SEATTLE, Oct 19 (Thomson Reuters Foundation) - As firefighters slowly gain ground on the deadliest wildfires in Californian history, experts predict the problem will get worse unless "rampant suburbanisation" near wildland is banned.
Many scientists believe climate change played a part in sparking the big U.S. blazes that erupted this month, killing at least 41 people and gutting 5,700 homes and businesses.
But others point to another likely cause - the growing number of U.S. residents who have moved to the western half of the country, spreading out from the historic cores of cities like San Francisco, Los Angeles, Phoenix and Denver.
"(This) has had an incredibly significant impact and because these landscapes are for the most part areas that have historically experienced wildfires," said Gregory L. Simon, an associate professor at the University of Colorado Denver.
From shrub fields in southern California to desert canyons in Arizona, or the Rocky Mountain foothills in Colorado, land experts say this expansion has created a growing number of areas where the built environment meets nature.
Nature has its own way of regulating itself - for example, wildfires caused by lightning are a naturally occurring phenomenon that serves an important role in these ecosystems by periodically burning out undergrowth and dead vegetation.
"Despite knowing this fire history, planners and city developers have given the green light to let these formerly open landscapes become regular suburban developments," Simon told the Thomson Reuters Foundation.
Money lies behind the move, he said, citing three distinct forces. Rural counties want to increase their tax base, home builders lobby hard to develop and high prices in city centers push moderate income earners to the urban periphery, Simon said.
Since the 1960s, the number of people in the United States living in these areas - known as "wildland-urban interface" (WUI) - has risen to 140 million from 25 million.
According to the U.S. Forest Service, 60 percent of houses built between 1990 and 2000 are in wildfire-prone areas.
Meanwhile, the number of housing units within half a mile of a national forest grew to 1.8 million in 2000 from 484,000 in 1940, National Geographic reported.
While climate change increases the risk of wildfires, human expansion into fire-prone areas exacerbates the problem, as most of the blazes are caused by people.
Despite the risks, the federal government in effect subsidises the cost by guaranteeing wildfire protection that far outstrips the contributions made by the local governments where these homeowners pay their property taxes, experts say.
Montana-based Headwaters Economics research firm said that since 2002, the average annual cost of wildfires to the federal government is $3 billion, up from $1 billion in the 1990s.
"Should we halt construction in the WUI?", Simon asked.
"Yes, that is what we should be doing. Will that happen? Probably not as much as we would like."
He noted as only 16 percent of privately-held land in wildfire-prone areas has been developed, the scope for further development is immense.
The Forest Service expects the 45 million people already living in these areas will grow by another 40 percent by 2030.
Simon's prescription is to discourage growth by providing fewer fire protection services and declare more areas as public land that cannot be developed.
Another solution would be to provide more affordable housing in central locations in the metropolitan areas closest to fire zones like Denver and San Francisco. Simon said he hoped the fires would encourage insurance companies to re-evaluate their underwriting standards and increase prices or stop reinsuring homeowners in risky areas.
In the Western states of Oregon and Washington, there is some evidence that such work is already underway. Those ideas may fall on deaf ears in wildfire country, however.
"I don't think we're going to try to reduce the amount of proposed construction," Bob Plumb, fire marshal of Chelan County, Washington, told the Thomson Reuters Foundation.
"I never like to get in the way and stop people from building something."
This rural county of apple orchards and lakeside holiday homes some 150 miles (241 km) east of Seattle has had its share of devastating fires.
In 2015, a fire burned 30 homes and several warehouses in Wenatchee, the county seat, and 30 resort homes on Lake Chelan.
Plumb was hired to help the county adopt a building code that would prevent future fires by requiring homeowners to keep vegetation clear from several metres around their houses.
But such regulation may not sit well with someone who bought a house precisely because it is nestled in a pine forest.
"Are people willing to embrace that? We're not sure," Plumb said. "It's a balancing act."
So can humans live responsibly in areas where wildfires have always been a factor?
"Yes I think they can, but it's going to take a substantial effort," said Tom Zimmerman, president of the International Wildland Fire Association.
Simon, however, has little tolerance for further development in the fire zone.
"Putting all the blame on climate change, high winds, and too much vegetation takes responsibility off the irresponsible city planners and developers who knew exactly the risks when they agreed to build these communities," he said.
"The biggest issue is the fact that we've continued to develop."