Wildfires caused by human activities are often more destructive

Wildfires caused by everyday human activities are often more destructive

Written by Virginia Iglesias & The Conversation US

The following essay is reproduced with permission. The Conversation is an online publication covering the latest research.

Investigators are trying to determine the cause of multiple wind-driven wildfires that destroyed thousands of homes across the Los Angeles area in January 2025. Given the location of the fire and the lack of lightning at the time, it is likely that the fire started in public infrastructure, other equipment, or other equipment. Human activity was involved.

California wildfires have become increasingly destructive in recent years. Research by my colleagues and I shows that wildfires in the United States are up to four times larger and three times more frequent than they were in the 1980s and ’90s. Fast-moving fires are particularly destructive, accounting for 78% of structures destroyed and 61% of fire suppression costs between 2001 and 2020.

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Although lightning strikes are a common cause of wildfires in the United States, the majority of wildfires that threaten communities are caused by human activities.

The deadly Maui Fire that destroyed the Hawaiian town of Lahaina in 2023 was caused by a broken power line. Metal from cars and lawn mowers dragging on the ground can cause a fire. California’s deadliest fire of 2024 started when a man drove his burning car into a ravine near Chico. The fire destroyed more than 700 homes and buildings.

Why are these wildfires so destructive and difficult to contain?

The answer lies in a combination of wind speeds, climate change, the legacy of past land management practices, and current human activities that are reshaping fire behavior and increasing fire risk.

perfect storm of fire

Wildfires rely on three key factors to spread: conducive weather, dry fuel, and an ignition source. Each of these factors has undergone significant changes in recent decades. Climate change is setting the stage for larger, more intense fires, while humans are actively fanning the flames.

climate and weather

Extreme temperatures play a dangerous role in wildfires. Heat dries plants and makes them more flammable. In these conditions, wildfires ignite more easily, spread faster, and burn more intensely. In the western United States, the amount of forest burned has doubled since 1984 due to drying due to climate change.

Compounding the problem is the rapid rise in nighttime temperatures, which are now rising faster than daytime temperatures. Cooler, more humid conditions used to provide a night’s rest, but these days have become less frequent, allowing fires to continue burning without respite.

Finally, wind contributes to the rapid spread, increased intensity, and erratic behavior of wildfires. Gusty winds can push heat and embers to the front of a fire front, causing it to expand rapidly. You can also start spot fires in new locations. Additionally, wind can speed up combustion by providing more oxygen, making fires more unpredictable and difficult to control. Fires, typically caused by strong winds, have become more frequent in recent decades.

fuel

Fire is a natural process that has shaped ecosystems for more than 420 million years. Indigenous peoples have historically used controlled burning to manage landscapes and reduce fuel accumulation. But a century of fire suppression has allowed dense fuel to accumulate over vast areas, priming larger and more intense wildfires.

Invasive species, such as certain grasses, can exacerbate the problem by creating persistent fuel beds that accelerate fire spread, often doubling or tripling fire activity.

Additionally, human development in fire-prone areas, particularly at the wildland-urban interface where adjacent areas have a mix of forest and grassland vegetation, has introduced new highly flammable fuels. Buildings, vehicles, and infrastructure often catch fire easily and burn hotter and faster than natural vegetation. These changes have significantly altered fuel patterns, creating conditions that are more likely to cause more severe and difficult-to-control wildfires.

ignition

Lightning can cause wildfires, but humans are responsible for increasing their rate. From unattended campfires to arson and sparks from power lines, more than 84% of wildfires impacting communities are caused by human ignition.

Human activities have not only tripled the length of the fire season, but also caused fires that pose a higher risk to people.

Fires caused by lightning strikes often coincide with rain or moisture storms, which slows the spread of fires. However, human-caused fires typically ignite under more extreme conditions, such as high temperatures, low humidity, and strong winds. This allows the flames to grow higher, spread faster in the critical early stages before crews can respond, and cause more serious ecological damage, such as killing more trees and degrading the soil. There will be consequences.

Human-caused fires often start in or near populated areas where flammable structures and vegetation create an even more dangerous situation. The house and surrounding materials (such as wooden fences and porches) can burn quickly, and embers can be thrown into the air, further spreading the flames.

As urban development expands into wilderness areas, the likelihood of human-caused fires and the amount of land potentially exposed to fire increases, creating a feedback loop that increases wildfire risk.

whiplash weather

A phenomenon known as “whiplash weather,” characterized by unusually wet winters and springs followed by extremely hot summers, has become particularly pronounced in Southern California in recent years.

A wet spring in 2024 encouraged plant growth, which was then dried out by summer’s scorching temperatures and turned into a highly flammable fuel. This cycle resulted in the largest fires of the 2024 season, some of which were caused by humans.

In Southern California, the dryness continued from fall to early winter, with almost no precipitation. Soil moisture in the Los Angeles area, where the fires began on January 7, 2025, was about 2% of historic levels for that time of year.

As the factors that can cause wildfires converge, the likelihood of increasingly severe wildfires occurring is greater than ever. Severe fires also release large amounts of carbon from trees, vegetation, and soil into the atmosphere, increasing greenhouse gas emissions, exacerbating climate change, and causing more extreme fire seasons.

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