California's experience with wildfire in a changing climate

By Alexander Gershunov

California is experienced with wildfire, fire starting in dry ecosystems fanned by roaring dry winds and often spreading to urban areas consuming the built environment. Many of the largest and most destructive wildfires have occurred in recent years. The deadliest was Camp Fire of November 2018, burning down the entire town of Paradise, killing 85 people, destroying 18,804 structures — mostly homes — and blazing through 153,000 acres. The Thomas Fire blazed through 282,000 acres, burning through most of December 2017 and into January 2018, the largest wildfire in California’s recorded history at the time and now the largest only in Southern California’s history. It destroyed over 1,000 structures and killed two people. Twenty-two more people were killed by mud and debris flowing from the burn scar triggered by the first rain event of winter that finally put out Thomas’ smoldering remains. All of these and many other fires started on the slopes of California’s coastal topography in strong, dry and gusty downslope winds promoting their quick progression. They were all ignited by power lines.

There are strong indications that some of the Los Angeles fires still burning today were also ignited by power lines. While many traditional sources of wildfire ignition (cigarettes, campfires) are declining due to education and messaging, power line ignitions are growing in frequency.

Strong gusty Santa Ana winds of Southern California and Diablo Winds in the north accelerate down hills, often gusting around 100 kilometers per hour. They can swing and contact power lines, snap old wooden power poles and trees and break off branches that can fall on power lines, all causing sparks to fly and ignite parched vegetation below. Such wildfires often start in hard-to-reach locations and spread uncontrollably. Burning in the sloping backcountry, downslope wind-driven wildfires are fast-spreading and devastating. Downwind, they expose heavily populated coastal areas to smoke impacts far from the fire. Sometimes, as we recently witnessed, they tragically burn down into the densely built environment where houses become fuel. As the population expands into the hilly backcountry prone to extreme Santa Ana winds, power line ignitions become more commonplace.

Climate change complicates the situation further. Warmer temperatures with more frequent and intense heat waves promote drier vegetation and stronger wildfires. Santa Ana winds start up around October, usually before the first rains of winter, when they encounter dry vegetation after the long dry (and warming) summer typical of California’s Mediterranean climate. Though Santa Ana winds peak in December and January, the traditional season for wind-driven coastal wildfires is October, because the first rainstorms of winter arriving typically by November moisten the vegetation, reducing fire danger. With climate change, we expect both the dry winds and the rains to start later and the coastal wind-driven fire season to shift into December and January — the peak season for Santa Ana winds, when wildfire risk in unseasonably dry vegetation grows, fanned by consecutive Santa Ana wind events common during winter. In Southern California, this wet season is the latest and driest on a record spanning 150 years. The Santa Ana winds, however, are on schedule.

Most mountainous regions around the world, including Korea, are subject to dry, gusty downslope winds which, when blowing over parched vegetation, create enormous fire danger. Global warming, directly, contributes to drier vegetation and elevated fire danger all over the world.

After inadvertently igniting the Witch Fire in 2007, San Diego Gas & Electric — the local power utility — started implementing proactive Public Safety Power Shutoffs (PSPS) in high-fire-risk areas, with oversight from the California Public Utilities Commission (CPUC) — the state agency that regulates California’s public utilities. As power lines have ignited many catastrophic wildfires beyond San Diego County since then, PSPS are becoming a standard practice among California’s power utilities, mandated by CPUC. Although effective when informed by skillful fire weather forecasts and timely warnings issued by the National Weather Service, the public left without power is severely inconvenienced and often endangered, especially in cases of compromised individuals whose life depends on electrically powered medical devices, complicating implementation of PSPS.

CPUC regulations attempt to reduce fire ignition risk while assuring public access to safe and reliable electricity — a difficult balance to maintain in fire weather conditions. LA fire investigations are ongoing, but indications are that PSPS were not effectively implemented by the local power utility in advance of the LA fires. While other states and countries are looking to learn from California’s experience with wildfire and its prevention, the LA fires suggest that California has not learned enough from its own experience. As the coastal population keeps expanding into the sloping backcountry where Santa Ana winds roar and climate change continues to dry out fuels further into the windy season, California’s experience with destructive wild-urban fires will certainly grow. As I write this on Jan. 22 — the middle of a wet season that hasn’t yet started — two big LA fires that ignited over two weeks ago are still burning and four new Southern California wildfires are being battled — fanned by the third significant Santa Ana wind event of 2025.

Alexander Gershunov is at the Climate, Atmospheric Science and Physical Oceanography (CASPO) Division of Scripps Institution of Oceanography at University of California, San Diego. He can be reached at weclima.ucsd.edu.