More than 80 percent of all wildfires caused by humans, new study says

People have triggered five out of six wildfires in the U.S. over the last two decades, tripling the length of the wildfire season and making it start earlier in the East and last longer in the West, a new study finds.Even as climate change worsens the nation’s...

More than 80 percent of all wildfires caused by humans, new study says

People have triggered five out of six wildfires in the U.S. over the last two decades, tripling the length of the wildfire season and making it start earlier in the East and last longer in the West, a new study finds.

Even as climate change worsens the nation’s fire season — making it longer and easier to burn more acres — researchers said human activities play an even bigger role.

In Southern California, large populations living in close proximity to fire-prone foothills and national forests make the region a poster child for human-caused wildfires that wreak havoc on life and property, said lead author Jennifer Balch, a fire ecologist with University of Colorado-Boulder.

“This problem isn’t going to go away,” she said during an interview Monday. “We spend over $2 billion a year fighting fires. We will have more and more people living in this mix between houses and wildlands by the year 2030.”

While fire experts have long blamed people more than lightning, the new work details the extent of human-caused ignitions and how they interact with global warming to make matters worse.

Scientists analyzing fire data from 1992 to 2012 found that 84 percent of all U.S. wildfires — but only 44 percent of the total acres burned — were started by people, either by accident or on purpose. And human-caused blazes have more than tripled the length of the wildfire season from 46 days to 154 days, according to a study in Monday’s journal Proceedings of the National Academy of Sciences.

“People are moving more and more into natural wild areas and essentially providing ignition for wildfires,” said Balch. The result? Fire seasons can be year-round, as long as people are around, she said.

The spark

Of the more than 1 million human-started fires since 1992, about 29 percent began by trash burning, another 21 percent were arson and 11 percent were from misuse of equipment, Balch said.

Last year’s Soberanes fire in California was sparked by an illegal campfire and burned for nearly three months. The blaze surpassed $200 million in firefighting costs, the most expensive in U.S. history.

One out of every five wildfires occurs on the Fourth of July from fireworks, Balch said.

The human connection

Last summer, Michael Spengler, 53, drove his pickup truck off Highway 39 in San Gabriel Canyon on June 20. The truck burst into flames and started the Reservoir fire near Morris Dam, just north of Azusa. The fire burned 1,146 acres. A separate fire above Duarte known as the Fish fire burned 4,253 acres. The cause remains unknown. Taken together, the two fires caused the evacuation of 1,376 homes and the cancellation of the 626 Golden Streets ciclovia-type event. The event will take place on Sunday, spanning 18 miles from South Pasadena through Duarte and Azusa.

In January 2014, the fast-spreading Colby fire above Glendora blackened 1,992 acres of hillsides, canyons and trees and destroyed five homes and 10 outbuildings. It was started by three men who fanned an illegal campfire with paper torn from a spiral notebook. A gust of wind sent the campfire out of control, lighting up the canyon. Clifford Eugene Henry Jr., 23 of Glendora and 22-year-old Steven Robert Aguirre of Baldwin Park received six months and five months in prison, respectively. The third man, Jonathan Carl Jarrell of Irwindale received 3 years probation.

The hottest spots

The Southeast is a hot spot for human-triggered wildfires. Kentucky, West Virginia, Tennessee had fire seasons that lasted more than 200 days and 99 percent of the wildfires in those states are caused by people.

The region is home to swaths of forests with trees that don’t catch fire easily or naturally so people are the main culprits, said study co-author Adam Mahood.

The climate connection

Climate change has lengthened the U.S. fire season by a few weeks, which is dwarfed by what humans do. But the study shows how both human-sparked flames and man-made climate change work together to make America burn more, especially during more frequent dry, hot weather.

“If a campfire grows out of control during a wet, cool period, then it probably isn’t going to grow into a large wildfire,” said University of Utah fire scientist Philip Dennison, who wasn’t part of the study. “Climate change loads the dice toward warmer, drier conditions that make it more likely that a fire will develop from human-caused ignitions.”

Fire danger near normal

Recent rains have added new grasses to foothills and canyons within the Santa Monica Mountains, San Gabriel Mountains and San Bernardino Mountains. While the grasses hold more moisture, they can quickly dry out after a hot summer and fall, Balch said.

“Moisture during winter and early spring allow grasses to grow. If that is coupled with a very dry season then you can get significant fires,” she said.

The change from five years of drought to a surplus of rains have sharply reduced the probability of winter fires to near zero in Southern California, according to the National Significant Wildland Fire Potential Outlook released by the National Interagency Fire Center in Boise, Idaho on Feb. 1. The forecast says grasses and soils will retain enough moisture to ward off a severe fire season through summer or later. The group casts the fire potential through May as normal.

Staff Writer Steve Scauzillo contributed to this article. Reach him at or by email at:

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