Energy release component percentile for vegetation in California, 29 July 2018. California's drier vegetation leads to hotter fires. Increasingly warm temperatures are creating record and near-record dryness in the vegetation, which causes wildfires to burn more intensely. Data: Northwest Climate Toolbox. Graphic: Los Angeles Times

By Rong-Gong Lin II and Ruben Vives
31 July 2018

(Los Angeles Times) – The northern Sacramento Valley was well on its way to recording the hottest July on record when the Carr fire swept into town Thursday.

It was 113 degrees, and months of above-average temperatures had left the land bone-dry and ready to explode. Within a few hours, hundreds of structures were lost and six people killed.

The destruction adds to California’s worst wildfire year on record — dozens dead since October, with more than 10,000 structures lost from San Diego to Redding.

There are many reasons for the grim totals, but experts say one common denominator connects the disastrous fires: California is facing extreme heat, the likes of which it has never seen in the modern historical record.

“The temperatures have just been almost inexorably warmer all the time,” said UCLA climate scientist Daniel Swain, and fires “burn more intensely if the fuels are extremely dry.”

In the past, there has been some reluctance among scientists to cite climate change as a major factor in California’s worsening wildfires. Human-caused ignitions and homes being built ever closer to forests have played a large role. But the connection between rising temperatures in California and tinder-dry vegetation is becoming impossible to ignore, according to experts who study climate and wildfires.

“The regional temperatures in the western U.S. have increased by 2 degrees since the 1970s,” said Jennifer Balch, director of Earth Lab at the University of Colorado, Boulder. “You’re seeing the effect of climate change.”

Average maximum and minimum California temperatures, 1895-2017. Data: NOAA. Graphic: Los Angeles Times

Neil Lareau, assistant professor of atmospheric sciences at the University of Nevada, Reno, said unusual warmth is now routine, and that heat “leads to drying things out quicker.”

Vegetation can have various degrees of dryness — a wet log in the woods could smolder before puttering out, while tinder-dry chaparral on a 110-degree day could explode when ignited, Swain said. Extremely flammable vegetation can create a particularly intense fire with the potential to grow much faster — leaving less time for firefighters to get a handle on a blaze and for people to escape.

“What that means is the fire has to do less work to ignite the vegetation right next to it. And it can spread faster, and it releases energy more quickly,” Lareau said. […]

As the Carr fire rapidly expanded, the Redding area was experiencing record-tying temperatures. In Southern California in October and November — in the middle of a punishing spate of wildfires — the average temperature was the hottest in more than 120 years of record keeping. San Francisco hit its all-time heat record in September, with a downtown reading of 106; in July, all-time temperature records shattered throughout Southern California, with Burbank hitting 114 and Van Nuys 117.

Redding’s temperature of 113 on Thursday wasn’t unheard of for that time of year, but Swain said it was the accumulation of intense heat over the previous weeks and months that added to the problem.

The resulting dry vegetation was a key factor in the Carr fire, Swain said. There was no wind preceding the blaze in Redding — no Santa Anas or Diablos whipping it up. Instead, the exceptionally dry vegetation produced intense heat that shot hot air up to 39,000 feet into the sky at speeds of up to 130 mph, Lareau said. That air was replaced by air moving in at the base of the fire, in a movement that appeared like a tornado.

Satellite view of burn scars from wildfires around Redding, California in July 2018. Photo: Los Angeles Times

“This fire vortex, this pretty terrifying tornado-like feature, and I don’t say that lightly … was made possible by the extreme heat produced by this fire,” Swain said. “To see that in the brush- and mixed-forest region immediately adjacent to a city of 100,000 people in California was pretty extraordinary.” [more]

The common thread in California's wildfires: heat like the state has never seen



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