Climate Sets the Stage, Humans Light the Match: The Real Causes of California Wildfires
Movies, news stories, and social media often reinforce the image of a dramatic lightning strike igniting a dry forest and sending flames racing across the hills, which can contribute to the perception that nature is the main culprit for causing wildfires. But this is far from the truth. Only about 5 to 10 percent of California wildfires are caused by lightning. Approximately 85–95% start because of human activity, like power lines, vehicles, campfires, fireworks, and sometimes arson. Understanding this is crucial to preventing fires, holding people accountable, and creating better policies. [1]
Why do so many people believe lightning is the main cause?
Part of this belief stems from the way wildfires are portrayed in movies and on TV. Lightning-caused fires are flashy, dramatic, and memorable, so they stick in our minds. Social media often oversimplifies wildfire reporting by labeling fires as natural disasters. High-profile lightning fires, such as the 2020 Lightning Complex Fires, which were a series of massive wildfires sparked by an unusual wave of lightning storms across Northern California, make it seem like these events happen all the time. The persistence of this myth is also explained by psychology. Humans naturally rely on mental shortcuts to process information quickly, and one of these is called the availability heuristic, first described by psychologists Amos Tversky and Daniel Kahneman. [2] According to this principle, people judge the likelihood of an event based on how easily examples come to mind.
Dramatic lightning-caused fires are easy to remember, so we overestimate how often they happen. Additionally, this belief is reinforced by defensive attribution, a psychological principle in which people assign responsibility to external forces to distance themselves from blame. As a result, there is a psychological comfort in believing fires are “acts of nature” rather than the result of human carelessness, even though one could argue that a loss of control to natural forces may be more frightening. It is often less threatening to believe that nature, rather than our own actions, is responsible.
The Fire Triangle
To understand why some fires ignite and spread, it helps to look at the fire triangle, a key concept in fire science. A fire needs three things to start: fuel, oxygen, and heat.
Fuel
Fuel is anything that can burn, such as dry grass, leaves, brush, or structures like homes and buildings, which are common in wildland–urban interface areas in California’s hot, dry climate. These structures often catch fire because they are built close to flammable vegetation and are built with flammable materials, such as wood siding and decks, and were constructed before strict fire-resistant building codes were in place, raising concerns about building standards and land-use policies.
Oxygen
Oxygen comes from the air around us and is almost always present in outdoor environments, making it a constant factor in wildfire ignition and spread.
Heat (The Spark)
Heat is the energy source that ignites the fuel and is often referred to as the spark. In California, this heat is frequently caused by human activity, such as sparks from vehicles, downed power lines, or careless campfires. When fuel, oxygen, and heat are all present, a fire can start and spread rapidly.
The Science of Fire Spread
Scientifically, this process works like this: when heat touches fuel, it causes a chemical change called pyrolysis, which breaks the fuel into smaller molecules, including flammable gases. These gases mix with oxygen in the air and ignite, producing the fire we see. The burning gases release additional heat, which spreads to nearby fuel, causing further pyrolysis and the release of more flammable gases. This creates a chain reaction that keeps the fire going. If any one of the three elements, fuel, oxygen, or heat, is missing, the fire cannot continue.
Lightning, Climate, and Human Activity
This framework also explains why lightning alone is rarely the main cause of California wildfires. While lightning provides a spark, humans are responsible for most sparks in areas where fuel and oxygen are abundant. Some people argue that climate change and natural conditions make lightning more dangerous. Hotter, drier weather does increase fire risk because it dries out vegetation, making it easier to burn. But even under these conditions, most fires still start because of human activity. Climate provides the fuel, but humans provide the spark. Lightning may make a dramatic story, but humans start most fires.
Knowing the true causes of wildfires matters because recognizing that humans are responsible emphasizes the need for stricter safety rules, improved infrastructure, and personal responsibility. This includes enforcing defensible space requirements around homes, which require homeowners to clear flammable vegetation near buildings. It also means upgrading aging power lines to reduce the risk of electrical sparks, and adopting fire-resistant building standards in high-risk areas, such as using noncombustible roofing, ember-resistant vents, and tempered glass. California has already begun taking steps in this direction through initiatives such as public
safety power shutoffs, which temporarily cut electricity during extreme fire weather to prevent power line ignitions. Other steps include updated wildfire building codes that require safer construction materials in fire-prone regions, and vegetation management programs in wildland–urban interface communities that reduce excess brush and trees near homes. By busting the lightning myth, we stop blaming nature and start taking action. Understanding the facts is the first step to protecting homes, forests, and lives.
References
U.S. Forest Service. (2013). Spatial wildfire occurrence data for the United States, 1992–2012 [Data set]. Forest Service Research Data Archive. https://www.fs.usda.gov/rds/archive/catalog/RDS-2013-0009.6
Tversky, A., & Kahneman, D. (1973). Availability: A heuristic for judging frequency and probability. Cognitive Psychology, 5(2), 207–232. https://doi.org/10.1016/0010-0285(73)90033-9
The Guardian. (2020, August 25). California fires: Firefighters work to contain two largest blazes as thousands more burn. https://www.theguardian.com/us-news/live/2020/aug/25/california-fires-firefighters-work-contain-two-largest-blazes-7000-others-burn-live-latest-news-updates
PBS NewsHour. (n.d.). California’s catastrophic wildfires in 3 charts.https://www.pbs.org/newshour/science/californias-catastrophic-wildfires-in-3-charts
Fire Prevention India. (n.d.). The fire triangle. https://firepreventionindia.com/the-fire-triangle/
Liu, J. (2019). California wildfire ignition causes from 1980 to 2019 [Research paper]. Bronx High School of Science, New York, NY.
