Arizona is among the West’s leading states in the growth of big wildfires, says a new report linking the trend to climate change.

This state is second among 11 Western states in growth in the number and size of large wildfires on Forest Service land since 1970, says the study from the nonprofit research organization Climate Central. Arizona is also predicted to have by far the largest increase in the number of days with high wildfire potential by 2050.

Arizona’s ranking is apparently due to the fact that it had the second fastest increase of all the states in spring-summer temperatures since the 1970s, one of the study’s authors said.

“Arizona is actually among the fastest warming states in the entire country,” said the researcher, Alyson Kenward, a Climate Central senior scientist and vice president. “In terms of future risk, Arizona is projected to get even hotter and drier and to be the driest in the West, which is why our projections are for the state to see the most high wildfire potential days.”

Across the West, the researchers found a direct correlation between dramatically rising large wildfire totals and sizes and rising temperatures that many other studies have linked to carbon dioxide and other greenhouse gases emitted into the air.

The Climate Central report is the most recent of three studies in the past year linking rising temperatures and the growth in large Western wildfires.

Climate Central noted that the 2015 wildfire season was the worst on record, nationally, with more than 10 million acres burned across the entire West. The group’s map shows 12 wildfires currently burning in Arizona — three in Southeastern Arizona. The study found the number of large fires annually in the Western U.S. more than tripled between the 1970s and 2010s.

The costs of fighting these fires has also soared. Federal spending on that topped $2 billion nationally in 2015, 15 years after hitting $1 billion, wrote the author of a second study, Anthony Westerling, an associate environmental engineering professor at University of California, Merced.

The Climate Central study was not a formal attribution study saying warming temperatures have caused the increase in wildfires. But the researchers said it’s clear that hotter weather generally increases wildfire risk.

It’s also not yet possible to blame an individual wildfire on human-caused climate change, even though “climate change may be creating ideal conditions for more Western wildfires,” the new report said.

“What strikes me is that across the West, the number of large wildfires and acreage burning is increasing and over that time, temperatures are increasing,” scientist Kenward said. “It’s not that wildfires are good or bad. They are a natural thing.

“But more and more in the West, people are moving close to areas where wildfires can burn. In the future we expect temperatures to keep rising and snowpack to melt earlier,” Kenward said. “The risk to wildfires will increase and more people will be exposed to higher risk.”

For Arizona, Climate Central found:

  • The number of large wildfires grew an average of 11 annually from the 1970s to the 2010s — second to Idaho’s 21. The report defines large wildfires as those covering more than 1,000 acres.
  • The average annual acreage burned in Arizona grew 186,000 acres from the 1970s through the 2010s. That’s also second to Idaho, which had an increase of 305,000.
  • Arizona’s average annual temperature increase over that period of 2.3 degrees was second to New Mexico’s 2.67.
  • Arizona ranks second in the number of people living in areas considered at risk for wildfires. About 2.886 million Arizonans live in what’s known as the Wildland Urban Interface.
  • Forty-five percent of Arizonans live in fire-risk areas — the fifth highest percentage among Western states.
  • By 2050, Arizona will have 34 more days with high wildfire potential compared to today. To predict the number of future high-risk fire days, the study used a drought index that measures the dryness of the top 8 inches of a forest floor, which serves as a proxy for the dryness of forest fuels.

Westwide, the average annual number of large fires has more than tripled from the 1970s to the 2010s, the new report said. The area that these fires burned has risen by more than six times in the same period.

Longer fire season

The link between Western wildfires and climate change was first explored in detail in a decade-old study written by Westerling and a group of other researchers at Scripps Institution in San Diego. He and several other researchers have agreed the link has strengthened since then, as temperatures, the number and size of wildfires have grown.

That was the conclusion of Westerling’s new study this spring that looked at wildfire trends since the 1970s. It covered National Park Service, Bureau of Land Management and tribal land as well as Forest Service land. It was published in the British journal Philosophical Transactions of the Royal Society of Publishing.

Looking strictly at forest land, he found dramatic increases in the amount of total acreage burned, the amount of fires and the length of the fire season across the West between the period 1973-82 and 2003-12. The entire West’s fire season, for instance, increased 84 days over that time, while that of the Southwest increased 112 days.

The Climate Central study, which also found a longer fire season today compared to the ’70s, concluded that the season “is approaching the point where the notion of a fire season will be made obsolete by the reality of year-round wildfires across the West.”

Westerling found that the number of wildfires in forests rose 556 percent across the West and 462 percent in the Southwest over his study period. The number of acres burned rose 1,271 percent Westwide and 1,466 percent in the Southwest. On nonforested land, the pace of growth isn’t so fast — 65 percent a decade increase in acres over three decades.

A third study, published last year, found that from 1984 to 2013 the amount of moderately and severely burned area in just the Southwest rose 10.2 percent a year. Just looking at forest land, the increase was 16.5 percent annually, said the study. Its 12 authors included Michael Crimmins, a University of Arizona associate professor of global change and soil, water and environmental science, and Thomas Swetnam, the now-retired director of UA’s Laboratory of Tree Ring Research.

Higher temperatures linked

All three studies, to one extent or another, echoed Westerling’s conclusion that, “These changes in wildfires are strongly linked to drying from warming temperatures and earlier spring snowmelt.”

In the spring and summer, hotter temperatures lead to the drying of fire-prone duff and downed wood on the forest floor and of live trees, Climate Central said. Typically, the years of the hottest spring and summer temperatures tended to be the years with the most wildfires, scientist Kenward said.

“Drier fuels are more likely to ignite from lightning strikes and human activity,” Climate Central said. “And in hotter and drier climates, those fires that do start are more likely to find ideal fire conditions over larger areas, leading to more area burning.”

Another factor the studies cited is declining snowpack. Across the West, more precipitation falls as rain rather than snow these days. That means less snowpack stored and less water available later in the year to keep fuels most.

“Likewise, hotter spring temperatures lead to earlier melting of the snowpack, causing a similar loss of water available in the hotter and drier times of the year,” Climate Central said.

Westerling said his research found that the areas with the earliest spring snowmelt accounted for 70 percent of the area burned in large forest wildfires and 43 percent of the area burned in nonforest wildfires.

“Indeed, most large forest wildfires in the West occurred in warmer years with earlier spring snowmelt,” Westerling wrote, although he didn’t find a strong link between the Southwest’s wildfire increase and the earlier spring snowmelt.

Fire suppression impact

Another factor that many authorities have traced to the growth of Western wildfires is the region’s prolonged history of fire suppression. It’s generally accepted that the tendency of firefighting agencies such as the Forest Service to try to extinguish every blaze in the past allowed tens of thousands of small trees to grow that are now obvious fuel for future fires. They tend to be much bigger and more severe than those in the past.

None of these studies looked in detail at fire suppression impacts on past or future fires. Westerling wrote that while management of the landscape can influence wildfire in many ways, “it is a warming climate that is drying out western U.S. forests and leading to more, larger wildfires and a longer wildfire season.”

Climate Central’s Kenward said that its projections for longer fire seasons in the future don’t account for any potential changes in wildfire suppression practices, or for changes in how humans living in wildfire zones would behave.

“We’re just saying that one of the things we absolutely need for wildfires is dry fuel, and we used a measure of how dry we’re expecting or projecting the fuel on the ground in the West to be” to predict future fire risks, Kenward said.