Wildfire goes wild in Arizona

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Climate and Wildfire

Wildfire goes wild in Arizona

Two walking firefighters with huge wildfire in the background
A lightning strike ignited the Yarnell (Arizona) Fire on Jun. 28, 2013. On June 30, 19 members of the Granite Mountain Hotshots, a firefighter crew, were killed by the blaze.

2013 shaped up as another hot, dry, fiery season in the American Southwest. On June 30, a sudden change in wind shifted a firestorm that killed 19 members of an elite fire-fighting crew in Arizona.

Meanwhile, the cost of fighting fires is exploding. According to the New York Times, “Twenty years ago, the Forest Service spent 13 percent of its budget on fighting fires. These days, 40 percent of its money goes to firefighting, and that is still not enough to cover the bills.”

As a result, the fund to prevent forest fires by clearing fuel around threatened communities has taken a 25 percent hit. As the west bakes and burns, are we seeing the fingerprint of global warming in the fire record? The evidence is persuasive, but not conclusive, says Max Moritz of the department of environmental science, policy, and management at the University of California at Berkeley. “People are kind of surprised when they talk to scientists to find there are not that many documented studies that actually do find shifts in fire regimes attributable to recent climate change.”

The fire record is short and spotty, making it “difficult to detect a trend in any given location and hard to attribute that definitively to climate change,” Moritz adds. “Even though most scientists who study the issue are convinced that there should be, and is, a connection, it’s a little more complex than that.”

As scientists pore over records of climate and fire, the future may include even more fire. In 2012 1, Moritz and colleagues wrote, “Given the strong linkage between fire and climate … there is little doubt that climate-induced disruptions to fire activity will occur in many areas.”

Initial image: Global annual average temperature over land and oceans. Despite the clear long-term warming trend, individual years do not always show warming, and some years show greater changes than others. These year-to-year fluctuations in temperature are due to natural processes, such as the effects of El Niños, La Niñas, and the eruption of large volcanoes. Courtesy: National Climate Assessment

Mouseover image: Statewide temperature trends in Arizona since 1920. Blue line shows a linear trend that started in 1970. Courtesy: Climate Central

Burning basics

Any understanding of how climate affects fire must be rooted in fire’s three fundamental requirements: fuel, weather and ignition, says Michael Flannigan, a professor of wildland fire at the University of Alberta. As temperatures rise, he says, wildfires become more likely:

  • Warm air can hold much more water, which increases evaporation from soil and organic material, and transpiration of water from living plants. “Unless there is an increase in precipitation, the warmer we get, the drier the fuels become, and that makes it easier for fires to spread,” Flannigan says.
  • At higher latitudes in both hemispheres, the dry season and the fire season are both extended by climate change.
  • Warm weather spawns more lightning, which sparks many of the most devastating fires.
Graph shows projected changes of U.S. soil moisture, with increase in west WA, west OR, CA, Southwest, and decrease in east WA, east OR, and ID.
When soil dries out, fires become more common and intense. Maps compare average change in soil moisture for two future periods to 1971-2000. Lower scenario assumes significant reductions in greenhouse gases pollution; higher scenario assumes continued growth.

We’ll return to a pressing question: are warming and burning trapped in a dangerous feedback loop? If warm weather is conducive to bigger wildfires that release more greenhouse gases (not just carbon dioxide but also methane), will more heat get trapped, creating more warming and more fires?

Question come easier than answers. Attaining a global view of wildfire is nettlesome, as fires come in all shapes, sizes and intensities. Before the satellite era, fires could burn without even being recorded. Long natural cycles, caused by processes like El Niño, skew winds, temperatures and precipitation across the planet, causing cyclic changes in burn statistics.

Human decisions matter: rapid homebuilding in beautiful, fire-prone landscapes is a key reason why firefighting cost the federal government $1.9 billion in 2012, up from $240 million in 1985.

Forests are complex places. According to the latest draft of the National Climate Assessment, “Climate change is increasing the vulnerability of forests to ecosystem change and tree mortality through fire, insect infestations, drought, and disease outbreaks. Western U.S. forests are particularly vulnerable to increased wildfire and insect outbreaks…”

Smokey the Bear’s all-out war on wildfire has left a lot of fuel in the American West, observes Jennifer Marlon, an associate research scientist at the Yale School of Forestry and Environmental Studies. “We have a really big fire deficit, because we have been putting fires out, and we are just starting to catch up. These forests want to burn in a sense, especially if they get hotter and drier.”

The area burned in North America “is definitely increasing,” says Flannigan. In the Western U.S., “Generally 1 percent of the fires above 200 hectares in size are responsible for 99 percent of the area burned; the tail wags the dog.” These intense “crown fires” burn the treetops and are difficult to contain.

Wildfire: More than just forests

In wildfires, the effect of climate warming varies by place and ecosystem. The clearest rise in forest fires is in “boreal” forests located in the north polar region. “There are two sides to the story,” says Guido van der Werf, a researcher who studies global satellite data at VU University Amsterdam. “It’s fairly safe for the boreal forest, and somewhat true for temperate forests, that if it’s dry and warm, you will get more fires. Temperature is on the increase, and forest fires are on the increase.”

But van der Werf says we often forget savannas, the vast grass- and shrub-lands studded with occasional trees that are the source of half of the carbon dioxide created by wildfire. Savannas, mainly in Africa, Australia and South America, “are going in the opposite direction as boreal forest,” he says. As more savannas are converted to agriculture, they burn less often, “so globally we are seeing a shift away from savanna dominance of fires toward the forests.”

The satellite record “is too short to say whether there is a clear trend globally, but the evidence is solid in the boreal forest,” says van der Werf. “It’s pretty simple: If it gets warmer, it gets drier and you get more fire.” And warmer and drier “is what most climate models suggest.”

Satellite view of Tasmania shows small red circles and smoke plumes
Since the end of December, 2012, a record-breaking heat wave fueled hundreds of bushfires raged throughout Australia. Some of the worst fires struck Tasmania, a large island off the south coast. NASA’s Moderate Resolution Imaging Spectroradiometer viewed numerous fires on January 6, 2013. Red outlines indicate hot spots.
Courtesy: NASA

Looking into history

Scientists can explore the past relationship of climate and fire by studying sedimentary deposits of charcoal and soot. These relics of old fires, which “form a natural archive that reconstructs wildfire over decades and millennia,” have been attracting attention recently, says Marlon. “Most people who have looked at long-term [climate] change looked at a particular site; they have not until the last five or 10 years put all the data together, to get an idea of what happened across entire continents or the entire world.”

“When global temperature rises, we see more charcoal in lake sediment, and that’s indicative of more fire.”

Marlon says a new global database of more than 800 charcoal records is helping fill in the blanks on the last 12,000 years. “This is the entire course of human civilization, and it shows a straightforward upward trend in fire.”

Clearly, that’s partly because glaciers were retreating as the last ice age faded, but the warming since they melted does carry a warning, she points out. “Forests, grasslands and range have burned a lot more. It’s a simple relationship: when global temperature rises, we see more charcoal in lake sediment, and that’s indicative of more fire on the landscape.”

Better than a crystal ball

In an effort to sort out what climate models tell us about future fire, Moritz and colleagues ran 16 climate models. The idea is to compensate for individual shortcomings, he says. “We don’t know which of the climate models is most likely to show a realistic scenario, and one way to tackle that uncertainty is to take them all, do an ensemble forecast, and then look for agreement in the results.”

Maps show increases in fire intensity, especially in North America, Central Asia, Middle East and North Africa.
A study based on a group of climate models predicts a widespread increase in fire intensity, especially starting in 2070.

The model results, Moritz explains, were used to nail down three groups of variables:

  • Primary productivity: How much biomass (fuel) will accumulate per year;
  • Fire weather: How hot, dry and long is the fire season; and
  • Ignition, what is likelihood that lightning will start a fire when the other conditions are ripe for it?

The study provides a theoretical basis for interpreting the current fire situation in terms of climate change, Moritz says. “The model is telling us what we can probably expect, and it’s what we are seeing now. More and more of us are concluding that there is probably a link with climate change. There are so many converging lines of evidence that lead to us say, ‘This probably is it.’ More and more people are worried about it.”

In a 2013 publication2, Flannigan and colleagues forecasted the severity of fires based on multiple computer climate models and emission scenarios. “We found across all the models and all the scenarios, a 150 percent to 300 percent increase in the severity rating across the globe by the end of the century,” Flannigan told us. If the 1 percent of fires that do the most damage become more frequent, “we are going to have problems.”

Just as we are learning that floodplains are dangerous, we need to heed zones with outlandish fire danger, says Flannigan. “There are going to be floods in flood-prone areas, and fires in fire-prone areas. We have to change the way we do things.”

Initial image: The colors compare temperature from 1991-2011 to the 1901-1960 average. Bars compare regional average temperatures to 1901-1960. The period 2001 to 2011 was warmer than any previous decade in every region. Roll over to see changes in precipitation.
Mouseover image: The colors compare annual precipitation from 1991-2011 to the 1901-1960 average. Most areas were wetter. Bars compare average precipitation by decade to 1901-1960.
Graph shows the chance of extreme drought increasing quickly during 21st century
Most of the U.S. and Mexico are likely to face increasing drought in the coming century, as measured by the Palmer Drought Severity Index under a mid-range emissions scenario.
Red: based on observed temperature and precipitation;
Blue: from the average of 19 climate models;
Gray (in background): individual results from over 70 model simulations.

Fiddling with feedback

Let’s return to the feedback question: Will warming beget fires that beget greenhouse gases and therefore more warming?

On a regional scale, positive feedback has been studied best in the Amazon. After forest cover is removed, the open land dries and is exposed to more sunlight and wind, and intense fires emitted gases and particles that changed local microclimates. “Some fires in the Amazon can get so big that the clouds and winds can affect the regional climate,” says Marlon. “At a global scale, if we start seeing a trend in wildfire that affects greenhouse gases, it could theoretically” become a noticeable and positive feedback.

Smoke rising from black peatland with meandering stream and grassland
Peat, shown afire in Alaska, can burn if it dries out. Once ignited, peat smolders and can burn undetected for very long periods, releasing large amounts of carbon into the atmosphere.
Courtesy Merritt Turetsky, University of Guelph

Peat, a little-noticed, partly-decomposed organic material that can store vast amounts of carbon for centuries, may also play a role in feedback. Peat fires in Indonesia have made news after soggy peatlands were drained to make palm oil plantations (ironically for a “low-carbon” biofuel) and the fires released vast tonnages of carbon into the atmosphere.

Peat is one example of stored carbon that can initiate positive feedback. “There is potential for positive feedback, especially if peatlands start to burn deeper and more frequently,” says Flannigan. “We had a fire in Alberta over the last couple of years that burned two meters of peat. This is legacy carbon, it’s been building up for thousands of years since the last ice age, and it can go up in smoke in hours, and be pumped into the atmosphere, and so a positive feedback is a definite possibility.”

Understanding the global picture of wildfire is difficult, Flannigan notes. “This regional variation in fire activity will continue. Not everywhere is going to have a bad fire year every year.” The various broad-scale oscillations of oceans and weather will cause surges and dips in fire levels, he says, “but the general trend is an increase in fire, and I see no reason for that to stop.”

— David J. Tenenbaum

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Terry Devitt, editor; S.V. Medaris, designer/illustrator; Yilang Peng, project assistant; David J. Tenenbaum, feature writer; Amy Toburen, content development executive


  1. Max Moritz et al, 2012. Climate change and disruptions to global fire activity. Ecosphere 3(6):49. http://dx.doi.org/10.1890/ES11-00345.1
  2. Global wildland fire season severity in the 21st century, Mike Flannigan et al, Forest Ecology and Management 294 (2013) 54-61
  3. The National Climate Assessment and Development Advisory Committee, January, 2013 draft.
  4. [Video] Fighting wildfires
  5. How climate change affects boreal peatfire
  6. The climate context behind the deadly Arizona wildfire
  7. [Slideshow] Australia is on fire
  8. Wildfire smoke: A rising health concern with climate change
  9. Peat fires drive temperatures up
  10. Wildfire safety tips