Of all the insidious threats faced by wildland firefighters — extreme heat, desiccated forests, unpredictable fire behavior and a nearly year-round fire season — what might be the most fearsome?

Humans.

People do dangerous things, things that start wildfires. Pushing a burning car into a gully. Mowing the lawn on a hot summer day. Miswiring a hot tub. Driving cars with flat tires. Burning toilet paper rather than packing it out of a campsite. Setting off smoke bombs at gender reveal parties.

In hot, dry conditions already primed for fire, people’s actions can quickly escalate from a harmless mistake — such as hammering a metal stake into the parched ground — to igniting a blaze that kills a firefighter.

People — whether purposeful, reckless or simply careless —  are responsible for about 95% of California’s wildfires. Last year alone, people caused more than 7,000 wildfires in California; nationally, it was more than 50,000.

“Humans are incredibly predictable,” said Cal Fire Battalion Chief David Acuna. “They drag chains and they leave campfires to burn or they have a flat tire but think they can just make it to the next exit.”

“It’s people not using appropriate judgment,” he added, “They are not looking at what they are doing and whether or not it’s going to start a fire. I believe in the ability of humans to recognize what is not in their best interest, but people constantly prove me wrong.”

Add to the list the Park Fire, a fierce fire raging across 600 square miles in four Sacramento Valley counties that is already the fifth largest in California history. Butte County authorities arrested a 42-year-old Chico man suspected of pushing a burning vehicle into a ravine.

Human action, tragically, also may have caused the 57,300-acre Borel Fire in Kern County, which began last week on the side of Highway 178, possibly sparked when a vehicle crashed, killing the driver.

In a summer of severe fires, with dangerous lightning storms possible in coming weeks, authorities wish people would stop doing stupid stuff. Nature doesn’t need any help starting fires.

That would include the man accused of driving his truck for more than four miles, minus a front tire, in Sonoma County last week. The metal scraping on the road sparked the Flora Fire, a small blaze east of Healdsburg. The fire was contained last Sunday and a suspect is in custody. Cal Fire lists the cause as “vehicle.”

A similar maneuver started the 2018 Carr Fire near Redding. The fire, which killed three people fighting the fire and five civilians and burned nearly 230,000 acres, was ignited by a spark caused by someone driving a trailer with a flat tire.

Another major human cause is related to institutions — the power utilities — rather than reckless individuals. “Since 2015, power lines have caused six of the state’s 20 most destructive wildfires,” according to a 2022 report from the State Auditor’s office. California’s most-deadly fire, the 2018 Camp Fire, which killed 85 people, was started by arcing transmission lines whipped by high winds.

Natural causes still play a significant role, especially lightning, which could become even a bigger threat with climate change.

Intentional arson is rare in wildfires

Arsonists purposefully setting fires is not common, accounting for about 10% of California fires, depending on the year. Last year 111 people were arrested for arson-caused fires in the state.

Thousands of human-caused fires fall into the unofficial category of people being knuckleheads: Folks doing something they think is safe or, absent any thinking, something they come to regret.

A few examples:

• A smoke bomb at a gender-reveal party sparked the deadly 2020 El Dorado Fire in San Bernardino and Riverside counties, which killed a firefighter and cost $42 million to suppress.

• A person using a lawnmower in Mariposa County caused the French Fire on July 4.

• Starting a fire to signal for help, a hunter inadvertently started the 2003 Cedar Fire near San Diego, which burned 280,278 acres, destroyed 2,820 buildings and killed 15 people.

• Faulty wiring on a hot tub in 2015 ignited a fire that burned for longer than a month in Lake County.

• Weapons fired in backcountry started the 2017 Detwiler Fire, destroying 134 structures and threatening Yosemite National Park. The same cause in the same area sparked the Telegraph Fire nine years earlier.

• A homeowner attempting to install a shade cloth on his property used a hammer to drive a metal stake in the ground. The sparks set off the Ranch Fire, which killed a firefighter and burned more than 410,000 acres in Colusa, Glenn, Lake and Mendocino counties in 2018.

Throw into the mix: escaped Mylar balloons sparking utility lines, unattended campfires, weedwackers, cigarette butts tossed out of car windows, trains, backyard fireworks. It’s a wonder that the state doesn’t burn down every year.

Dispatching crews to human-caused fires takes resources away from the fires that nature creates, said Adrienne Freeman, a spokesperson with the U.S. Forest Service.

“The answer is to use common sense,” she said. “It’s important not to place blame, but if you have a flat tire and you are 500 feet from the gas station, you may want to push (the car instead.) Think about it in the bigger picture.”

Freeman said a particular numbskull decision she’s seen is, evidently, common: backcountry campers setting fire to used toilet paper so they don’t have to pack it out.

Vehicles, lawn equipment are leading causes of wildfires

“The unsung reason is parking on dry grass. It’s common,” she said. “People pull up to a river access and see all these cars parked on the same strip you are parked on. What do you do? You park there.”

California firefighters respond to countless brush fires every summer that were started from a hot tailpipe making contact with high and dry grasses next to a road. The flammability of grasses is so well known that even those in the fire service are on guard not to spark fires when they use or move equipment.

“You can actually start fires while fighting fires,” Freeman said, noting that heavy-duty fire engines and other fire vehicles can kick up rocks and cause sparks. “You are moving your dozers to respond to a fire,  you have to be very careful.”

Dave Winnacker, fire chief of the Moraga-Orinda Fire District east of Berkeley, said human-caused fires commonly involve using equipment outdoors unsafely and vehicle accidents. His department operates a rigorous fire awareness program but sometimes education and warnings are not enough. In June, the fire district instituted a near-total outdoor fire ban.

Many California counties suggest mowing grass early in the day or late afternoon,  but it’s not mandatory.

“We rely on our education and spreading the word. At the end of the day, some people are going to get it and some are not,” said Andy VanSciver, public information officer for the Ventura County Fire Department.

Julie Cart is a reporter with CalMatters.