Why wildfires started by human activities can be more destructive and harder to contain

Wildfires are becoming increasingly destructive across the U.S., as the country is seeing in 2024. Firefighters were battling large blazes in several states from California to North Dakota in early October 2024, including fires burning near homes and communities.

Research shows wildfires are up to four times larger and three times more frequent than they were in the 1980s and ‘90s, with some consuming hundreds of thousands of acres in a single blaze.

Lightning strikes are one cause, but the majority of wildfires that threaten communities are sparked by human activities.

Metal from cars or mowers dragging on the ground can spark fires. So can power lines touching trees. Officials confirmed on Oct. 2 that a broken power line started the deadly 2023 Maui fire that destroyed the town of Lahaina, Hawaii. 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.

Although the number of fires in 2024 has not been unusually high, the acreage burned has far surpassed the 10-year average, displacing thousands of people, destroying homes and straining firefighting resources.

What makes these wildfires so destructive and difficult to contain?

The answer lies in a mix of changing climate, the legacy of past land-management practices, and current human activities that are reshaping fire behavior and increasing the risk they pose.

Fire’s perfect storm

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.

Climate and weather

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 U.S., 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.

Fuel

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.

Ignition

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.

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. 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.

2024 fire season’s whiplash weather

The record-breaking summer heat in 2024 intensified fire hazards, with vegetation rapidly drying out and leaving landscapes parched in many areas. In addition, a phenomenon known as whiplash weather, marked by unusually wet winters and springs followed by extreme summer heat, has been especially pronounced in Southern California.

A wet spring fostered vegetation growth, which then dried out under scorching summer temperatures, turning into highly combustible fuel. Severe heat waves, along with the associated lack of nighttime cooling, created conditions where fires not only spread faster, but were also more difficult to contain.

This cycle has fueled some of the biggest fires of the 2024 season, several of which were started by humans. Atmospheric instability during some of these fires also led to the formation of pyrocumulonimbus clouds – massive, fire-fueled thunderheads that can generate their own weather, including lightning and tornado-like winds that drive flames even further.

As these factors 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.

Virginia Iglesias receives funding from the National Science Foundation.