Study: Dry Seasons — Not Brush — Spur Wildfires

A new study finds that heat waves are the most significant driving force in western wildfires. Since 1980, when fire activity increased sharply, the weather has mattered far more than the amount of built-up brush and other factors that are often blamed for destructive fires. Researchers say hot and early spring seasons make the forests tinder-dry by summer — and more likely to burn out of control. As a result, they say, global warming could intensify fires in the American West.

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Forest fires in the western United States have gotten noticeably worse in the past few decades, and a new study blames climate change. A report published online by Science magazine finds that most fires are the result of long, dry, and hot summers, and those have become much more common since 1980. NPR's Richard Harris reports.

RICHARD HARRIS reporting:

Wildfires in the west cost about $1 billion a year, both in efforts to fight them once they take off, and in efforts to thin out vegetation to help reduce the intensity of fires to begin with. Anthony Westerling at the Scripps Institution of Oceanography, decided to take a closer look at what's driving the increase in western forest fires.

Mr. ANTHONY WESTERLING (Researcher, Scripps Institution of Oceanography): Conventional wisdom has been that vegetation management is such a big part of the story. It turns out to be a small part of the story.

HARRIS: Westerling gathered records for forest fires throughout the west since 1980. He then gathered information about rainfall, temperature, snowmelt, and other data to see if climate and fires were linked.

Mr. WESTERLING: What we found, was that temperature is very important for forest fires in a large part of the western United States, and in particular, the northern Rockies. Warm years have most of the fires; cool years have very few fires.

HARRIS: The long-term heat wave in the west has not only dried out the trees, turning them to tinder, but it has caused snow packs to melt, weeks and weeks earlier, than usual.

Mr. WESTERLING: An early snow melt of a month or more has a huge increase in the length of the dry season, so you have more opportunities for a fire, as well as drier fuels, for them to burn in.

HARRIS: It's not clear how much of the extended heat wave in the west is being caused by human-induced global warming, and how much is part of a natural, 30-year climate cycle, but Westerling says his findings don't bode well.

Mr. WESTERLING: If we have climate change, anything like the scenarios that are being projected by scientists around the world, then the kind of fire seasons that we've experienced in recent years that have caused a lot of damage and led to a lot of policy changes in the western United States, are going to become much more frequent.

HARRIS: Some of those policy changes have to do with thinning the forests, as a way to reduce the intensity of forest fires. Westerling says his findings show that's not necessarily a good strategy.

Mr. WESTERLING: It doesn't mean that it's a pure waste of time, but it means it needs to be targeted to the areas where it's going to make a difference.

HARRIS: Thinning still makes sense in the forests of the southwest, for example, where the buildup of vegetation, not climate, is the driving force behind forest fires.

Mr. WESTERLING: But in parts of the northern Rockies, for example, where most of the increase in large fires has occurred, that's not going to be an effective policy.

HARRIS: These results come as no surprise to Doc Smith at Northern Arizona University. He's been involved in fighting western forest fires for many decades.

Mr. DOC SMITH (Arizona University): There's just overwhelming evidence that this climate shift is at the root of many of our fires. Now what? Now what are we going to do?

HARRIS: Smith says it's still vital to thin certain forests, like the Ponderosa Pines, where decades of fire suppression have led to a huge buildup of tinder. But as for the climate, well, that's not something that can be modified anytime soon. Richard Harris, NPR News.

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