The causes of Southern California’s terrifying wildfire siege
The wildfires raging across Southern California have scorched tens of thousands of acres of land, destroyed hundreds of homes, and disrupted countless lives.
While the scenes of flames screaming up hillsides, and smoke trailing hundreds of miles out over the Pacific are dramatic, they’re not unprecedented. In fact, they’re not even that rare. However, they are likely to be more damaging in the future, and more common in certain parts of California and the American West in general, thanks to climate change and other factors.
Wildfires, particularly during the fall, are endemic to California. In Southern California, the biggest blazes tend to flare up when the dry, warm Santa Ana winds blow, sometimes unceasingly for days on end. This week, for example, the Santa Ana wind event began on Monday, and is not forecast to stop until Friday night.
Much of the time during these events Southern California gets lucky, and there are no major blazes to speak of.
It’s not one of those years.
This year is proving to be California’s worst wildfire season on record. In October, more than 40 were killed and at least 10,000 homes lost when fires tore through Sonoma and Napa Counties, devastating the community of Santa Rosa, in particular. Known as “El Diablo” winds in this area, the offshore airflow was instrumental in propelling the wildfires faster than people could get out of the way, let alone allow firefighters to squelch the flames.
Santa Ana winds result from a particular weather pattern featuring a high pressure area anchored over the Great Basin, and a low pressure area southwest of California, over the Pacific Ocean. The air pressure contrast between these two systems results in a funneling of air from east to west. As the air descends from the higher elevations toward the coast, it warms and dries, leading to extremely low humidity.
These background conditions set the scene for a major fire, provided there’s a spark. This year, there’s been added fire risk from repeated heat waves that have sent temperatures soaring into the triple-digits as late as October, further drying out vegetation that is now serving as fuel for the fires.
“It’s the coincidence of an ignition,” that determines whether a Santa Ana event will lead to major fires, said Jon Keeley, a research scientist with the U.S. Geological Survey, in an interview. “And the two big causes of ignitions during Santa Ana wind events are power lines that blow down from the strong winds and arson.”
The winds themselves can initiate a fire by downing power lines, something suspected in the fires that affected Northern California in October. Power outages preceded the Thomas Fire in Ventura on Tuesday, as well.
Importantly, the winds also make the fires nearly impossible to contain, blowing hot embers well ahead of the fire line, allowing blazes to hop across 10-lane highways with unsettling ease.
The National Weather Service has been warning of the likelihood for “extreme fire behavior” with any fire that starts during this long-lasting wind event, going as far as designating the fire danger as “extremely critical” in the Los Angeles area.
Once the fires get going, they can create their own weather, sucking in air from surrounding areas and lofting it upwards, promoting further fire growth and unpredictable behavior.
Longer term trends
This week, the high pressure area, or “ridge” in the jet stream, is extremely pronounced, part of a tendency toward more extreme weather patterns across the country. Recent studies indicate there may be connections between altered weather patterns, which are favoring drier conditions in California, and climate change.
Last summer was the state’s hottest on record, following a wet winter, which in turn followed the worst drought in modern California history. These episodes of weather whiplash have left the Golden State’s trees stressed, making them more susceptible to toppling over from strong winds.
Multiple studies have found that long-term climate change, which is mainly due to human emissions of greenhouse gases, is raising the risk of large wildfires in the Western U.S. However, according to the USGS’ Keeley, this doesn’t apply to the lower elevations of southern California. His research has shown that the warming climate is leading to more frequent and larger fires in Northern California, particularly in high elevation forests, he said.
There are broader, non-climate forces at work, too. Population growth is placing more people in harms’ way, including by building in areas at the so-called wildland-urban interface, or WUI, where areas that traditionally burn as part of their natural ecology butt up against homes and businesses. In short, urban sprawl is not helping solve our wildfire problem.
Keeley emphasized this point, saying that it’s not just climate change, but global change that will alter wildfire severity and risk going forward. Wildfires are likely to have more catastrophic human impacts due to population growth along with climate change, he said.
For example, he cited a 1964 wildfire that burned almost the exact same area as the deadly Tubbs Fire in Santa Rosa. However, back then, the blaze caused no fatalities and only destroyed about two-dozen homes. Since that fire, though, he said the population in that area grew fivefold.
“We need to be thinking about all the global changes that are taking place,” Keeley said.
In the near-term, though, the Santa Ana winds are forecast to increase on Wednesday night, and may hit 70 miles per hour in some spots of Southern California on Thursday.
In other words, this latest round of fires, which is a sign of the state’s heritage and future challenge in a warming world, may yet grow worse.