Gauging the Impact of Climate Change on Hurricanes

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Typhoon Haiyan slammed into the Philippines with sustained winds of 195 miles per hour, making it one of the strongest tropical storms to date. How much does climate change impact these big storms? Can we expect more in the future? How reliable is our current data on hurricanes?


This is SCIENCE FRIDAY. I'm Ira Flatow. Typhoon Haiyan barreled into the Leyte Gulf of the Philippines this week killing thousands of people. It brought 13-foot storm surges, 195 mile per hour winds. Super typhoons, Franken-storms, superstorms, we call them hurricanes, and in Asia they're typhoons. But it seems like each year a record-breaking tropical storm batters some coastline worldwide.

Are these storms actually getting bigger or is this all part of a natural cycle? Models of climate change predict more intense storms. When can we point to a storm and say, just as predicted by climate change research? That's what we'll be talking about this hour. Our number is 1-800-989-8255. 1-800-989-TALK. You can also tweet us @SciFri and go to our website, at ScienceFriday.com to join the discussion there.

Let me introduce my guests. Kevin Trenberth is a distinguished senior scientist in the climate analysis section at the National Center For Atmospheric Research in Boulder, Colorado. Welcome to SCIENCE FRIDAY. Welcome back.

KEVIN TRENBERTH: I'm good. Good day to you, Ira.

FLATOW: Thank you. Gabriel Vecchi is a research oceanographer in the Geophysical Fluid Dynamics Lab at the National Oceanic and Atmospheric Administration in Princeton, New Jersey. Welcome to SCIENCE FRIDAY.

GABRIEL VECCHI: Oh, thank you. Great to be here.

FLATOW: Kevin, can you explain what factors made Haiyan such a strong storm?

TRENBERTH: Well, this, you know, the Pacific Northwest is where it's most active in the world for typhoons and for tropical storm activity. That's where the largest, I suppose we can call it a pool of really warm water exists in the world and that water is also very deep so the upper parts of the ocean, not just the sea surface temperatures, are among the warmest in the world and it's been particularly warm in recent times.

Since 1992, we've had altimeters on spacecraft that have been measuring the global sea level locally to millimeter accuracy and so we've got really global measurements of sea level. Previous to that, we only had measurements at islands and coastal stations. And since then, the global sea level has gone up about two and a half inches and in the region just east of the Philippines, it has gone up about eight inches overall.

And so the winds throughout the Pacific have sort of piled up the warm water even more in that region than is occurring elsewhere in the world and, you know, this sort of set the stage for a lot of, if you like, heat potential in the ocean, which is what these storms feed upon.

FLATOW: So there you point out a lot of dots and then you connect the dots. Can you connect the dots that this tropical - well, this Typhoon Haiyan is a result of global warming then?

TRENBERTH: Well, I don't think that's quite the right question. You know, the real question is, is global warming playing a role and I think, you know, my view on this nowadays is that the environment has changed, especially the water - the ocean environment. The oceans are simply warmer than they used to be by about one degree Fahrenheit, say prior to about the 1970s.

And the air above the oceans is warmer and moister. It's warmer by maybe by a little bit more than one degree Fahrenheit and there's about 5 percent more moisture on average over the oceans than there used to be, say, in the 1970s and earlier. And so this environment is where all of the storms are developing. It adds to the fuel that these storms feed upon, especially with regard to precipitation and especially with regard to hurricanes where there's a more direct relation between the subsequent intensity of the storms and this fuel that's actually feeding them.

FLATOW: Gabriel, do you believe also that there is a, if not a direct link, an indirect link?

VECCHI: I think my view on it is perhaps slightly different from Kevin's. I think that the things that made Haiyan in particular intense, which were the reservoir of warm water that Kevin pointed out that we have in the West Pacific, it's unusually deep and warm recently. It's more southerly trek. Typically storms that hit the Philippines hit further north and this one hit further south closer to the Equator, closer to warmer waters.

It had a very fast track so it was able to move away from a cold wake that a storm tends to generate in its aftermath. And then, as it hit the Philippines, there's some indications that it might have interacted with the mountains of the Philippines and really gotten perhaps a boost. All of these factors are incredibly interesting and worth exploring, but none of them have been, in particular, connected to global warming.

So while all other things equal a warmer planet should give us warmer storms, I think for this particular case, the connection is more difficult to make. I mean, you know...

FLATOW: Well, how many storms does it take to make a connection?

VECCHI: That's a very good question and that depends on how large of a signal we expect to see and how large the noise is behind it and that currently is a topic of considerable scrutiny and debate with the scientific field. You know, there was recent - recently the IPCC, the Intergovernmental Panel on Climate Change, released its fifth assessment report, and in it there was an assessment of where the current scientific literature stood on our ability to detect trends in hurricane and typhoon activity.

And given the data that we have available and the signal that we expect global warming to give us, which is still of uncertain magnitude and sign, we weren't able to give it anything beyond low confidence. We didn't have confidence to say whether we have seen trends yet.

FLATOW: Kevin, you agree?

TRENBERTH: Well, I would put it a little differently. What Gabe is really saying here is that there's a very large amount of natural variability from one year to the next, from one location to another, and this is the historical record and so it's very hard to see small trends in that large variability. In addition, the observational record is contaminated by large changes over time.

You know, we only have satellite measurement since about 1970 and especially in the Pacific Northwest and other parts of the world outside of the Atlantic, the record is really quite shaky before then. But even after 1970 when we've got satellite imagery, the quality and the kinds of observations available from satellites have changed as well and so we have more information at different wavelengths, like in the microwave and the infrared and the visible wavelengths and things like that.

And so we can make much better estimates of what's actually happening within a storm now than we used to be able to do. And so from the observational record, you really can't say in a satisfactory way just how things have actually changed even and this is a part of what leads to the uncertainty in any detection.

So I think what we fall back on then is more our understanding of how these systems work, the fact that they're very dependent upon the sea surface temperatures and the ocean temperatures below the surface and then we can do various kinds of experiments. And Gabe and his colleagues at GFDL have done some of these where you change that environment. You change the sea surface temperatures. And there's been some interesting experiments recently of that nature using Superstorm Sandy, for instance.

FLATOW: So I'll ask it again, and so what point - how many Superstorm Sandies or Haiyans do you need before you can point to a storm and say, well, you know, these superstorms are due to global warming? Is it only going to be 50 years from now and looking back on the data to get enough storms, enough data collection? Or are you going to just - people are going to say, you know, those scientists don't even know, why should I believe in this?

VECCHI: So Ira, I think, well, maybe we can move in a direction where I think both Kevin and I would agree that the community has a relative amount of confidence that by the end of the 21st century we should be able to see the impacts of global warming on tropical storm and typhoon and hurricane and cyclone intensities.

And presently, because of the data issues that Kevin mentioned and - and in the West Pacific there's the added data issue that there used to be aircraft reconnaissance flights that went out and then they stopped. And so now, even though we have better satellites, we have not as good aircraft observations in the West Pacific. So these changes in the observing system have made it very difficult for us to say how the world has changed now.

And so the answer would be sometime between now and the end of the 21st century.

FLATOW: So we should wait till the end of the 21st century before we take any abatement because we don't know there's a connection.

TRENBERTH: Well, no. You know, I certainly stated it differently before. I think there is a component of these. It's maybe a modest component. The way in which we contract this most readily is through precipitation. And so the precipitation - the heat that gets released when precipitation forms in these storms is what drives the storms. And we have the best statistics for precipitation.

And so across the United States, for instance, we know now that when it rains it rains harder than it used to. And so you have tropical storms like - well, hurricanes like Hurricane Irene, which was, you know, the year before Sandy, where there was just tremendous amounts of rainfall right up into Vermont. And I think there's very clear evidence that the rains are harder and there's a - and the rains are harder particularly in tropical storms.

And so this is what gives these storms a bit of extra fuel. Now, the effect I believe that we can justify overall is perhaps in the 5 to 10 percent level. So it's not the sort of thing where we're saying that global warming is responsible for these storms. But to - but I think there's a good case that can already be made that there is an influence and that influence matters. Because when you have a really strong storm, you go to the next level. You break records and things break as a consequence of that.

FLATOW: Okay. We're going to take a break, come back and talk more with Kevin Trenberth and Gabriel Vec. Our number, 1-800-989-8255. Talking about tropical storms, hurricanes, cyclones and their connections to global warming. Stay with us. We'll be right back after this break.


FLATOW: This is SCIENCE FRIDAY. I'm Ira Flatow. We're talking this hour about climate change and hurricanes with my guests Kevin Trenberth and Gabriel Vecchi. Our number, 1-800-989-8255. Let's go to the phones. Lots of people would like to talk about this. Sean in Crestwood, Kentucky. Hi, Sean.

SEAN: Yes, I worked for a couple of decades in aviation weather services with the FAA. And the night Katrina hit, I happened to be working the shift. And besides the upper air winds, I was actually tracking the ocean, the surface temperatures, which were really warm. And you could just see not even (unintelligible) that night. The geography was channeling the surge into the bay there. You could just watch it go over the really high water temperatures at the time and just build.

And I have another question for them. After 9/11 and when they grounded everything, they noticed there was a five degree warmer nighttime temperature when there's aviation. It cooled five degrees more when they had grounded all the airplanes. And obviously we can't replicate this, but could that kind of effect where we actually are having warmer nighttimes, it's not cooling - and there's been some talk - could that be also increasing the size of these storms because they don't get to sort of relax and get smaller but keep building over the night, which is what they used to not do.

I know that more with thunderstorms than hurricanes, so I'll listen to your guests off air.

FLATOW: Great question. Thanks for calling. Kevin, what do you think?

TRENBERTH: Well, certainly he's referring especially to the idea that a jet aircraft put water vapor at high levels in the atmosphere and there is often a cloud - the thin cirrus cloud that forms. And, you know, water vapor and thin cloud can indeed act as a little bit of a blanket, just like carbon dioxide does in the atmosphere. It traps some of the heat. And the biggest effect is very likely on the nighttime temperatures.

You know, he referred specifically to this event that occurred after 9/11 when there was no aircraft. And certainly what he said was true but it's still very hard to say how much of that might've just happened anyway. And that's the part which always makes this a little bit difficult. But, yes, there are some effects of those kind that are playing into the overall climate change that we're dealing with.

FLATOW: Go to the phones to Gabriel in Portland, Oregon. Hi, Gabriel.

GABRIEL: Hi. It's good to be on the air. I wanted to say that this is exactly why the scientific community is losing the debate on climate change with conservatives and others. We're I'm willing to make these conclusions on these huge storms and these wildfires. Katrina and Sandy are just a two examples, the most recent one in the Philippines, wildfires in Australia, droughts. We're not willing to make any conclusions. We're saying that the data isn't adding up just yet.

We have an idea, but if we're going to win this debate, we actually need to make the conclusions now because it is that big of an issue for us.

FLATOW: Gabriel, what do you say to that?

VECCHI: Oh, that's - I hear that statement often. And I think it is - there's a very intuitive sense that as the planet warms we should have more hurricanes and stronger hurricanes and typhoons, and a sense that every event does carry that signature of global warming. But I think viewing the scientist's role in this process as having a stake in the outcome of some debate is perhaps misinterpreting what our role is, at least in my opinion.

Our role is to provide society with the best predictive understanding about what climate is likely to be doing in the future, what it has done in the past, and what it will do near term and why, in order that then society can have the debate. But - and so in order for us to be most effective in that role, we do have to be as explicit about the uncertainty that does exist.

FLATOW: Right. But you say you need to be predictive. But you already have - if I read your writings, and most scientists predict, is that hurricanes will be more intense. There'll be more rain from tropical storms. There'll be this and that. You've predicted that, but then at what point do we then point to something and say, well, we made that prediction, why aren't we - I'm just taking the side of the listener. If you made that prediction, why can't you say that's what we predicted?

VECCHI: Well, so Ira, I think it's correct, we have made predictions. But the predictions also have a definite timescale attached to them. So the predictions that are most clear are about the end of the 21st century. At least when it comes to hurricane intensity. However, Kevin - I want to go back to the point he raised before the break, which I think is very interesting. He started pointing to a different aspect of storms, then intensity, precipitation, which may be something that we have an expectation that will change more in a way that we can see more easily.

So rather than trying to look at intensity, perhaps, we should try and look at these other characteristics of storms, things that we could more easily connect to climate change. And so that - precipitation is one of them and there's another series of them. But then when we come back to this individual storm, when we look at the things that made it be as strong as it was, I come back to the fact that really none of them are things that in this specific context do we predict global warming should've caused.

FLATOW: Kevin, any...

TRENBERTH: Yes. Can I challenge Gabe a little bit on that? I want to push him a little bit, because actually, in his lab they've done some very nice experiments with their own models. And in recent times they have been able to replicate reasonably well the changes in number from one year to another - in the Atlantic, for instance. And therefore it says that the large scale environmental variables that are influencing the hurricanes are actually amazingly predictable and well known.

And, you know, the number one variable is certainly the sea surface temperatures. And so there are some other factors that are also important. But, you know, we know the sea surface temperatures are higher and we know that that is caused by global warming, by the human influence. And so there are things we can point to that, you know, I think we can say more than has come across from Gabe right now, and I've heard him in the past make more definitive statements as well. So I'm challenging him.

FLATOW: All right, Gabe. Give you a chance to rebutt.

VECCHI: So well, I'm very familiar with that work that was involved in a lot of it, that came out of my lab. And Kevin is correct in saying that the past history of year-to-year variations in hurricane activity in the Atlantic and also in the east Pacific and also the west Pacific is in some sense very predictable, if you know sea surface temperatures. But there's two crucial factors that we've come to learn.

One of them is that it is not the absolute sea surface temperature, the total warmth in the ocean that matters, but rather the patterns of sea surface temperature that in our work comes out as the most relevant factor. So not whether the Atlantic is warm or cold that leads to an active season, but whether the Atlantic is warmer or colder than the rest of the world that leads to an active season in the Atlantic.

And the second point to keep in mind is the factors that change sea surface temperature in the Atlantic, and especially the patterns of sea surface temperature globally, are more than just global warming. There's year-to-year variations due to El Nino. There's year-to-year variations due to a variety of other factors. And so we need to be very careful about what we mean and what the question is.

I think the particular intensity of Haiyan in 2013, I would be very, very cautious about connecting it in any direct or indirect way to global warming. If we start talking about the aggregate rainfall associated with hurricanes or perhaps even the rainfall that came from individual storms in the Atlantic, there the connection becomes a lot cleaner.

FLATOW: All right. Well, we'll be checking back with you. Thank you, gentlemen, for taking time to be with us today.

VECCHI: You're welcome. Thanks for having us.

FLATOW: Gabriel Vecchi is research oceanographer in the Geophysical Fluid Dynamics Lab. That's at NOAA in Princeton. And Kevin Trenberth is distinguishing scientist in the climate analysis section at the National Center for Atmospheric Research in Boulder, Colorado.

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