Blazes in forests rely on three key elements to spread: conducive weather, dry fuel and an ignition source. Each of these factors has undergone pronounced changes in recent decades
Investigators are trying to determine what caused several wind-driven wildfires that have destroyed thousands of homes across the Los Angeles area in January 2025. Given the fires’ locations, and lack of lightning at the time, it’s likely that utility infrastructure, other equipment or human activities were involved.
California’s wildfires have become increasingly destructive in recent years. Research my colleagues and I have conducted shows US wildfires are up to four times larger and three times more frequent than they were in the 1980s and 1990s. Fast-moving fires have been particularly destructive, accounting for 78% of structures destroyed and 61% of suppression costs between 2001 and 2020.
Lightning strikes are a common cause of US wildfires, but the majority of wildfires that threaten communities are started by human activities.
A broken power line started the deadly 2023 Maui fire that destroyed the town of Lahaina, Hawaii. Metal from cars or mowers dragging on the ground can spark fires. California’s largest fire in 2024 started when a man pushed a burning car into a ravine near Chico. The fire destroyed more than 700 homes and buildings.
What makes these wildfires so destructive and difficult to contain?
The answer lies in a mix of wind speed, changing climate, the legacy of past land-management practices, and current human activities that are reshaping fire behaviour and increasing the risk they pose.
Wildfires rely on three key elements to spread: conducive weather, dry fuel and an ignition source. Each of these factors has undergone pronounced changes in recent decades. While climate change sets the stage for larger and more intense fires, humans are actively fanning the flames.
Extreme temperatures play a dangerous role in wildfires. Heat dries out vegetation, making it more flammable. Under these conditions, wildfires ignite more easily, spread faster and burn with greater intensity. In the western US, aridity attributed to climate change has doubled the amount of forestland that has burned since 1984.
Compounding the problem is the rapid rise in nighttime temperatures, now increasing faster than daytime temperatures. Nights, which used to offer a reprieve with cooler conditions and higher humidity, do so less often, allowing fires to continue raging without pause.
Finally, winds contribute to the rapid expansion, increased intensity and erratic behavior of wildfires. Wind gusts push heat and embers ahead of the fire front and can cause it to rapidly expand. They can also create spot fires in new locations. Additionally, winds enhance combustion by supplying more oxygen, which can make the fire more unpredictable and challenging to control. Usually driven by high winds, fast-moving fires have become more frequent in recent decades.
Fire is a natural process that has shaped ecosystems for over 420 million years. Indigenous people historically used controlled burns to manage landscapes and reduce fuel buildup. However, a century of fire suppression has allowed vast areas to accumulate dense fuels, priming them for larger and more intense wildfires.
Invasive species, such as certain grasses, have exacerbated the issue by creating continuous fuel beds that accelerate fire spread, often doubling or tripling fire activity.
Additionally, human development in fire-prone regions, especially in the wildland-urban interface, where neighborhoods intermingle with forest and grassland vegetation, has introduced new, highly flammable fuels. Buildings, vehicles and infrastructure often ignite easily and burn hotter and faster than natural vegetation. These changes have significantly altered fuel patterns, creating conditions conducive to more severe and harder-to-control wildfires.
Lightning can ignite wildfires, but humans are responsible for an increasing share. From unattended campfires to arson or sparks from power lines, over 84% of the wildfires affecting communities are human-ignited. Human activities have not only tripled the length of the fire season, but they also have resulted in fires that pose a higher risk to people.
A burned-out washer and dryer are all that remain recognizable in the debris of what was once a home. Burned tree trunks are in the background.
Lightning-started fires often coincide with storms that carry rain or higher humidity, which slows fires’ spread. Human-started fires, however, typically ignite under more extreme conditions – hotter temperatures, lower humidity and stronger winds. This leads to greater flame heights, faster spread in the critical early days before crews can respond, and more severe ecosystem effects, such as killing more trees and degrading the soil.
Human-ignited fires often occur in or near populated areas, where flammable structures and vegetation create even more hazardous conditions. Homes and the materials around them, such as wooden fences and porches, can burn quickly and send burning embers airborne, further spreading the flames.
As urban development expands into wildlands, the probability of human-started fires and the property potentially exposed to fire increase, creating a feedback loop of escalating wildfire risk. A phenomenon known as whiplash weather, marked by unusually wet winters and springs followed by extreme summer heat, was especially pronounced in Southern California in recent years.
That dryness continued in southern California through the fall and into early winter, with very little rainfall. Soil moisture in the Los Angeles region was about 2% of historical levels for that time of year when the fires began on January 7, 2025.
As the factors that can drive wildfires converge, the potential for increasingly severe wildfires looms ever larger. Severe fires also release large amounts of carbon from trees, vegetation and soils into the atmosphere, increasing greenhouse gas emissions and exacerbating climate change, contributing to more extreme fire seasons.
The Conversation
(Virginia Iglesias is Interim Earth Lab director, University of Colorado Boulder.)
