Dramatic footage captured aircraft battling the inferno, dousing the flames and releasing vivid pink fire retardant in their path. Entire neighborhoods were razed to the ground, but what truly captured international attention was that among the destroyed homes were the opulent mansions of world-renowned actors like Mel Gibson and Anthony Hopkins.

On 7 January 2025, a series of 31 wildfires began, causing widespread devastation across Los Angeles and the surrounding areas. By 16 January, the fires had claimed 27 lives (with many still missing), forced over 200,000 evacuations, and destroyed or damaged more than 12,400 structures, mostly homes, across over 40,000 acres (160 square kilometers). Ten days later, the Palisades and Eaton Fires, already the second and fourth most destructive in California's history, remained active.

Human settlements have been built within or close to a chaparral ecosystem, where wildfires are a natural occurrence. Chaparral is characterized by drought-resistant plants such as shrubs, small trees, and bushes, including sagebrush. These plants are well adapted to arid conditions, often having small, waxy leaves to reduce water loss. Fire is a natural part of this ecosystem, with some plant species adapted to regenerate from seeds after a fire.

The fire triangle, taught in school chemistry, highlights that three ingredients are needed for a fire: heat (or a source of ignition), fuel, and oxygen. If any of these components is removed or reduced, the fire cannot continue. For example, smothering a fire to remove oxygen or cooling it with water to reduce heat can extinguish the flames.

Wildfires are caused by a combination of natural and human factors. Natural causes include lightning and volcanic eruptions: lava flows and hot gases from volcanic eruptions can ignite nearby vegetation. Organic matter like hay, compost, or mulch can spontaneously ignite due to a build-up of heat. For example, the Wennington wildfire that destroyed 18 homes in east London on 19 July 2022 (the hottest day on record) was started when a compost heap in one of the gardens caught fire. One resident claimed her house burned down “within two minutes.”

Human causes include arson, carelessly discarded cigarettes, and campfires that aren’t properly extinguished. Controlled burns can sometimes escalate, while faulty electrical equipment or downed power lines may create sparks that ignite nearby vegetation. Heat from vehicle exhausts and sparks from machinery like lawnmowers or chainsaws can also cause fires, as can sunlight magnified by discarded glass.

Several factors have conspired to make these wildfires worse than usual. The previous two Californian winters were wet, promoting increased growth of chaparral vegetation. However, this year Southern California has faced severe drought, with the driest start to the rainy season and the driest nine-month period on record before the fires began, leaving vegetation extremely dry. This also depleted local water supplies and reduced the window for safe controlled burns, which typically help reduce available fuel before fire season. The Santa Ana winds exacerbated the problem. They reached 45 meters per second in places, five times faster than Usain Bolt can run the 100m.

In winter, high pressure forms over the Great Basin desert, which includes parts of Nevada and Utah. Cold, dense air sinks from the surrounding mountains, such as the Sierra Nevada and the Rockies, and becomes trapped in the basin. The surface pressure increases as the sinking air warms and compresses.

When a low-pressure system forms over the eastern Pacific, air from the Great Basin flows downhill towards the Californian coast along the pressure gradient. According to one meteorology journal, “a popular rule of thumb used by forecasters is to measure the difference in pressure between Los Angeles International Airport and Las Vegas; a difference of 9 millibars is enough to support a Santa Ana event.” As the air descends, it warms by about 1°C for every 100m, causing its relative humidity to drop to as low as 5%.

When this air reaches the towering Transverse Ranges, it moves through mountain passes and canyons to the coast, following the path of least resistance. The air speeds up due to the Bernoulli Effect; this is similar to how water exits a hosepipe faster when the end is squeezed.

The wind blowing from the mountain passes is warm and very dry, which dries out the vegetation. It can reach the speed of a category 1 hurricane as it enters valleys and the coastal plain. These conditions cause fires to spread rapidly, making them difficult for firefighters to control. The winds also carry embers long distances, igniting new fires several kilometers from the original ignition point or ahead of the main fire front.

If people are determined to live in areas that are prone to wildfires, they need to construct buildings using fire-resistant materials; many homes destroyed in the Los Angeles wildfires were made of wood. A defensible space of up to 30 meters around properties should be cleared of flammable materials like wood piles and dead vegetation. Trees should be thinned out. Fire-resistant plants, such as succulents, lavender, and certain grasses, should be used, as they are less likely to ignite. Firebreaks made from gravel, rocks, or other non-flammable materials can be created in areas with dense vegetation.

Other precautions include keeping gutters clear of leaves and using gutter guards to reduce debris build-up. Ember-resistant screens or mesh (3mm or smaller) should be installed on chimneys and similar areas to prevent embers from entering the home. Double-glazed windows with tempered glass and fire shutters can protect windows from flying embers. Swimming pools and ponds can serve as water sources for firefighting, while sprinklers can help keep landscaping moist to reduce fire risk. Additionally, firefighting equipment should be accessible in case residents need to evacuate.

Wildfires account for 70% of global biomass burning annually. While fires are a natural part of ecosystems, the 2021 IPCC Sixth Assessment Report highlights that climate change is increasing both the frequency and severity of wildfires, leading to larger areas being burned and longer wildfire seasons.

Research from Climate Central shows that Southern California has experienced more fire weather days since the early 1970s. Other studies suggest that wind-driven fires in the western USA are spreading faster and burning larger areas due to drier vegetation.

Jon Keeley, a Professor of Ecology at UCLA, notes that while the number of Santa Ana wind events has remained steady since 1948, their timing has shifted, with more occurring in December and January. He argues that the growing destructiveness of fires is partly due to urban sprawl into fire-prone areas and the expanded power grid. In extreme weather, power lines are more likely to fall or spark fires, and this now accounts for the majority of fires in Southern California. Indeed, Jeremy White, writing in The New York Times on 13 January 2025, published evidence that power lines are responsible for most destructive wildfires.

There is little doubt that climate change is increasing the frequency and intensity of wildfires, so we need to adapt by making our homes and infrastructure more resilient to future fires.

• January 2025 California wildfires have been fuelled by meteorological conditions strengthened by human-driven climate change: Zenodo
• Longer, More Intense Fire Weather Seasons: TinyURL
• Wildfire statistics: California Fire
• Abatzoglou, J.T., et al. (2023) Downslope Wind-Driven Fires in the Western United States. Earth’s Future, 11: DOI
• Keeley, J.E., et al. (2021) Ignitions explain more than temperature or precipitation in driving Santa Ana wind fires. Sci. Adv., 7: Science
• White, J., Many of California’s Most Destructive Fires Were Caused by Power Lines, New York Times, 13 Jan, 2025: NYTimes