"It's not really being controlled,"says Lee Frelich, director of the Centre for Forestry Ecology at the University of Minnesota. "You can't control fires of that intensity. When you get rain or lower wind speeds or higher humidity, and fires go down, firefighters can go in and take control.
"But when fires make their big runs, on hot days with high wind speeds and low humidity, that's when most of the acreage is burned. And there's no control."
Traditionally, the state fire season ran from June to September, but it now falls between May and October. The fire risk is heightened by increasingly earlier snow melt in the Sierra Nevada Range.
Ten other major fires are raging in the tinder dry state - 98 per cent of California is in drought - and similar conditions throughout the Trans-Mississippi West have sparked 50 serious fires. The National Interagency Fire Centre in Boise, Idaho, rates the national risk level at four out of five.
Fires are almost as ubiquitous as freeway gridlock in California. The Rim Fire has yet to equal the state's largest event, in 2003, when 15 fires torched 303,532ha, killing 15 people. In 2006, the worst national fire season, more than 96,300 fires consumed 3.9 million ha. This year 31,900 fires have burned 1.2 million ha in the US, with two months to go.
Increasingly, fire crews risk their lives protecting those who opt to live in harm's way. Last month 19 men died in a vain bid to save a remote Arizona community, left like a "moonscape".
In Colorado more than a million people live in "red-zoned" areas. Protecting them all is a daunting proposition in an era of budget cuts. This year the Forest Service has spent US$967 million ($1.24 billion) fighting fires in 10 states, leaving just US$50 million, enough for a few days. The agency diverted US$600 million from elsewhere last week.
The problem is exacerbated by a century of fire suppression in a region where fires are part of the natural cycle. In the past, explains Matthew Hurteau, assistant professor of Forest Resources at Penn State's College of Agricultural Sciences, fires often crept through forests behind fronts small enough to step over. Many trees survived. Preventing wildfires created denser forests where more trees competed for less water, compromising their immune systems and inviting deadly insect infestations that killed trees. The result was more fuel for today's fast and hot fires.
Efforts are being made to thin forests, often using prescribed fires, a tactic that ultimately reduces tree mortality from wildfires. When Greg Aplet, senior forest scientist with the Wilderness Society, visited Yosemite's northwest edge (where crews are trying to hold the Rim Fire) in July he found open forest, cleared by controlled burns. But the adjacent unburned national forest was full of fuel and "looked like a bomb ready to go off".
Given their epic scale, thinning the West's combustible forests is impossible. "Ultimately many dense forests will burn," predicts Frelich. By century's end forests, like the boreal that stretches into the Arctic, will likely be savannah.
Meanwhile, scientific studies show that climate change, with predictions of extreme heat events and diminished snowpacks, has dramatically upped the fire risk. Drier forests are also more likely to be ignited by lightning strikes.
A 2006 University of Arizona report found extreme Western wildfires have increased since 1985, due to warm springs, high summer temperatures and early snow melts. California's Sierra Nevada may see a four-fold increase in fires by 2100. Less trees means less carbon storage, accelerating this cycle.
Last year, a University of California Berkeley study that examined 16 climate models, found wildfires will multiply unless carbon emissions were reduced.
This trend mirrors projections elsewhere. And a paper in Ecosphere, the journal for the Ecological Society of America, found fires will increase at high and mid-latitudes but decrease around the equator. Climate change affects global water and air currents and will also heighten the fire risk.
This week, as the Rim Fire menaced the Hetch Hetchy Reservoir, which supplies 85 per cent of San Francisco's water, plus hydroelectric power - raising fears ash would contaminate water and forcing the city to buy power elsewhere, California Governor Jerry Brown did his best to reassure voters.
The threat to urban life emphasised that climate change, once seen as an "environmental" issue, is a far bigger challenge. Yet although fire managers widely accept climate change makes the fires more deadly, lawmakers have shied from acting. Part of the problem is that it is hard to pin the Rim Fire squarely on climate change, even though this is decisive.
With no end in sight huge resources are being thrown against the Rim Fire. Yet even if this battle is won the longer war is daunting as climate change loads the dice.
"We are recognising that we cannot fight these fires forever," warns Aplet.
"We're increasingly ineffective at stopping them. And we're blowing through unbelievable amounts of money in a futile effort to try."
