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The largest single fire in modern California history was the so-called Cedar Fire in San Diego County during October 2003. It burned 280,278 acres (438 square miles), destroyed 2,820 buildings, and killed 15 people.

A satellite image of the 2003 Cedar Fire in San Diego County, known as the largest single fire in modern California history.

Photograph courtesy Jacques Descloitres, MODIS Rapid Response Team, NASA GSFC

Melody Kramer

National Geographic

Published August 28, 2013

The large wildfire burning in and around Yosemite National Park has already consumed more than 184,000 acres, and shows no signs of slowing down. The blaze, which has been dubbed “Rim Fire,” is now the largest fire in the Sierra Nevada mountain range and one of the largest in California’s history. (Related: “With Rim Fire Near, a Look at Yosemite’s History With Fire.”)

The Rim Fire is one of more than 30 blazes currently churning across the West. And a combination of higher temperatures, untamed underbrush, less rain, and more developments in the region means that the number and intensity of wildfires is likely to increase in the coming years, says Don Wuebbles, a professor of Atmospheric Sciences at the University of Illinois.

“This probably is the new normal,” he says.

“If we look at how the climate has changed over the past 50 years—with warmer temperatures increasing beyond what we used to see in the early part of the 20th century, and changes in precipitation—fires will continue to happen and get worse and worse,” says Wuebbles, who co-authored a draft federal report linking climate change to an increase in severe weather trends.

The numbers certainly back him up: Wildfires are roaring through twice as many acres per year on average in the U.S. than they were 40 years ago, U.S. Forest Service Chief Tom Tidwell told the Senate in June.

That number could very well double again in the next 30 years, says Wuebbles.

“I would say that this [fire season] is the new normal but it may not be the new normal for long,” he says. “Thirty years from now, we may look upon this as being a much better period than what we may be facing then.”

Fueling Fires: Climate Change

All wildfires need three things to burn: ignition, fuel, and the right climate, says Erica Smithwick, the director of Landscape Ecology at Penn State University and an expert on fire patterns.

“But if you play with any of these things, you’re going to manipulate the fire,” she says.

Take climate, for instance. Climate models indicate that in some portions of the West, future temperatures could rise by as much as 7ºC (12.6ºF), says Smithwick.

(Related: “Why Is the West Ablaze?”)

“If precipitation also increases significantly, it may help mediate the changes caused by temperature,” Smithwick says. “But most likely, an intense warming will contribute to more forest fires because there will be more combustion potential.”

States in the West have already seen temperatures jump. Over the past three decades, Arizona—which saw huge wildfires earlier this season—has seen its ten-year average temperature increase by 2.3ºF, compared to 1.6ºF for the entire U.S. And California—currently home to ten blazes—is experiencing the driest calendar year so far on record, says Christopher C. Burt, the author of Extreme Weather: A Guide and Record Book and the weather historian for Weather Underground.

“I’m sure that’s one of the causes of the fires this year,” he says. “And the reason the Yosemite fire is so large is because it’s so dry there.”

The changes in temperature—combined with a longer growing season—lead to what Wuebbles calls a “perfect storm of wildfires.”

“You might get to the point where in some parts of the West, there are no more forests,” says Wuebbles. “When you see more long periods without precipitation and you combine that with warmer temperatures and earlier snow melts, you see a trend in a lot of acreage burning.”

Watch how photographers capture unique images of fast-moving forest fires.

Fueling Fires: Changes in Vegetation

Increasing temperatures and drought conditions then create more fuel for the fires. In Arizona, two decades of record-level droughts have increased area vegetation to record numbers.

And in many parts of the West, a history of fire suppression—that is, fighting fires in areas where they were once allowed to burn—has also increased the amount of dry vegetation. This vegetation then builds up and allows more dangerous and uncontrolled fires to spread, putting both humans and nature at risk.

“In some of these systems, you would have fires burning in the understory—or the floor of standing forests—and they wouldn’t kill the trees,” says Smithwick. “But if the understory starts to grow, then the fire could be carried up into the canopy and even trees like sequoias could be at risk.” (Related: “How Sequoias Survive Wildfires.”)

In many areas susceptible to wildfires, including Yosemite, the National Park Service conducts “prescribed burns,” both to clear unsafe vegetation and restore the local ecology. But the changes fall far short of what once occurred naturally. And changes in climate mean that the vegetation that is cleared may come back in a completely different form.

“The forests will adjust,” says Stephen J. Pyne, a fire historian and the author of several books on fire management. “But if the climate changes, you’re not going to get the same stuff back.”

As a result, Pyne says the entire system will change.

“It’s very likely that we’re into an era where the fires of the past will not be the fires of the future,” he says. “But it’s equally likely that the fires of today will not be the fires of the future.”

Fueling Fires: More Development and More People

As fuels increase and temperatures rise, the number of people moving to areas that border wildlands—a location called a WUI, short for wildland-urban interface—also continues to increase. The number of housing units within half a mile of a national forest, for instance, grew from 484,000 in 1940 to 1.8 million in 2000.

“There’s a powerful economic incentive for local governments to encourage local land developments in a WUI,” says Lloyd Burton, a professor at the University of Denver who studies environmental and disaster management law and policy. “But then a wildfire starts, and it outstrips the local government’s firefighting capabilities.”

“So when there’s a wildfire, the local government calls in the federal government—and basically half of the U.S. Forest Service’s budget goes into firefighting,” he says.

Some 70,000 communities in the U.S. are at high to moderate risk from “uncharacteristically large wildfires,” estimates the U.S. Forest Service. When a fire does break out, their elite firefighting units—known as hotshots—then go in and battle the blazes. (Related: “Who Are the Hotshots?”)

Often, says Smithwick, these hotshots are battling fires to protect homes or structures in areas that once would have been allowed to burn.

“You have people in the way of these fires,” she says. “And some of these fires shouldn’t be put out in the first place—they reduce the fuels and cleanse the system.”

But as residential developments continue to expand into the WUIs, the firefighters are supposed to protect them, says Burton, who recently wrote about the costs of living in the forest in the Denver Post.

“It puts more and more of these firefighters at risk,” he says. “It’s a situation that’s become increasingly untenable.”

The Future

And the situation will not improve—as long as communities don’t mitigate the risks, says Burton.

“There are some counties in Arizona and New Mexico and Colorado that compel residents to take mitigation measures—like building structures that are made of fire-resistant materials—and others that have done nothing at all,” he says.

Meanwhile, out in California—which did pass a statewide fire-mitigation law after a deadly 1991 wildfire—waits with baited breath for the rest of fire season.

“I predict that this September and October will be horrific,” says Burt, the weather historian, who recently wrote about the worst wildfires in U.S. history. “We’ve received only 65 percent of normal precipitation in the burn areas. When the offshore wind flow develops in September and October, our summer wildfire season will really begin.”

17 comments
Alec Sevins
Alec Sevins

As with so much else, human overpopulation is the root problem on these basic levels.

1) It compels people to expand into more fire prone areas, which are then protected from natural fires to protect artificial structures. Animals have simple homes, often underground, and ancient mechanisms for getting through fires. But human dwellings are rarely robust.

2) More people burning fossil fuels boosts CO2 levels, which causes warmer temperatures and exacerbates fire threats.

Thus, we have vicious cycles brought to you by "economic growth," a cover story for mindless human overpopulation. You don't hear about this in Presidential speeches or corporate action-plans because the concept of limits to growth doesn't go over well with the money-obsessed public, even when they keep getting "burned" by growthism's side-effects.

samantha hall
samantha hall

After reading these comments I agree with those who have stated our suppression of natural wildfires in the past have hurt the wildfires of the future (seems to be worse).  Back in the 1500-1800's I know for a fact that the state of Vermont had LESS trees in the state than it does now.  Don't know about other states, but farm fields were more prevalent in Vermont in the old days than they are now. Could be due to the moisture level in the forests, but Vermont doesn't burn that easily now.  Anyway....my other point is residential areas.  I know of many of these areas in Arizona that put up a big stink about prescribed burns cause "the smoke will ruin their atmosphere".  Well, now look what happens, wildfires run rampant in these areas cause the citizens have balked at letting the wildland firefighters do their jobs to prevent these big disastrous fires from happening by simply burning the overabundance of fuels on the ground.  The Granite Mountain tragedy could of possibly been prevented if only the town of Yarnell let them do the prescribed burns they wanted to years ago.  Fire is a tricky subject with us, most see it as a harmful entity as others see it as a good thing, which I agree.  There are many species in the plant world that depend on fires to reproduce.  A part of me wants to say let mother nature do what it needs to...while another part says, I don't know.  I think there is still much more we can learn from natural wildfires.  There is a plateau in the Grand Canyon where man has not touched any of the fires that burn.  Nature has done it's thing and the forest is one of the healthiest in the states.  What does this tell us?

Sean Hao
Sean Hao

"But the changes fall far short of what once occurred naturally. "

I don't understand, you mean the changes to forest are huge or small? I'm dubious about it.

Bea Moeller
Bea Moeller

"...a combination of higher temperatures, untamed underbrush, less rain, and more developments in the region means that the number and intensity of wildfires is likely to increase in the coming years..." And 2 of those things we could affect--yet what's talked about is "climate" which we can't immediately do anything about. It's looking as if excuses are being made for not acting to clear underbrush and curb development!

Bob Burnitt
Bob Burnitt

A certain amount of the fires are NORMAL, nature has utilized fire for a very long time, some species actually NEED fires for their seeds to germinate.  However, what is NOT normal is MAN believing the Balance of Nature does not apply to him and he THINKS he has the :right: to BREED in to infinity.  Furthermore, you DELIBERATELY build  a home in a Flood Zone, or a Tinderbox or a Tsunami Zone, or Hurricane Zone, you deserve what you get.  Where I live I cut the vegetation AWAY from the House and outbuildings and fences so when WE have "wildfires" we do not lose everything.  And NOW where I live we do have runaway fires.  We did not have them in the past because people used to farm and ranch here as WE did.  The land is now TOO EXPENSIVE to Farm.  But Real Estate SPECULATORS have bought up the land, they do NOTHING with it, they allow the brush and WEEDS to grow wild, and every 8 or 10 years Wild fires Clean their mess up.  We did NOT have that in the past.  Bob Burnitt Ellis County Texas

Bryan Bird
Bryan Bird

This story is not entirely accurate. From 1500 to 1800, an average of 145 million acres burned every year nationwide – about 18 times the recent annual burn total. By the 1930s, 50 million acres in the lower 48 were burned annually by wildfire and by the 1970s the number of acres had dropped to 5 million. See USDI Review and Update of the 1995 Federal Wildland Fire Management Policy January 2001. Chapter 1, page 6.http://www.nwcg.gov/branches/ppm/fpc/archives/fire_policy/history/index.htm

Alec Sevins
Alec Sevins

@Bea Moeller: You may know that "curbing development" in the long run requires more human birth control. 80 million more people are currently added to the world each year and they demand a lot of acreage. Not a day goes by when nature isn't shrinking overall. The buzzword "balance" remains a joke. "Sustainable growth" is another bogus concept.

Growth "management" is a losing strategy with only so much physical space to build on. Everyone and their brother would like to head for the hills, not admitting that they are embedded in the problem.

Arthur Firstenberg
Arthur Firstenberg

@Bryan Bird 

Your history is also not accurate.  The 1930s were abnormal.  A 10-year drought began in 1930.  Those were the years of the Dust Bowl.  Thousands of destructive crown fires burned an average of about 39 million acres of forest per year in that drought.

Your figures for prior centuries come from a single unsourced graph and are completely wrong.  The Medieval Warm Period, from the 1300s to the early 1500s, was accompanied by relatively frequent fires, but only twice as frequent as today according to some estimates.  Fires were less frequent during the Little Ice Age that followed, and were virtually absent from 1790 until the mid-1800s.  The average fire rotation between 1500 and 1800 has been variously estimated to be between 60 and 300 years.  Assuming the fire rotation was 100 years, the average acreage burned in what is now the continental U.S. during those centuries would have been about the same as it has been since the mid-20th century, i.e. between 3 and 9 million acres per year.

The 1930s were not only an abnormal decade, but they occurred during a longer period of unbridled misuse of our forests.  The late 19th century to the mid-20th century was an aberration, characterized by extensive clearcutting and massive burning of logging slash that resulted in widespread catastrophes and uncharacteristically large amounts of fire.

Ionescu Emanuel
Ionescu Emanuel

@Bryan Bird Umm, question: how many trees were in North America between 1500-1800 and how many were by 1930'?

I have the impression that there were way more trees between 1500-1800, which in turn partially explains why there were also more burned acres.

Russell Scalf
Russell Scalf

@Bryan Bird  Yes, the great decrease in fire acreage is actually part of the point. We have fewer fast, low-intensity fires and a century and a half of fire suppression and fuel build up. So that when a fire does happen, it's very high intensity, crowns out and is very destructive. A forest area can burn several times in the course of a century and the big trees mostly survive. Very few trees will survive this one.

Bob Burnitt
Bob Burnitt

@Bryan Bird   Fires are a NATURAL part of Nature, but MAN does not think the Balance of Nature applies to him because he is such a "Technological Genius".  The "religions" of the world promote OVER BREEDING.  There are too many PEOPLE and the problem has gone VERTICAL.  We are moving in to a very dark time, BB

Bryan Bird
Bryan Bird

@Melody Kramer @Bryan Bird I know, they are based on recent history not the actual fire record. So, they are extremely biased. No doubt, we are seeing larger individual fires and some unusual fire behavior in recent history, but it does need to be put into context.

Bryan Bird
Bryan Bird

@Russell Scalf Only in certain forest types. Your perspective glosses over the various Fire Return Intervals that characterize different forest types. Just a portion of Western U.S. forests demonstrated the  "fast, low-intensity" fire behavior your mention. Many other forest types experience mixed severity and even high severity, stand replacing fire. there is even science now demonstrating that high burn severity fire ecosystems (basically forests of dead snags) are rare in the Sierra Nevada and in response so are some wildlife species that depend on burned forests. Crown fire is not always a destructive event.

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