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Mounting evidence links our region's increase in destructive wildfires to climate change, researchers say.

Seven years ago this month, scientists here and elsewhere who published a groundbreaking, well-publicized study on Western wildfires said the buildup of small trees in forests was more responsible for the explosion in Southwestern wildfires than hot weather.

But one of those scientists, UA tree ring lab director Thomas Swetnam, says he now believes weather - warming, drying and extreme wind events - is the biggest factor driving our devastating fires.

Not all fire researchers agree, but many do.

"The relationship between warming and wildfires in the Southwest is really, really clear now," says Los Alamos National Laboratory researcher Park Williams.

He, Swetnam and other scientists also remain concerned about the immense fuel buildup in the forests, chaparral and grasslands, due to a century-old practice of fire suppression. Researchers say that problem could get worse because, for budget reasons, the federal government plans to cut spending for removing fuels from the forests and deserts by about 40 percent, or $203 million, for the upcoming fiscal year. The proposed Obama administration budget for fire suppression and preparedness would rise, however, by about $210 million.

Last week's Yarnell Hill Fire that killed 19 firefighters about 30 miles southwest of Prescott was sparked by a lighting strike during a spell of extreme heat - about 10 degrees above normal - that has been linked to climate change, along with dry weather and high winds.

But fire scientists agree that no one blaze can be attributed directly to global warming, just as they agree that no single heat wave can be tied to global warming. Overall, though, the region's increased fires and the warm, dry conditions accompanying them are consistent with what's happening or what's been predicted to happen with human-caused climate change, says Glen MacDonald, director of UCLA's Institute of the Environment and Sustainability.

At the same time, today's vegetation buildup on Southwest wildlands is essentially "embodied solar energy," which a big fire releases all at once, says Donald Falk, a fire researcher at UA's School of Natural Resources and the Environment. In some huge fires, that energy is similar in scale to when a nuclear bomb explodes, Falk says.

"I don't think you can say the cause is more climate change or more fuel accumulation," he says. "Those two factors are interacting to produce these fires."

More acreage burned

Nationally, wildfires burned an average of twice as many acres per year than they did 40 years ago, U.S. Forest Service chief Thomas Tidwell told a Senate hearing in Washington, D.C., last month.

Since Jan. 1, 2000, about 145,000 square miles have burned nationwide - about 25 percent larger than the size of Arizona, The Associated Press reported recently.

In Arizona and four other Southwestern states, the number of acres that burned in large wildfires on federal lands has increased dramatically since 2000, said a recent report on climate change prepared by University of Arizona researchers. By 2050, the amount of burned areas will rise 43 percent in Arizona and New Mexico, that report said, citing other studies.

Overall, "recent climate trends of warming and drying conditions have corresponded to major increases in the extent and severity of forest die-off and fire in the Southwest," research ecologist Craig Allen of the U.S. Geological Survey, testified in Congress last year. That link is based on evidence dating back many thousands of years, Allen says.

Already, warmer weather has increased Arizona's fire season by two months in recent decades, UA's Falk says. With less winter rain and warmer springs, snowpack melts earlier. Soil and plants dry more quickly.

"The first forest fire this year in Arizona was fought in February, in the Douglas Ranger District," Falk says. "That used to be the time of winter rains and snowpack."

his view changed

Back in 2006, the UA's Swetnam and other researchers documented that hotter temperatures and earlier spring snowmelts were contributing to larger and more destructive Western wildfires.

But Swetnam said back then that warming's effects in the Southwest weren't as severe as those of fire suppression, particularly when compared with the northern Rockies, because their lodgepole pine forests didn't have the history of fires and fire suppression as the Southwest's lower-elevation ponderosa pine forests.

Now, Swetnam says his view about Southwest fire causes has been changed by more years of extreme warming and drying here, and even more extreme fast-moving wildfires. Second, researchers have a better understanding of how rising temperatures trigger responses in the forests, such as drought and bark beetle-caused tree deaths, he says.

Swetnam, director of the UA's Laboratory of Tree-Ring Research, adds he's more convinced than ever that human-caused warming is linked to increased wildfires. He says he's been pushed in that direction by recent papers published by top climatologists attributing seasonal and long-term heat waves, and heat-aggravated drought, to human-caused warming. But, he adds, "I don't think the linkage is unequivocal yet."

This year, Swetnam and Los Alamos' Williams collaborated with other researchers on a new study that also spells out climate's impacts on forests. The study says drought-related stresses on forests such as bark beetle outbreaks that make fires more destructive are directly connected to temperature through what's known as a vapor pressure deficit.

A vapor pressure deficit, which is aggravated by heat, describes how strongly the Earth's atmosphere is trying to pull water out of our ecosystem, Williams says.

"Think of it as the sponginess of the atmosphere. The sponginess increases exponentially as you warm the atmosphere," Williams says. "It's like seeing a puddle on the streets on a hot day evaporate more quickly than on a cold day."

But it's tough to say if climate or fuels buildup is the biggest factor driving wildfires, because fires are affected by how we manage the forests, he says.

increased management

If the fuel buildup were reduced, impacts of heat and drought on fires wouldn't be as severe, Williams and Swetnam say.

"We need to vastly scale up our investment in vegetation and fuels management," Swetnam says. "We spend tens of millions of dollars in suppression costs on a single large wildfire, but only a tiny fraction of that amount beforehand to properly manage those same forests."

Thinning saved the town of Alpine from burning down in the 2011 Wallow Fire, but more thinning won't make the fire problem go away, particularly as long as people continue to build homes in the forests, UA's Falk says.

"We have to break the whole cycle - thinning forests followed by a change of land use and a change of the fire burning policy to allow fires to burn over landscapes."

If that happened, would climate change still be a factor?

"Absolutely," Falk says. "But not as explosively."

On StarNet: Track the worst Arizona wildfires over recent years at

"The first forest fire this year in Arizona was fought in February, in the Douglas Ranger District. That used to be the time of winter rains and snowpack."

Donald Falk,

fire researcher at UA's School of Natural Resources and the Environment

Contact reporter Tony Davis at or 806-7746.