Let’s Break It Down: Climate Change and Wildfires

Beginning in October 2017, a total of 9,054 wildfires burned across the state of California, making the 2017 California wildfire season the most destructive on record. According to the California Department of Forestry and Fire Protection, 1,381,405 total acres were burned, costing California over $13 billion. The fires first ignited in Northern California in October, before spreading to southern California in December 2017, with some fires reaching full containment only this month. Much of the recently burned areas in Southern California were cleared of vegetation, making the area prone to the mudslides that occurred after heavy storms last week, killing at least 20.
The significant intensity of the wildfires along with the destruction they wrought has raised questions about whether or not climate change has contributed to the annual fires and their effects.
In order to better understand the role, if any, that climate change had on the recent fire season in California, the Falconer spoke with Dr. Jon E. Keeley, a current research scientist with the U.S. Geological Survey, former director of the ecology program for the National Science Foundation and Adjunct Professor in the Department of Ecology and Evolutionary Biology at University of California, Los Angeles. Keeley has been published more than 350 times in national and international scientific journals and books, and his research focuses on the ecological impacts of wildfires.

Falconer: How did 2017’s wildfires compare to previous wildfires in the state?

2017 was a phenomenal year for two reasons. The fires in the North Bay of San Francisco, Sonoma, Napa County Fires — the fires themselves were not unusual, because we’ve had big, wind-driven fires in that area before. What was very unusual was the number of people who died, I think it was 44, last count I heard, and over 7,000 homes burned, and that is just phenomenally large for any fire in California and certainly the most catastrophic fire we’ve ever had in terms of human loss and property loss. So that was just big in terms of the losses, the fire itself wasn’t unusual. The other big fire event was the Thomas Fire in Ventura County and that did not have an extraordinarily high loss of life and property, but it was a phenomenally large fire and, in fact, in recent history, it’s the largest fire on record. We have evidence that there were larger fires in the 19th century, but in recent history, the Thomas Fire is the biggest that’s been recorded. What was unusual about that fire was the fact that the Santa Ana winds, which are generally associated with most of our big fires in Southern California, those wind events usually last typically two or three days. And if you live in San Diego, I’m sure you’ve experienced Santa Ana wind events before. What was unusual about the Thomas Fire is that wind event lasted at least twice as long as a normal wind event, and that was very unusual. I’ve been following Santa Ana wind events and fires for my whole career and I’ve never seen anything that compares to the length of that wind event, and that almost certainly is the reason that the fire grew to such a large size. Now one of the things that’s of interest is that even though the fire was much larger than the Napa or Sonoma fires, there was far less damage in terms of loss of lives and property. And part of that, I believe, is related to the importance of citrus and avocado agriculture in the Ventura area. In many places, that fire would burn into those orchards and because those trees have high fuel moisture, the fire would generally burn out, and so many people were buffered by those orchards and almost certainly reduced the amount of losses. 

Falconer: Can you explain what the Santa Ana winds are and their specific effect on wildfires?

The Santa Ana wind is due to a high-pressure cell that moves from the north down into the Great Basin, and that high-pressure cell — when it coincides with a low pressure cell off the coast — is what drives the Santa Ana winds because everything moves from a region of high density to a region of low density. When the density of air is very high in the Great Basin and the pressure is low off the coast, then that drives these winds from the interior to the coast. [The winds are] very high-speed winds — the velocity is sometimes 50 miles per hour — and they’re very dry winds. Wind is probably the number one factor that affects how big a fire gets, and it doesn’t matter where you are, whether it’s in California or near the western U.S. or anywhere in the world; the wind is what ultimately determines the size of the fire, because the wind picks up burning embers and carries them as much as a mile away from the forefront, and that’s what causes the fires to get out of control.

Falconer: Was the timing of these past two fires significant enough to be different from previous ones? In other words, was it normal for the fires to have occurred at the times that they did?

A lot of people have tried to make something out of the fact that these fires burned in December, but that’s based on very limited understanding of fire history in Southern California. December wildfires driven by Santa Ana winds are not unique to this event. There is an article that was just published this week, I believe, in which someone was recounting five December Santa Ana wind-driven fires that he remembers historically in the Santa Monica mountains. So we’ve had fires that have been fairly large, like 10, 20 acres in size in December before so that’s not unique at all. Normally we don’t get them in December because normally we get rains that occur in December and that increases the fuel moisture and decreases the probability of a fire, but it is not all that unique to have a dry December. In the last hundred years we probably had half a dozen Decembers with little or no rainfall. To summarize, it was atypical, but it was by no means unique.

Falconer: When are fires most likely to occur?

The worst fires that we have are in the autumn, so typically September to December are the months where we have the worst fires, and that’s because the Santa Ana winds don’t occur year round; they occur typically in the autumn going into the winter. We can get fires [during] other times of the year, but a lot of it has to do with the type of vegetation. If you have a landscape that’s dominated by grasses — grasses will dry up very rapidly — you could have rain and a week later the grass would be dry enough to burn. So in areas with grasses, the fire season can be much longer. In fact, in some areas the fire season seems to be 12 months long. But that’s only in areas [that are] dominated by grasses.

Falconer: Is there a reason why these two fires occurred in California, first in Northern California and then in Southern California?

Yes. It’s because California has probably one of the worst fire climates in the world, so there’s just much greater probability for fires [here] than in other parts of the world. The important thing to understand about California’s fire climate is we have a drought that is normal and occurs typically from May to October and we get it every single year. So, every single year we have the possibility of a fire. When you look at fires in other parts of the West, for example [in] the Rocky Mountains … a couple times a decade they will have an extreme drought and then they get big fire events, and so what’s different about California is that we have a predictable fire climate every single year. In other parts of the West they don’t have such a climate every year and so the probabilities of fire events are different. The other thing is that we have these Foehn winds, the Diablo winds and the Santa Ana winds in the South, and that definitely contributes to California having what a lot people think of as one of the worst fire climates in the world.

Falconer: Going off of that, are fires this year predicted to be as intense as those seen in years prior? 

No, nobody predicted this. [That was] largely because we get Santa Ana winds every single year. In fact, I did a study that was published [recently in 2017] where I examined the number of Santa Ana wind events every year for the last 30 years and compared it to the amount of area that was burned, and the relationship was zero. There is no relationship between the number of Santa Ana wind events and fires, and that’s because we get [the winds] every year but we don’t get a bad fire every year. The reason we get a bad fire is because somebody starts a fire during the Santa Ana wind event, and so ultimately you can’t really predict when people are going to start fires. Humans account for 84% of all wildland fires, and you can’t predict when people are going to start fires, and so that’s why [these recent fires] weren’t predicted.

Falconer: Is there evidence that wildfires are becoming worse due to climate change?

The answer is, in some parts of the western U.S., yes, fires are beginning to worsen and it appears to be because of climate change. What we found is that in places like the Sierra Nevada, about 50 percent of the variation [in wildfires] is explained by climate. Although this is consistent with recent global warming, we found an equally strong relationship between fire activity and climate in the first half of the 20th century. It is hypothesised that the fire–climate relationship in forests is determined by climatic effects on spring and summer fuel moisture, with hotter and drier springs leading to a longer fire season and more extensive burning. Now we did the same analysis for Southern California, and we found actually no relationship between the temperature in any year and how much area burned that year, and I believe the reason is in places like San Diego County, it’s hot enough and dry enough to produce a large fire. The fact is that somebody either accidentally or intentionally starts a fire during a Santa Ana wind event and the major causes are either arson, wood burning, or power lines that get blown down by the Santa Ana winds, and so in Southern California it’s just the timing of ignition which usually results in a big fire event, so we don’t see any reason to expect that global warming is going to cause worse fires in Southern California. What is going to likely impact fires in the future is population growth, because in San Diego County, 99 percent of all the fires are started by people and over the next 30 years we’re going to see perhaps a 50 percent increase in population … and that means there’s a higher chance that somebody will light a fire in a severe event. And so, we do expect fires to get worse. The important point about climate change and fire is that [climate change] almost certainly is going to be important in forests in the West. It is not likely to be nearly as important in California, and population growth is likely to have a bigger impact on future fires [in California], mainly because every year it’s hot enough and dry enough for a bad fire. If the temperature gets warmer, we don’t see any reason to expect that the climate is going to be any more suited to a bad fire than it is already. But in Southern California, the number one cause of wildfires is someone starting a fire during a severe wind event, and since we’re expected to see an increase in population growth in Southern California, we should expect a greater chance of someone starting fires during those wind events.

Falconer: Does climate change affect the Santa Ana winds?

In terms of climate change affecting [the winds], there really is no evidence that climate change is going to affect the Santa Ana wind events. There have been two studies that have been published [that investigate this question] and they show relatively slight changes. People have tried to model what the future Santa Ana winds will look like with global warming, and they haven’t really found any convincing evidence that it’s going to change much. There’s slight differences, like maybe a 10 percent greater chance that there will be Santa Ana winds in later years, but there really is no convincing evidence that global warming is going to have a big impact on these winds. Now that doesn’t mean they won’t, that just means we don’t have any evidence of it.

Falconer: Is there evidence that suggests conditions such as drought or the wind patterns will change because of climate change?
The evidence for droughts isn’t nearly as clear as the evidence for global warming. In other words, we have very good evidence that, in the coming decades, temperatures are going to get warmer. So that’s pretty clear, from most of all the models that have been done. What isn’t so clear is what the impact will be on rainfall. And so we don’t have real clear evidence that droughts are going to be worse in the future. Now the impact of droughts might be worse because if you have limited rainfall but higher temperatures, the impact on the vegetation is going to be worse. And so there is an indirect effect of global warming on drought stress in plants, for example, because temperatures are going up. When you have a dry year, the plants are going to be stressed more. Now that could have an effect on fires, [and] we don’t have evidence of this, but I believe that one of the factors in the Thomas fire was the fact that between 2012 and 2014 we had a severe drought in California. We believe that one of the impacts of that drought was to cause the dieback of a lot of chaparral shrubs, and those dead shrubs are still on the landscape, which might have contributed to the fires getting worse.

Falconer: As California’s population grows and more land becomes developed, do you think it will become harder for citizens to avoid wildfires?

I think that’s a very good possibility, and what it says is that the wildfire problem needs to be addressed as much by land planners as it is by firefighting agencies.

Falconer:  Do you think the recent fires will convince people who are skeptical of global warming and its effects that it is actually happening?

Well, it’s hard for me to predict what people will do, so I have no idea how people will respond. But like I said, I don’t think global warming is the primary effect affecting these big fires we recently had in California. So, even though there are newspaper articles out there saying that global warming is reflected by these big fires, I think they’re wrong. I don’t think those big fires had anything to do with global warming; I think they just had to do with these extreme wind events that we get, the fact that somebody ignited a fire during those wind events, and the fact that we have very high population density, which puts people at risk to these big fires. So like I said earlier, there’s no question that global warming is impacting fires in western forests, but there’s no evidence that it had any role in these big fires in California this year. And the important thing is that I hope people don’t look at these big fires this year as inevitable results of global warming; I hope they understand what actually happened [in the fires,] and certainly one of the major factors is population growth. 
Falconer: What do you think is the main thing that people all around the world can learn from these past two recent fires in California?

I think the main thing to understand about these fires is these big fire events are really a normal feature of this landscape, and the fires themselves by and large really haven’t changed over time —what has changed is population growth, and more people living in risky environments are being made susceptible to these fire events. So that’s probably the important thing to understand about them; the fires are a natural part of this landscape, [and] the population growth and development is expanding into these watersheds and putting more and more people at risk. So, I hope what people would understand would be the need for better land planning.


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