A firefighter takes cover as high winds whip embers, as the Camp Fire burned out of control through Paradise, fueled by high winds in Butte County, Calif., on Thursday. (Peter Dasilva/EPA-EFE)

This story has been updated.

In mere hours last Thursday, the Camp Fire - California’s most destructive on record - transformed Paradise into hell. At least 29 people died in the blaze and over 200 are missing. Still only 25 percent contained, the inferno has now consumed an estimated 111,000 acres and burned through nearly 7,000 structures.

A second blaze, dubbed the Woosley Fire, in Southern California, has surged to over 85,500 acres since its inception late Thursday, and destroyed 177 structures.

How did these wildfires grow so explosively? It began with scant rainfall and abnormally warm temperatures which parched the landscape and created tinderbox conditions. Then came howling winds that fanned the flames, once the fires were sparked. And, in an environment of rising temperatures, climate change increased their potential intensity.

California’s fire season typically begins in late spring, around the end of May into early June. It lasts until wintertime rains arrive in November or December, when “atmospheric rivers” carry tropical moisture into the otherwise arid region.

But this fall, hot and dry weather has persisted longer, and the rains have yet to come — fitting into a trend toward longer fire seasons. Increasingly, fire season isn’t constrained to the warm months, but occurring year-round.

Climate change has “shortened the rainy season at both ends" said Daniel Swain, a climate scientist at the University of California Los Angeles, in an email.

Paradise has received just 0.88 inches of rain since May 1, compared with a historical average of over 7 inches during the same period.

“If Northern California had received anywhere near the typical amount of autumn precipitation this year ... explosive fire behavior & stunning tragedy in #Paradise would almost certainly not have occurred,” Swain wrote in a tweet.

When it’s this dry, the turbulent Santa Ana winds, a seasonal breeze that can crescendo to a full-on gale, pose a serious danger.

The winds swirl around a semi-permanent high-pressure ridge that establishes itself each summer along the northern reaches of the Sierra Nevada, around Reno. Spinning clockwise, this system pushes wind down the chain of mountains westward toward the coast.


Landsat 8 imagery composite showing the location of the Camp Fire near Paradise, Calif.

When air moves downhill, it is compressed because of the greater weight of the atmosphere — and subsequently warms. But this “downslope” warming dries out the air, and the relative humidity percentages can tank into single digits.

It’s this process that ushers a bone-dry airmass into the heavily populated spine of the Golden State. On Thursday, the National Weather Service office serving the San Francisco Bay Area measured record low levels of atmospheric moisture for the time of year.

This dry downsloping is often accompanied by 60-mph winds. When the Santa Ana winds arrive, any seemingly innocuous flame can quickly become deadly.

The warming climate only makes a hot, dry weather pattern — prone to fire-fanning winds — more extreme. It intensifies a process known as evapotranspiration in which plants release water into the air. The warmer the air, the more water plants release.

These warmer temperatures translate to the vegetation drying up, littering the ground as crisp, brittle plant matter ripe for fueling a blaze. The soil is also starved of moisture, offering little protection against the spread of fires.

“[L]ocal vegetation was at record dry levels for the time of year when the fire started, and that is part of the reason why this event ultimately became as bad as it did,” Swain said.

California is warming — fast. Faster than many other states.

The January through October period ranked as the fourth-warmest on record. The five warmest years on record have all occurred in the last five years.

California’s Forth Climate Change Assessment projects a spike of 5.6 to 8.8 degrees in daily average temperatures by 2100 — far above the national mean.

Los Angeles has warmed 3.2 degrees since the 1940s. Farther north, the increase has been even more noteworthy.

As temperatures have warmed, Swain noted that autumns in California have trended drier in recent decades.

With a longer fall dry season, and wintertime precipitation falling in shorter but more intense bursts, the stage is set for routine drought during the summer months, which then sets up dangerous wildfire conditions. It’s a vicious cycle.

Last year, the U.S. Department of Agriculture announced that a record 129 million trees — “mostly conifers” — died as a result of drought. The agency cited an “increased wildfire threat” for 2018 in its December 2017 report, adding that 56 percent of its national budget was consumed by firefighting efforts.

Going into the 2018 fire season, experts already knew it would be bad. The 129 million dead trees covered a historic 8.9 million acres, the department said.

But the last piece of the puzzle? Where we build. Irrespective of climate change, wildfires will inevitably claim more structures if we continue to build and expand their targets.

Experts refer to the wildlife-urban interface — WUI — the approximately half-mile radius at the periphery of communities where the forests and humans “meet.” According to a paper published in the Proceedings of the National Academy of Sciences in March, there has been a 41 percent increase in the number of homes built in the WUI between 1990 and 2010 — totaling 43.4 million homes.

This, the authors explain, could be a grave problem. “Wildfire problems will not abate if recent housing growth trends continue.”

Swain said the intrusion of wildfires into populated urban areas “used to be really rare historically, but has been happening with alarming frequency recently.” Combined with the effects of climate change, that factor is leading the trend toward larger and more destructive fires in California, he said.