Study Shows 84% of Wildfires Caused by Humans

Over the last 21 years, debris burning, arson and campfires have combined with climate change to make the fire season much longer

Wild Fire
Wikimedia Commons

In the last decade, mega-wildfires have become routine news. In 2015, fires burned a record 10 million acres of U.S. wildlands, and 5.5 million burned in 2016, including major fires in California and a blaze that started in Great Smoky Mountains National Park that damaged 2,400 buildings in Gatlinburg, Tennessee, and killed 14 people. While wildfires are a natural phenomenon usually sparked by lighting, it turns out the recent destruction isn’t all Mother Nature’s fault. A new study shows that 84 percent of wildfires in the United States are started intentionally by humans or by human activity.

According to a press release, researchers from the University of Colorado, Boulder’s Earth Lab took a deep dive into the U.S. Forest Service’s Fire Program Analysis-Fire Occurrence Database, analyzing all wildfires recorded between 1992 and 2012. The researchers found that humans caused more than 1.2 million of the 1.5 million blazes in the database.  

The cost of those human-induced fires is staggering. The researchers estimate that man-made fires have tripled the average fire season over the past 21 years from 46 days to 154 days. It now costs over $2 billion per year to fight the fires, and that figure does not include the impacts to recreational lands or local economic impact that fires can have.

“We are playing a really substantial role in shifting fire around,” Jennifer Balch, fire ecologist at the Earth Lab and lead author of the study in PNAStells Christopher Joyce at NPR. “I think acknowledging that fact is really important particularly right now when we have evidence that climate is changing, and climate is warming, and that fires are increasing in size and the fire season is increasing.”

Thomas Swetnam, professor emeritus at the University of Arizona who studies forest fires, tells Doyle Rice at USA Today that it’s not necessarily the case that more people are maliciously setting fires or that Smokey Bear has failed in his mission to educate the public. Instead, Swetnam says that climate change is the biggest driver of increased fires. An increase in drought, fuel buildup in unburned forests, earlier springs and higher temperatures are all contributing to more combustible forests. So the same actions that might have caused a small, easily extinguished fire decades ago are now creating dangerous infernos.

“[This is a] very well done study," he said. "We have known for a long time that fires set by people are an extremely important factor in the wildfire problems, but this study shows in detail how important people are in lengthening the fire season and contributing to increasing numbers of large wildfires.” 

Rice reports that debris burning starts the most human-caused fires, at 29 percent, with arson the cause of 21 percent of fires. Equipment use causes 11 percent of fires, while campfires and children playing with fireworks or matches each cause 5 percent of fires. The Fourth of July, predictably, is the biggest day for wildfires, with 7,762 fires ignited on that date over the 21-year study period.

Balch tells Joyce that there is a solution. She suggests conducting more prescribed burns on forest land to decrease the amount of fuel in the forests after 100 years of fire suppression.

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