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From NPR News, this is TALK OF THE NATION: SCIENCE FRIDAY. I'm Joe Palca.

Wildfires are a costly natural disaster. Not only do they destroy homes and businesses, but putting them out cost more than a billion dollars each year. In recent years, large wildfires in the United States are growing in number and strength, especially in the northern Rocky Mountains. That's a conclusion from a new study in the online edition of the journal, Science.

The researchers blame the changes on rising temperatures, so is this an example of climate change in action? And if it is, what can we do about it?

Well, joining me to answer those questions is Anthony Westerling. He's the lead author on the Science article and Assistant Project Scientist in the Climate Research Division at Scripps Institution of Oceanography at the University of California San Diego in La Jolla. And he joins me from the campus of UC San Diego. Welcome to the program, Dr. Westerling.

Dr. ANTHONY WESTERLING (Assistant Project Scientist in the Climate Research Division, University of California San Diego): Thank you very much, Joe. And...

PALCA: I'm sorry, go ahead.

Dr. WESTERLING: I was going to give a plug for my new university, because I'm actually Assistant Professor at UC Merced now in northern California.

PALCA: Oh, is it a UC campus, the University of Merced?

Dr. WESTERLING: Yeah, it's a brand new one. It's still under constriction.

PALCA: Well, there you have it. Okay, and to our audience then, if you'd like to join our discussion about wildfires, give us a call or if you want to go to the University of California at Merced, give us a call. Our number is 1-800-989-8255. That's 1-800-989-TALK. And if you want more information about what we'll be talking about this hour, go to our Web site at www.sciencefriday.com, where you'll find links to our topic.

So, Dr. Westerling, let's start by - the reason that you undertook this study. I mean, what made you suspect that forest wildfires were growing in strength and number?

Dr. WESTERLING: Well, anecdotally, we all have some idea that must be the case. I mean, just from the media reports and the very active fire seasons we've had in recent years, but nobody had been able to take a comprehensive look at it across the region, and so a big part of this effort was to put together the records of wildfires from park services and forest services in the western United States, so we could do that.

PALCA: I'm wondering if that data - I mean, it's collected not for research purposes but I guess for, you know, monitoring of some sort, but is it adequate, is it accurate enough to give you the kind of information you need to do a scientific assessment?

Dr. WESTERLING: Well, you know, a lot of the data's pretty messy, but what we find is that the bigger fires are actually recorded pretty well. The little ones aren't that important and they just fill them in pretty quickly, you know? And so a lot of information can be missing, for example, about a small fire that didn't do much damage. But the large fires have bigger implications economically and for the ecosystem and people pay a lot more attention to them, and so those records are actually much higher quality and so that made our job a lot easier.

PALCA: So you started this in about 1970. Why then, and was it a steady increase or was there a jump at some point?

Dr. WESTERLING: We started our study in 1970 because there was this convenient set of data from both the forest service and the park service that was consistently reported from 1970 onwards, so this was limited to parks and forests in the western United States that had the 34-plus year record now, starting in 1970.

PALCA: And - I'm sorry - steady (unintelligible) incline or...

Dr. WESTERLING: No, no, not at all. Actually, it sort of jumps up rather quickly in the mid-1980s, and then from there on we have sustained periods of warmer temperatures and more fires. But then when it does cool down occasionally, we get a lot fewer fires in.

PALCA: I got it.

Dr. WESTERLING: So, if you want to think about it in terms of the temperature effects - and I'll get into why we think it's temperature in a moment - just to give you some idea, temperature doesn't have to increase that much and can be fairly gradual but then it can have a sort of a non-linear effect where it switches from not a lot of fires to a lot of fires, because of its effects on snow. Right? So, you've seen and ice cube.

PALCA: Snow? Okay. All right.

Dr. WESTERLING: Yeah, when you're, when the water, when it's, when the temperature is below freezing, nothing's melting, and then as soon as it goes above freezing it's melting. So that's how you get these sudden effects, even though the underlying factor that's pushing it can be gradual and not a sudden switch.

PALCA: Okay. Okay, well, but that's interesting. So what you're, so you're saying that, okay, so its usually 85 degrees on a typical summer day in the west, in a forest, and you're saying that if it just goes up to 87, that might be enough to, on a consistent basis, that might be enough to...

Dr. WESTERLING: Not at all, actually.

PALCA: Okay. All right.

Dr. WESTERLING: What I'm saying is that that, that doesn't make a big difference, right? It's just an incremental increase on a temperature that's already warm. What does make a difference would be raising the temperatures in spring, when you still have snow on the ground at the higher elevation. And melting that snow out earlier, so then you've...

PALCA: Oh, I get it. Okay.

Dr. WESTERLING: ...got a place that's usually got a short summer, and it's usually kind of cool and wet, and you melt it out earlier, say a month early or something like that, and then you could be increasing the length of the summer season by 50 percent or something. And then what happens is not only does it get drier overall, it's dry for longer. So the vegetation is stressed for a longer time. The forest dries out more than the usual, and it's dry for longer so there are more opportunities for a fire to ignite and then spread.

PALCA: Okay. We're talking about the growing strength and the growing length of the forest fire season. Give us a call, our number is 800-989-8255. We'd be happy to have you join the conversation.

So it seems to me that the typical explanation for this is that, you know, with human encroaching on forest land, there's more clearing, and there are natural fires aren't occurring with the same frequency, and so you're seeing more of an anthropogenic effect of human impact from, you know, construction and development. But you're saying something else.

Dr. WESTERLING: Actually, one of the biggest discussions was not just about that, but about the effects of fire suppression and logging and things like that over the 20 century, and perhaps even earlier, on the density of the forests. So in some places forests were quite open and they became much thicker over time. And that has certainly increased the severity of fires, and perhaps also the frequency of fires in some locations.

But what we found was that the biggest increases in the frequency of these large forest fires occurred in forests that were not affected by this kind of thing.

PALCA: Hmm.

Dr. WESTERLING: So these were forests that were a little bit wetter, and they already were densely forested, and any effects of management were kind of marginal. And these were the places that had the biggest increases in fire.

PALCA: All right. Well, let's invite some of our listeners to join the conversation, and we'll go first to Jim, in Chico, California. Jim, welcome to SCIENCE FRIDAY.

JIM (Caller): Thank you. I am a forest policy analyst for the Sierra Nevada Forest protection Campaign, and I'm also active with the Butte(ph) County Fire Safe Council, and so we deal with Sierra Cascade western slope issues mostly. And one of the things that I notice in media depictions of fire is often they will only mention the total acreage that has burned, and not break it out from the medium intensity, low intensity, high intensity fires.

For instances, a few years ago we had a fire here called the Story Fire. It burned about 55,000 acres. The paper depicted it as 55,000 acres destroyed, burned up, when, in reality, only 5,000 acres of it burned at high intensity and 50,000 acres burned in a way that was actually advantageous, kind of overcoming the effects of fire suppression that have occurred during the past century.

And so I hope that that's part of the discussion...

PALCA: Well, Jim, let me, Jim, I certainly, let me ask Dr. Westerling. I mean, when you were compiling your data, were you going from news reports or were you looking at the kind of data that Jim says is more accurate to really...

Dr. WESTERLING: Well, one of the problems is that we wanted to look at a longer time period, and the fire severity data is most available for the most recent fires and not for a lot of the older ones. So what we focused on was more the number of fires that passed a certain threshold that got above 1,000 acres in size, rather than the actual explicit sizes of individual fires, partly to get around the problem that he's describing.

PALCA: Okay, Jim, thanks very much for that call. Let's take another call now and go to Jaya(ph) in Folsom, California. Jaya, I hope I said that right. Welcome to the program.

JAYA (Caller): Hi. Thank you. My question is, what percentage of fires are caused by human error, and is this something that can be prevented through education, for instance?

PALCA: That's interesting. Thanks for that call. Dr. Westerling, were you able to parse that out at all?

Dr. WESTERLING: Yeah, I don't have an overall figure for the whole thing. Part of the reason is because it varies so much from place to place. If you're in the mountains of above California cities, for example, most of the fires are human caused. But you get into a more remote location, you have a lot more lightning fires. And the issue for us was that it turns out to be, from our perspective, largely irrelevant, what causes the fire ignition, because if you think about it in an extreme case, if it's raining or there's a lot of snow on the ground, you're not likely to get a really big fire from an ignition, so the issue for us was not necessarily what caused the ignition, but were the conditions, given that ignition, were the conditions appropriate such that it could grow to a large size and overwhelm the suppression resources that could be focused on it in a short time.

PALCA: So what are the consequences of this longer fire period and more intense fires? Is it just a bad thing for forests or is it a bad thing for some broader picture?

Dr. WESTERLING: Well, there are a couple things you could identify. One is to do with our water resources here and the other is with climate change. Thinking about water resources in the west, this is a dry climate in most of the western United States, and we rely on a lot of water that comes snow melt in the late spring or early summer.

So we have a lot of reservoirs constructed, but a lot of the water is actually stored as snow up in the mountains for free by Mother Nature. And when you warm things up, you know, it tends to melt earlier and it makes it more complicated to manage the water resources because when you melt it too early, you have a problem where you have to manage those reservoirs for flood control rather than just retaining the water. And so you can't retain as much of the water that you might want to save for later use.

And this fire problem could potentially exacerbate this, because if you repeatedly burn these forests, our watershed at greater regularities than before you can reduce the capacity of the forest and of the soils to retain the water. And so what happens is that when you get the runoff, it's much more abrupt, punctuated, flashier. You can get flooding more easily, for example. And it will make this problem of managing this tradeoff between flood control and resource management more difficult. So that's one complication.

PALCA: We're talking with Dr. Anthony Westerling. He's the author of a new report in the journal Science, I guess it's the online edition of Science, called Science Express, that looks at the historical changes over the last 35 or 30 years in forest fires, both in their intensity and duration of the fire season.

I'm Joe Palca, and this is TALK OF THE NATION from NPR News.

So I guess, you know, we've eventually got to address the question of, okay, it's warmer. Why?

Dr. WESTERLING: Yeah. You know, in this study we don't actually address the causes. We're looking at, you know, the ultimate cause. We're looking at proximate causes. So the immediate cause that we see is the warmer temperatures and the earlier spring snow melt. And with that in some places, reduced rain and snow, you know, reduced precipitation as well.

We don't have a long enough record there that we can say this is evidence, for sure, that climate change is occurring. You know, we rely on other people looking at other records, say long term temperature records and things like that to establish that. What we do say is, when you look at this, this is exactly consistent with what you would expect to see under a warming climate caused by climate change.

And regardless of whether or not these changes we've experienced are due in all or in part or not at all to climate change caused by humans, we anticipate that that will occur or continue to occur in the future. And so, as temperatures increase in this region, especially in the spring and early summer, we would expect that these very active severe fire seasons that we've had in the mountain forests of the west would become much more frequent.

PALCA: So in other words, things are going to get worse before they get better.

Dr. WESTERLING: Things are going to get worse, yes.

PALCA: Okay. Let's take another call now, from Bernard, in Philadelphia. Welcome to the program.

BERNARD (Caller): Hi. Can you model this behavior using non-linear chaos mathematics? James Gleick's book fifteen years ago, Chaos: Science - Making of a New Science, talked about modeling population behavior, economic weather behavior, using the dynamics of non-linear systems.

PALCA: So do you want to give us a 25-word explanation of non-linear chaos mathematic - no, never mind, I'll just...

(Soundbite of laughter)

PALCA: I think we'll leave that, we'll leave that to sort of - more of an open question for right now. But Dr. Westerling, what about that?

Dr. WESTERLING: It's an interesting question. You know, I haven't tried to model it directly mathematically. What we have been doing is trying to make projections about what fire would look like under different climate change scenarios. And we use hydrologic models that simulate what the snowfall and the runoff and the soil moisture retention would be like given temperature and precipitation from these climate models.

BERNARD: Because you...

Dr. WESTERLING: And so that gives you - and these are non-linear models, so that gives us a non-linearity of the hydrology. And then the wildfire we're modeling on top of that with just straight statistical models. And those are actually non-linear as well. We use statistic models and (unintelligible) and things like that. So it's not quite what he's talking about, but there's a level of sophistication.

PALCA: Right. Well, I'm sure, Bernard, if you want to take a crack at that, it's certainly an open topic. You can probably take a crack at it yourself. But here's the other thing that I'm wondering. What does all this say about mitigation efforts? You know, are the things that are more on the ground, building and structures and management, are they sort of hopeless in the face of a growing global climate change that might be pushing this forest fires?

Dr. WESTERLING: Well, the things that we're already doing we do pretty well. And there are two categories: one is fire suppression and putting out the fires, fighting the fires. And the other one is trying to manage better the vegetation, so thinning and ecological restoration of forests that have been changed by past land use and management effects.

And those are both appropriate policies that need to be pursued in places where they're appropriate. Specifically, you know, a thinning a forest is a really important thing to do in forests that have been thickened I guess is the way to put it, because of our past actions. But because this has been driven so strongly by temperature - and in a lot of cases, in places that have not been greatly affected by the past management, that's not likely to be adequate to reverse trends or to moderate future increases.

And the same thing with suppression. Suppression is important, and there are probably a lot of things that we could do to reduce risks around structures and communities and things like that throughout the west. But you have to think about the fact that we're already dedicating tremendous resources to fire suppression, thousands of men and women fighting fires in the western United States. And in these severe fire years a lot of money is being spent. There have been many years above a billion dollars for these expenditures in recent decades.

PALCA: Dr. Westerling, I think that's where we're going to have to leave this discussion. Thanks very much.

Dr. WESTERLING: Okay.

PALCA: Anthony Westerling is assistant project scientist in the Climate Research Division at Scripps Institution of Oceanography, and he's just joined the University of California at Merced. So thanks for coming on the program.

Dr. WESTERLING: Thank you.

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