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Raging wildfires: Climate changes to blame for record season?

Many scientists suggest that climate changes could be causing certain kinds of wildfires for which the West's forests are not well adapted. This year could help researchers better understand and fight the trend.

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"I'm sitting here in Idaho, and it's unusual for our mountains to be green this time of year," Ms. D'Amico says.

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Several factors have contributed to this year's remarkable fire season, explains Laura McCarthy, senior policy adviser for fire and forest restoration at the Nature Conservancy office in Santa Fe, N.M.

Fire policies for most of the 20th century focused on snuffing out every blaze. This reduced the frequency of ground-hugging, low-intensity fires that would clear the forest floors of shrubs, small trees, and ground litter. Without those fires, forests became more densely populated with trees and shrubs of varying heights – ladders for flames to reach the pitch-rich crowns of mature trees.

Despite efforts to restore forests to more-natural fire regimes, "about 80 percent of our forests in the Lower 48 states ... are in either a moderately or severely degraded condition, which means they're experiencing too much, too little, or the wrong kind of fire," Ms. McCarthy says.

Moreover, the climate change that scientists have documented in the West has extended the fire season.

Shorter-term climate conditions also have contributed to this year's fires, according to Martin Hoerling, a researcher at the National Oceanic and Atmospheric Administration's Earth System Research Laboratory in Boulder, Colo. La Niña conditions in the tropical Pacific from late last summer through this spring pushed the average storm tracks across North America farther north than usual, he explains.

Thus, the southern tier – from Arizona through northern Florida and Georgia – has been experiencing droughts ranging from extreme to exceptional (see map).

The onset of the hurricane season in the Gulf and the North American monsoon season in the Southwest may ease things somewhat, Dr. Hoerling says.

Beyond climate issues are land-use practices, McCarthy adds. Where logging has occurred, smaller trees more vulnerable to fire are left behind. Where livestock have grazed, the ground is denuded of the grasses and "fine fuels" that could sustain useful low-level fires. And urban encroachment has put more people into contact with forests, increasing the likelihood of fires set directly or indirectly by humans.

In the Southwest, where ponderosa forests play a vital role economically and ecologically, intense crown fires such as the Wallow stand a fair chance of altering the landscape for decades, if not longer, researchers say.

This year's fires could also have greenhouse-gas implications. A wildfire destroyed 26,000 acres of ponderosa pine in the Coconino National Forest near Flagstaff, Ariz., in 1996. Over the following decade, the burned area turned into a net source of carbon dioxide for the atmosphere, researchers from Northern Arizona University found.

With so much at stake, scientists are trying to figure out the right blend of techniques and the right treatment intervals for restoring more-natural fire regimes to ponderosa forests and beyond.

A team led by Northern Arizona University's Peter Fulé used data from a forest restoration site near the Grand Canyon to model the effects of potential treatments over the next 100 years.

Under a steady climate, the team found, forests could retain their resilience to wildfires if low-level fires were set every five years or so. With a warmer climate, they found that prescribed burns needed to take place only every 20 years, on average; vegetation's growth slows as the climate warms.


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