A wayward smoke bomb from a gender reveal party sparked a major blaze near Los Angeles in September, just one of many recent wildfires ignited by people. Now, an analysis of high-resolution satellite data from hundreds of California wildfires shows human-caused blazes spread much faster and kill more trees than ones ignited by lightning.
The findings highlight how fires that start differently can behave in distinct ways, with effects far beyond the amount of land torched, says Sean Parks, a fire ecologist with the Rocky Mountain Research Station’s Aldo Leopold Wilderness Research Institute, who was not involved with the study. “This focus on high severity rather than just area burned is important.”
Fire has always been a part of California’s natural history. But several centuries of human settlement have created new conditions that promote its spread. Studies have shown human ignition is to blame for 84% of all wildfires in the United States, and 97% of all those that threaten homes.
Human-sparked fires always seemed more extreme, says Stijn Hantson, a fire ecologist at the University of California, Irvine, who led the new research. But quantitative measurements of how fast they spread and their impacts on ecosystems in California had not been explored, he notes.
To probe those differences, Hantson and colleagues compiled daily high-resolution satellite data for 214 wildfires that burned in California between 2012 and 2018. They drew perimeters around detected hot spots for each wildfire from day to day. Then, the researchers measured the distance between each day’s perimeter to the next day’s hot spots to calculate on average how fast the fire grew daily until it was extinguished.
Human-sparked fires typically spread about 1.83 kilometers per day, more than twice as fast as the 0.83 kilometers per day for lightning-induced burns, the team reports today at a virtual meeting of the American Geophysical Union. The faster spreading fires also burned more intensely and killed “double or triple” the trees as slower, lightning-caused ones, Hantson says.
“These human-caused fires have a disproportionate impact on the ecosystem,” he says. “And though the ecosystem is fire-adapted, it’s not adapted for 80% of trees to die, like we’re seeing with some of these really intense fires.”
However, there is no inherent difference in the chemistry of a human-sparked blaze. “A fire is a fire,” Hantson says. “It’s the surrounding things that matter.” Causes of fires ranging from improperly discarded cigarettes to sparking power lines could ignite a blaze on any given day, he says, whereas lightning strikes and dry thunderstorms only happen seasonally.
The researchers tracked meteorological data, including wind speed and moisture evaporation, and found that human-caused fires were more likely to start on days with extreme weather conditions, such as such as gusty winds, that favor a rapid spread. They also found that human-kindled fires were more associated with drier, less-forested landscapes, devoid of the live vegetation that could limit fire spread.
This adds to scientists’ understanding of how humans are extending the fire season, says Nathan Mietkiewicz, an ecologist with the National Ecological Observation Network who was not involved with the research. Fire management needs to evolve, he says, to take this into account. “That could mean putting more fire on the ground” with controlled burns, Mietkiewicz says, cutting down the potential fuel and intensity for future wildfires.
Hantson and his colleagues plan to apply the same analysis to fire data from this year, California’s largest fire season on record. More than 1.6 million hectares have burned this year in the state.