New evidence shows rising temperatures accelerating frequency and intensity of Northern California wildfires
As they toil to extinguish the last of the LNU Lightning Complex fires burning across the North Bay, firefighters tasked with that Herculean labor also must contend with another adversary: the excessive heat warning issued by the National Weather Service.
Asked why it was going to be so hot over the holiday weekend, meteorologist Drew Peterson spoke of a “600-decameter high-pressure ridge” squatting over Northern California.
Think of that high-pressure system, Peterson said, “as a 20,000-foot-high mountain of hot, dry air sitting over us.”
“We’ve had them in the past,” he said of the Denali-sized dome bringing furnace-like conditions to inland areas this weekend. “But the frequency with which we’re seeing new, record temperatures is unusually high. And it’s a direct result of warming across the planet.”
We know by now that climate change has extended California’s fire season, which begins earlier and ends later, if it ends at all. What’s becoming clearer, according to climate scientists, meteorologists and fire experts, is that even modest increases in temperature have dramatically ratcheted up wildfire risk.
The ingredients required for a catastrophic wildfire “are lining up more frequently,” said John Abatzoglou, assistant professor of fire science at UC Merced. Longer fire seasons combined with parched fuels are resulting in more extreme fire danger days, or as Abatzaglou put it, “many more spins on the roulette wheel.”
Rising temperatures, drier fuels
While many Northern Californians speak of wildfires, evacuations and degraded air quality as “the new normal,” Daniel Swain doesn’t find that expression useful. It suggests a stability that does not exist.
“We have not arrived at some ’new normal,’” said Swain, a climate scientist at UCLA and author of the influential Weather West blog.
“The reality is we’re not in some new, stable-but-warmer state. We just continue to move progressively in the same direction,” he said.
That direction is hotter and drier. California has warmed 1.7 degrees Celsius over the last 125 years — though some parts of the state have heated up much more than others — with most of that temperature increase occurring over the last three or four decades, Swain said. “That doesn’t sound like a lot, but it is a lot.”
“I think a lot of folks underestimated what this would actually mean on the ground” in terms of fire risk. “We’re seeing some pretty major impacts already.”
During an interview Wednesday, Swain pointed out the second-, third- and seventh-largest fires in California history were burning at that moment.
He was referring to the SCU Lightning Complex, the LNU Lightning Complex and August Complex fires, respectively. (The latter blaze, roughly 45 miles northeast of Ukiah, had charred 298,629 acres in the Mendocino National Forest as of Friday, and moved up to No. 4).
While no one thought climate change would decrease fire risk, Swain said, he and his peers didn’t realize that “even very incremental changes in the atmosphere can really amplify what’s going on.”
“You may have heard of the Clausius-Clapeyron relationship.” he said to a reporter, hopefully.
The reporter had not.
“It’s an exponential function that describes moisture as a function of temperature.”
For every one-degree increase (Celsius) in temperature, he explained, the amount of water vapor that could be in the air increases by 7% — “like compounding interest.”
What does that have to do with wildfire risk?
As the amount of water vapor that could be in the air goes up, the amount that is actually in the air can’t keep up. That gap creates a vacuum that sucks moisture out of trees and plants. It makes them drier, more parched, more flammable.
It resulted, during those infamous August lightning storms, in an exceptionally high ignition rate: between 4% and 6% of each strike resulting in a fire, Swain estimated, due to the dryness of the vegetation.
The fires spawned by that lightning moved with a speed that surprised Swain.
“We’ve seen some of these crazy fires in the past, moving very fast, taking out neighborhoods and killing people,” he said. “But almost all of those have been wind-driven events” — conflagrations abetted by offshore winds coming from the east and north.
The blazes that blackened over a million acres in Northern California over the past three weeks did so without help from the kinds of offshore winds that accelerated the 2017 fires and the Kincade fire last year.
The winds whipping these latest blazes were generated, in large part, by the fires themselves. “That’s part of the story,” Swain said, “when you have drier fuel that burns hotter, more intensely.“