to when I was a young child, I was always interested in nature, asking questions about the environment and how organisms respond to their environment. So, my background began in biology and then ecology as I considered sort the environmental effects on organisms. And more specifically, to get back to the question of what started me being interested in hurricanes is an interest in the longleaf pine ecosystem. If you look at the distribution of longleaf pine in the southeastern United States, it's sort of a coincidence, with the frequency of landfall hurricanes. And sure enough, as part of the natural ecology of longleaf pine, are the effects of hurricanes on the longleaf pine ecosystem. Several years ago, I was interested in looking at some data from a colleague of mine that looked at the age patterns or age structure of longleaf pine in southern Georgia. And he saw sort of these peaks and troughs. Troughs where the old peaks from maybe 200 years ago or 300 years ago was best explained by the occurrence of hurricanes. And so, I had a data set that was similar, but it was in another part of the region -- it was in North Carolina. And I saw similar peaks and troughs in the old data from longleaf pine. And I thought, well, I wonder if that's related to hurricanes. So, I looked into hurricane history. It turns out that the National Hurricane Center has an archive data set that goes back to 1850. I started researching what are the frequencies of hurricanes going as far back as we have records, into the present. And what I was starting to see was a pattern that suggested that perhaps they're increasing in frequency and intensity over the many decades and centuries that we have records.
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Sisskin: Although most of us have experienced one. What exactly is a hurricane? Let's go to the basics.
Gilliam: Yeah, so a hurricane is a system that develops from the rapid release of surface heat from the ocean. And when that occurs, the result is often again all the different factors that give rise to hurricane formation. When you have that, a rapid rise creates a low-pressure system. And when that occurs, along with the Coriolis effect, that creates a vortex, a spinning, if you will, then you have the beginnings of hurricane formation. It starts as a depression. We call it a depression because it's a low-pressure cell. So just imagine a rapid up, rising, or uplifting of air that's going to create low pressure that's going to draw air into it. If you superimpose the movement of air into the low-pressure center along with the Coriolis effect, that's going to create a vortex. That's what begins that kind of motion. And if there's enough heat that's released and you have enough low pressure, then that's going to cause more and more movement of air into that -- increasing air speeds, decreasing pressure, and that's what leads to the formation of a hurricane.
Sisskin: So, what are the other factors that could lead to hurricane formation?
Gilliam: Okay, as I said, the main sort of what we call the heat engine is the heat that is absorbed in the surface of the ocean. Along with that, you need to have a lot of moisture in the atmosphere. And that is quite often found in tropical systems. You also have to have, this is kind of a fancy word, what's called a high lapse rate. That's just a fancy word for what happens when you have a temperature that decreases from the surface up into the atmosphere. So, a high lapse rate is what is needed for a hurricane. That just means that air temperature decreases very rapidly as you get away from the Earth's surface. Again, you have to have this vortex to form, and that's the result of the Coriolis effect, one of the interesting things. So I published a paper I'll talk about later on, and I was able to access storm tracks, historical storm tracks for the last many, many decades. And even though the equator, right in the middle of the tropics, has the highest temperature of water. There are no storms that form there because there is no Coriolis effect at the equator. So that's another thing that is required. The other is sort of something that you can't have to have a hurricane form, which is high winds aloft. And so we call that wind shear. For example, that's one of the reasons why we've had a kind of cessation of storms recently in the last several weeks. We have really warm ocean temperatures right now. But we have systems like the high pressure that's dominating our weather recently here in Pensacola, that's creating high winds at high altitude, and that basically sort of shears off the formation of storms.
Sisskin: Let's talk a little bit about climate change. You talked about the factors that control the strength and path of hurricanes. How could this be influenced by climate change?
Gilliam: Two variables. Of all the different factors that give rise to tropical cyclogenesis, two factors that are related to climate change would be the surface sea temperatures as well as the humidity of the upper atmosphere, both of which have been shown to be increasing over the last several decades. If you look at comparisons of numbers and intensities of storms, they are significantly, in a statistical sense, increasing over time. And so, one of the things I put in this paper I published a couple of years ago is what is considered average. Back in the mid-19th century into the early 20th century, on average, we would expect fewer than 10 storms a year. Now it's about 14 to 15. So, it's kind of what we call a new normal. I won't say I chuckle, but I always kind of think about this. When the predictions come out for a given year, and they'll say it's going to be an average year only about 15 storms. I think, well, that's twice what it used to be back 100 years ago. So, it's clear that the number of storms has increased. But also, if you look at the intensity of storms ... in other words, the category 3, 4, and 5, those are the major hurricanes. We have a higher frequency of major hurricanes now than we used to as well.
Sisskin: So, you've been talking about how climate change influences hurricanes. What is the evidence for and against climate change?
Gilliam: Well, the evidence is quite clear that we have warmed the climate on a global scale. If you look at long-term patterns of air temperature, it was fairly unchanging for about a hundred-year period from the mid-1800s to about 1950. But if you look at the period from that point to the present, there's a highly significant increase in air temperature. Of course, the variable that's more important for tropical cyclones is surface sea temperature. As it turns out, that surface sea temperature change parallels almost exactly what we see in air temperature. So, surface sea temperatures are going up on a global scale as well.
Sisskin: In the last few years, we have seen records every year on rapid intensification storms that are going from tropical storms to category 3, 4, and 5 hurricanes in 24 to 48 hours. Do you see this as the wave of the future?
Gilliam: Yes, I think it's kind of a really dangerous combination of the fact that both of those are happening. Rapid intensification and more slowly moving storms. As I said, one of the problems with Sally in this region was how slow it moved, and that put us in the eye wall for such a long time.
Sisskin: Can tropical cyclones ever be beneficial?
Gilliam: Yes, that's a really good question. I developed a sort of conceptual model with a colleague of mine, Dr. Bill Platt, who's retired, from Louisiana State University. And this model that we talk about refers to two different disturbances that maintain the structure and function of longleaf ecosystems. Fire is essentially unrelated to tropical storms, but fire is an important part of longleaf systems that primarily affects the ground cover, where you find all the diversity of the longleaf ecosystem. Once a longleaf stem gets above a certain height, there's not much that's going to kill it. It's very resistant to disease and to attack by insects. So, the main cause of mortality of mature pines would be tropical storms. As they come through, they have an effect of toppling over some of these larger stems. And that, even though it sounds damaging, is actually beneficial in terms of increasing light availability that allows more juvenile ponds to survive. In the past millennia, these ecosystems evolved with frequent storms. And so that's again a part of the ecosystem.
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Sisskin: We have some listener questions that were sent in. And the first one is: When are storm names retired?
Gilliam: OK, good question. For one thing, storm names are not generated by the National Weather Service or the National Hurricane Center, but instead by the Meteorological Organization. And so those are set up to repeat themselves. I guess it was ever since 1979 that they started this. Every six years, you get the same names that are applied to the storms. Whether one gets retired has to do with the damage and loss of human life. And once you have a storm that seems to fit those criteria. There is a petition, sort of an application to the world organization, to retire that storm.
Sisskin: Interesting.
Gilliam: And what I found interesting is that I will talk to my classes about the science behind climate change and how that relates to tropical cyclogenesis. And I put up the names of the storms from a few years ago to a few years from now, just to ask them (if) your name is up there. For example, Frank will never be a storm. Frank is going to be for Pacific storms, not for the Atlantic Basin.
Sisskin: Why?
Gilliam: I don't know. That's just the way they set that up. And so, the F storms have been things like Fran, Francis, and Felix. But I will point out somewhat humorously that amongst the recent retirees is my wife, named Laura, our son's name Ian, and our dog's name Fiona. So that's kind of interesting.
Sisskin: Now, here's a question that I've always been curious about. If a hurricane has a closed circulation, you know it's going around in a circle, right? And when you see it coming, the northeast quadrant and the east side is very, very strong. But very often the west part is absolutely nothing, barely any winds at all. If you're on the west side of the storm, you practically have a sunny day. How is that possible?
Gilliam: Okay, so from, let's say, an aerial tropical storm, quite often looks symmetrical, but the energy content of those storms is not symmetrically distributed. So, you have the highest energy in that front right sector. And the analogy I would use would be, if you remember the scene in “A Charlie Brown Christmas” where they're on the ice and they're kind of going around and around in circles, that one part of the going around, you go faster, and then you slow down. So, basically, think about that. That it's going fast when it goes forward and slower when it's going back, because it's going against, as it comes backwards, it's going against the forward motion of the storm. So that creates an asymmetrical distribution of energy.
Sisskin: And so, we can keep hoping that we're going to be on the west side.
Gilliam: Yes, absolutely.
Sisskin: OK, this was an interesting one. Given the cuts across the federal government at the National Weather Service and FEMA, no more satellite data from the Defense Department, and other disruptions in the prediction of planning for, and dealing with the aftermath, how are citizens going to be affected by changes that have taken place at the federal level?
Gilliam: Well, that's going to be just speculation, on my part, but I know that the ability for citizens to be prepared has a lot to do with the facts and the data that are collected by these government agencies. And as they are further and further winnowed down in their size, they're just going to be less and less effective about getting the data that we need to make predictions and to make preparations.
Sisskin: I understand that the models we have now we are as accurate at five days. We used to be at one or maybe two days prior to landfall. And this is due to all of the research done ah at NOAA and the National Weather Service.
Gilliam: I've been really impressed, as long as I've been tracking these things, is how much improvement there is in predictability. And so, I think that what's happening now with the cutbacks is really just going backward. We're going to be losing predictable information.
This interview has been condensed and edited.
