Volatile weather patterns marked by shortened winters, stifling heat waves and prolonged droughts. New housing developments encroaching on fire-prone lands. Shrinking budgets for fire-prevention measures.
That dangerous combination of factors helps explain the increasingly voracious wildfires that have ripped through the western United States in recent years, say scientists, lawmakers and historians.
While the deaths of 19 firefighters Sunday in Arizona marked the most lethal firefighting incident in generations, the 8,400-acre blaze that led to the tragedy has become more the norm than the exception.
"On average, wildfires burn twice as many acres each year as compared to 40 years ago. Last year, the fires were massive in size, coinciding with increased temperatures and early snow melt in the West," U.S. Forest Service Chief Tom Tidwell told lawmakers on Capitol Hill last month, adding, "The last two decades have seen fires that are extraordinary in their size, intensity and impacts."
Opinions differ on the precise reasons for the phenomenon. But broad agreement exists that climate change, coupled with economic development and state and federal policies on fire prevention, has played a significant role in shaping the fires raging across Western landscapes.
"This is the cost of how we live today," said Stephen Pyne, a professor in the School of Life Sciences at Arizona State University and a well-known fire historian. "Context does matter. Everything that's out there, fire reacts to. . . . How we develop things, what kind of vegetation we have, how we live on the land, what fire protection measures we take."
The paradox of destructive wildfires of recent years is that the West actually suffers from a "fire deficit," Pyne said.
"We're getting large, high-intensity fires where they shouldn't be," he said. "In an ideal world, we would get three or four times more fires than we're getting, but they would be on a smaller scale. More landscape, but less intensity. We have too many of the wrong kind of fires."
Figures from the Idaho-based National Interagency Fire Center show that federal spending on firefighting has risen dramatically over the past two decades. In 1993, the data show, federal agencies spent about $240 million fighting fires on nearly 1.8 million acres of land. Last year, the government spent nearly 10 times as much, about $1.9 billion, to combat fires on about 9.3 million acres.
In addition, NIFC research shows that of the largest documented wildfires in U.S. history, most took place either before the early 1900s, when the government settled on a policy to fight all wildfires, or in the past two decades.
The trend seems unlikely to change anytime soon. The Quadrennial Fire Review, a wildfire crystal ball of sorts that comes out every four years, predicted in 2009 that the effects of climate change would lead to "greater probability of longer and bigger fire seasons, in more regions in the nation" - in particular, shorter, wetter winters coupled with warmer, drier summers. The report also foresaw strained fire agency budget resources at all levels - federal, tribal, state and local.
A 2013 report that looked specifically at climate change in the Southwest noted that relatively wet conditions during the 1980s and 1990s gave way to drought around 2000, contributing to wildfires of "unprecedented size." Five states experienced their largest fires on record at least once in the past decade, including Arizona's record-breaking 500,000 acre Wallow Fire in 2011, started by an abandoned campfire.
Higher-than-average temperatures since the 1970s are also of concern. In 2011, the National Research Council projected that the total area burned by wildfires would jump 380 percent for every 1.8-degree increase in temperature.
Last month, Tidwell told lawmakers that his agency would continue to fight wildfires throughout the West despite the across-the-board 5 percent budget cuts imposed by Congress, but that it would mean making changes, such as hiring fewer firefighters this season.
Just days before the fatal fire in Yarnell, Ariz., a group of Western senators raised alarms after the Obama administration proposed sharp cuts to fire-prevention programs.
Sen. Lisa Murkowski, R-Alaska, Sen. Mark Udall, D-Colo., Sen. Ron Wyden, D-Ore., and Sen. James Risch, R-Idaho, noted that the Forest Service had repeatedly cut fire- prevention efforts such as clearing underbrush in national forests, only to spend massive amounts battling the destructive fires that result each year - money often intended for other uses.
"When the budgeted amount is insufficient, the agency continues to suppress fires by reallocating funds from other non-fire programs," the lawmakers wrote to Obama's budget director, among other Cabinet members. "This approach to paying for firefighting is nonsensical and further increases wildland fire costs.
Climate change expert Michael Crimmins of the University of Arizona said he and his colleagues are trying to analyze what happened in Yarnell and what might have been done to prevent such a disaster.
"The Yarnell Fire was a really rare combination of several different factors that you often don't see here in the Southwest that made it particularly tragic," Crimmins said, citing drought effects, record-high heat and the brimming thunderstorm season.
Thunderstorms usually bring relief in terms of wildfires, but in this case, they brought lightning - which sparked the fire - and chaotic, strong winds that changed the spread of the fire in unpredictable ways.
But in other ways, the Yarnell fire might be further evidence of a new normal. The National Interagency Coordination Center, comprised of representatives from various federal agencies, said that nearly two dozen other uncontained wildfires are burning throughout the country this week. Experts say fire seasons are starting earlier and ending later than in the past.
Michael Kodas, who is writing a book about the global increase in wildfires, said there's only so much humans can do to intervene. He said the century-old practice of trying to fight all wildfires has left large swaths of land more flammable than ever, crowded with trees and undergrowth ready to act as fuel.
In addition, he said the shortened winters and deepening droughts show no signs of abating. Nor is it likely that people will stop building homes and infrastructure in mountains and forests, where every grill, power line and cigarette represents a potential new ignition source.
"We're definitely seeing a very distinct surge in the size, speed and severity of wildfires," Kodas said. He added that while it's possible for governments to take some meaningful prevention measures and for residents to build homes with features such as metal roofs and buried propane tanks, that new wildfire reality is probably here to stay.
"To some extent," he said, "we are going to have to live with it."