As of Thursday afternoon, the nearly 400,000-acre Wallow fire in eastern Arizona was still 0-percent contained. The burn area, bigger than Chicago and New York City combined, is already the second-largest fire in the state’s history. Arizona’s largest blaze happened in 2002, making the state seem either particularly unlucky or part of a broader trend. Are large American wildfires becoming more common?
Yes, at least in the West, home to most of the nation’s largest wildfires. A 2006 paper in Science found a dramatic increase in large U.S. forest fires (defined as those larger than 988 acres) from 1970 to 2003 in an area roughly comprising the 11 western-most contiguous states. Not only were there almost four times as many fires from 1987 to 2003 than from 1970 to 1986, but the fires in the later period burned nearly seven times more land. The researchers also found that the average time between discovering a fire and containing it increased by almost a month, from 7.5 days to 37.1 days.
The paper’s authors attribute much of the increase in large conflagrations to longer and hotter fire seasons—with snow in the West melting earlier and temperatures rising higher than in the past. As a result, trees, shrubs, and grasses are drier for longer, meaning there’s more opportunity for them to catch fire. The lengthening of the fire season has been drastic: The average time between a year’s first reported wildfire and its last was 78 days longer during the years 1987 to 2003 than 1970 to 1986, an increase of 64 percent. A study of Canadian wildfires from 1920 to 1999 found similar results, with total burn area increasing over the last three decades.
While shifts in temperature help explain the overall rise in mega-blazes, changes in population and land use might be more important in certain regions, including the Southwest. Broadly speaking, more humans mean more potential sources of ignition; indeed, the Forest Service says people (rather than lightning, sparks from rock falls, or other natural causes) started the Wallow fire, possibly via an unattended campfire. The urge to put out any uncontrolled fire may also share the blame. In certain landscapes, naturally occurring fires may burn grass, trees, and other combustibles in conveniently small sections, creating over time what ecologists call a “mosaic“—alternating patches of recently burned and unburned land. When a fire starts in one patch of the mosaic, it quickly runs out of fuel. But for much of the late 19th and early 20th centuries, we tried to suppress even the smallest natural blazes, converting the mosaic into a more homogenous swath of flammable material.
Though fire-management techniques are shifting toward a greater tolerance for small wildfires, aggressive fire suppression is still standard practice in populated regions. Ponderosa Pine forests, which have fueled the Wallow fire, are notoriously prone to the broken-mosaic problem. Historically, these forests experienced frequent, but low-intensity surface fires rather than severe canopy blazes.
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Explainer thanks Lisa Elenz of the U.S. Forest Service and Dr. Brian Oswald of the Arthur Temple College of Forestry and Agriculture.