A new analysis concludes that severe winds and power line failures are to blame for Southern California’s deadliest and devastating fires, a finding with implications for high-risk Northern California as well.
Better maintenance of utility lines and better planning for where people build and live could reduce future fire damage, according to the study published in Wednesday’s issue of the journal. science advance.
“We need to recognize that fires are a problem for people,” said John E. Keely, a fire ecologist at the US Geological Survey’s Western Ecological Research Center.
Recently, attention has been paid to the role of hot weather and dense forests with dry twigs and logs. The study found that, while important, those factors are only associated with California summer fires, which are typically ignited by lightning in remote areas where people don’t live.
For example, the fuel-powered 2013 Rim Fire in Stanislaus Forest and nearby Yosemite National Park, the 2012 Rush Fire in rural northeastern California and the 2015 Rough Fire in the Sierra Nevada Range – ignited in the months of July and August – were all lightning. It started falling, yet no one was hurt.
In contrast, autumn and winter fires are usually wind-driven and ignited by power lines near communities. The two largest Southern California fires of the past decade – the Thomas Fire and the Woolsey Fire – were both triggered by power line failures.
The frequency of these wind-driven fires has increased in recent decades, along with the expansion of the state’s power grid to serve the growing population in areas affected by fires.
“This is a welcome advance in our understanding of the conditions during Santa Ana wind events in regions of Southern California,” said Noah Dieffenbaugh, a professor in the Stanford School of Earth, Energy and Environmental Sciences.
To understand the factors that determine the burned land area, Keeley and colleagues from the Scripps Institution of Oceanography at San Diego and UC San Diego, analyzed 71 years of daily windspeed data from 1948 to 2018, as well as daily . Monthly temperature and precipitation measurements cover the same time period.
The team focused on the Santa Ana winds, strong and very dry downslope winds that originate in the Great Basin and blow over coastal southern California.
But the findings are also relevant to Northern California, Kelly said. Northern California’s autumn and winter winds—called north winds, Diablo winds, or mono winds—follow a similar pattern, according to Keeley.
Two of Northern California’s most devastating fires — the 2017 Tubbs Fire in Napa, Sonoma and Lake Counties and the 2018 Camp Fire in Paradise — were wind-driven. One Kindle in October; Second, in November. Those two blasts alone took the lives of 108 people. October 1991 A fire in the Oakland Hills killed 25 people and destroyed more than 3,200 homes, turned into an inferno by the Diablo winds.
“It’s the same thing,” he said. “If you look at the causes of those fires, they were all started by people, and that’s largely because that’s when you get to these extreme wind events.”
The study found that all Santa Ana wind-driven fires were the result of human ignition, either intentionally or accidentally.
Over the past 71 years, the sources of these ignitions have changed markedly – with arson, and especially power line failure, becoming more frequent in the second half of the 20th century.
The study found that high temperatures and low rainfall in the week or month before the fires played a small role in determining how much of an area burned. Even wind-driven fires on more than 12,000 acres were not linked to the weather.
While dry fuels affect the size of major fires in Southern California’s summer, they play a lesser role in the fall, Kelly said.
“This does not mean that the moisture content of the fuel is not critical, but just that it is already at a very low level at the beginning of autumn,” he said.
Last year, a Stanford study found that hot, dry weather increases the risk of a longer, more dangerous wildfire season. Climate change, prolonged warming and declining autumn rainfall have increased the potential for danger, it found.
Dieffenbaugh said the new study does not contradict the Stanford research. The Stanford study was not a detailed analysis of a sub-region of the state, rather it analyzed conditions in different parts of the state during the autumn season and did not focus specifically on the Santa Ana wind events.
“All wildfires result from a confluence of conditions,” Dieffenbaugh said. “Weather conditions are significant contributors to high risk, but they are not the only contributors.”
“Ignition matters – a forest fire never started without ignition,” he said. “The wind absolutely matters.”
To mitigate the risk, Keeley welcomed steps taken by San Diego County so that all new developments have underground power.
“It really only applies to San Diego County,” he said. Even with the older development, “there’s still a lot of above-ground power (lines).” And there are still many power line-ignited fires in the rest of the state.
Public safety power shutoffs, also known as PSPS, are an important response to severe weather, he said, but it is only a band-aid.
“Until we start to recognize that population growth is at least as important as climate change, we are never going to solve these problems,” he said. “We need to deal with how we develop areas, and where we put people.