When you read about a monstrous wildfire like California’s Carr fire growing to over 190,000 acres, burning up over 1,000 homes and killing eight people, it’s strange to think that it began the size of a dinner plate. The fire (named for where it started: Carr Powerhouse Road) that grew into one of the biggest fires in California history, was sparked by a flat tire. A tire on a car traveling fast can shred and leave the car riding on the metal rim, throwing sparks as it rolls over asphalt. That’s what happened: A startled driver pulled over on the shoulder, causing dry grass to catch on fire.
If that same tire had blown out on a cooler, more humid day, or even in the morning when there was dew on the grass, it probably would have been a non-event, just another broken down car on the side of the road. Maybe the grass would have begun to smolder a bit, but easily could have been stomped out. But this occurred on a 105-degree afternoon with 20 percent humidity, the right conditions just waiting for a spark.
Every fire starts out small. I once saw Bureau of Land Management fire crews in Boise, Idaho, nearly corral a two-acre fire started by tracer rounds at a gun range, only to lose control of it when the winds picked up and a fire engine at a crucial point broke an axel. Several hours later, the small burn had spread to 15,000 acres and was running into the woods, where even more fuel awaited.
Remember Smokey the Bear’s famous line about humans preventing forest fires? It’s mostly true. Eighty-four percent of wildfires are caused by humans, according to a 2014 study from the University of Colorado’s Earth Lab. Very few of those are arson. Most are accidents that do little more than light a spark: a flat tire, a gun discharge, power lines flapping in strong winds, or even someone sharpening a lawnmower blade with a grinder. Those routine events happen all the time, all year long, but if they happen on the hottest, driest, and windiest days of the year, they can turn into disasters.
According to on-the-ground estimates, between 2,000 and 3,000 firefighters have been battling the Carr fire since it started. They’re a collection of state and municipal fighters, with some federal officials as well. During fire season, those men and women are on the road almost constantly, working 24-hour shifts and sleeping in tents. Keeping firefighters safe includes protecting them from fatigue, says Jay Walter, a field operations section chief with the Arizona Department of Forestry and Fire Management.
Walter, like many firefighting professionals in western states, has spent much of this season in California, shuttling up and down Interstate 5 following orders from incident teams trying to make the best use of the people available. The top priorities of fighting fires are always worker and public safety, so firefighters are required to take days off and get enough sleep, which makes the overall force stronger to deal with ever-growing fires, and over a longer fire season.
The Cost of Fires
Fighting wildfires in the United States costs billions of dollars every year. Researchers sound alarms about the increased fire risk due to climate change. But that overlooks the direct role that people play in igniting wildfires. Fires that start on their own—through a lightning strike, for example—are relatively small. Human-started fires burn seven times more land, and in the past decades have tripled the length of the fire season.
“National and regional policy efforts to mitigate wildfire-related hazards would benefit from focusing on reducing the human expansion of the fire niche,” the Earth Lab researchers reported. Making matters worse, a 2014 study predicted that by 2050, nearly 645,000 houses in California will stand in zones very prone to wildfires. Add in hotter, drier weather in the future, and you can imagine what awaits those houses.
Ground zero for much of this research is California, which routinely provides the ingredients for a fire. This year’s season is particularly bad after a record heat wave in July dried out much of the state’s vegetation. Researchers think that’s likely to happen in future years too because the trend is already established. The number of large fires has increased steadily since the 1980s, and there’s no reason to think the trend will reverse, or even stay the same, especially when more humans around mean more flat tires and more lawnmower blades.
Climate change also brings another meaningful change. Since California’s overall climate is warmer than when this burned-up vegetation became established, the same species are unlikely to come back. Instead, the grasses that grow will be grasses better adapted for hot weather—species better suited to desert weather that hold less moisture and dry out quicker.
Fire scientists like to call any vegetation that burns “fuel,” and have come up with a standard called fuel moisture to determine how dry the vegetation is. It’s a percentage of the weight of water divided by the dry weight of the vegetation, which means that the thinner the vegetation, the more easily it reacts to the ambient temperature and humidity. A hot day will dry out grass much more quickly than it will a two-inch-in-diameter chaparral branch no matter how much rain that vegetation received a month before. Fuel moisture less than 30 percent is considered dead.
It’s those light fuels like brown dry grass that blanket the spaces between the homes on the California hillsides. That’s not original to the state, but it’s good news for a fire looking to spread fast. In the driest parts of the country, it's often not if an area will burn, but when.