Potent fires have been erupting in California since mid-August.

That's when a lightning siege of over 13,000 strikes, from passing storm systems, started igniting the land. The siege sparked some 625 blazes. But the warming California environment is a big reason why certain blazes rapidly grew into some of the largest fires in Golden State history.

A measure of how quickly trees, shrubs, and grasses dry out — known as "vapor pressure deficit," or VPD — is currently at the first or second highest August levels in California (and the greater Southwest) in over 40 years, said John Abatzoglou, a fire scientist at the University of California, Merced. This dryness enabled a historic 2020 California fire season, as the state's vegetation, or fire fuel, is primed to burn.

"Fuels are incredibly dry," said Abatzoglou.

2020, while extreme, isn't a fluke. Fuels have been drying out in California for decades. "When we talk about how climate enables fire activity, we’re often talking about how dry fuels are," explained Abatzoglou. "This year is embedded within a long-term uphill climb toward warmer, drier, and smokier climates."

Temperatures in California have been rising since the late 1800s. This increases VPD, which is a measure of how much moisture is in the air versus how much moisture the air can hold. When the temperature goes up, the atmosphere can hold more water. But, in California during the summer — a land with a famously dry, warm, and sunny Mediterranean climate — there's generally little moisture in the air. So plants and trees are subjected to more heat, but little moisture. This parches vegetation, turning it to tinder.

"Fuel moisture really drives the fire business," said Mike Flannigan, a fire scientist at the University of Alberta. "If fuels get drier, it's easier for a fire to start and for a fire to spread."

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In August, a heat wave of rare intensity (stoked by a large scale, persistent, warm weather pattern one meteorologist called a "heat ridge of death") settled over California and other Western areas. The event perhaps even set the record for the highest temperature ever recorded on Earth. Such an extreme event amplifies VPD and the associated parched vegetation.

"It's all about the extremes in the fire business," said Flannigan, referencing long, dry, and hot weather events. "When you get that, you’ve got a fire problem." Under such conditions, grasses and pine needles can dry out in a matter of hours, he said. A big heat event will eventually parch almost everything. "Unless you're in a swamp, with seven to 10 days of warm weather you can have a raging inferno," said Flannigan.

Unfortunately for California in August, dried out fuels met a rare lightning siege. "Wegot very unfortunate timing," said Abatzoglou. Around four of every 100 strikes ignited blazes. And some of the blazes inevitably exploded. The state fire agency, Cal Fire, said Monday it was battling over two dozen major fires.

The big picture in California is clear. In research published last year, Abatzoglou and other fire scientists found that between 1972 and 2018, the amount of land burned in California increased fivefold, largely due to summertime burning in forests. "Increased summer forest‐fire area very likely occurred due to increased atmospheric aridity caused by warming," the researchers concluded, as warming significantly increased the vapor pressure deficit.

In parched forests, grasslands, and tundra, rapidly spreading flames mean ever-larger fires. The evidence is conspicuous. The 10 biggest fires in California history have all burned in the last 17 years, since 2003. The three largest blazes have burned since 2018 — two of which are currently burning (the LNU Lightning Complex and the SCU Lightning Complex).

The relentlessly warming climate isn't the only factor driving California's dramatic increase in burned area. Some fires, like the deadly Camp Fire in 2018, are enhanced by notoriously potent fall winds. Importantly, forest mismanagement has also allowed forests to grow unnaturally overcrowded. Decades of suppressing flames has inhibited the natural clearing of dense forests. This creates an abnormal amount of fuel (a problem the state and U.S. Forest Service just agreed to tackle by thinning 1 million acres of California forest annually.)

Still, a warming climate creates conditions for these fuels to easily spread flames. What's more, in 2020 some large fires have taken hold in areas that just burned in recent years, notably around Lake Berryessa in Northern California. And, some of California's largest and most destructive fires, specifically those in Southern California, aren't burning in forests but in the region's iconic oak shrublands, known as chaparral.

California's 2020 fire season isn't nearly over. Rains typically don't arrive until around November. Meanwhile, dry, gusty winds pick up in September, fanning any flames over parched fuels.

"We’re coming to the prime time for the fire season," said Flannigan. "It could get a lot worse."