Humans are responsible for 84 percent of all wildfires in the United States, a new study suggests.

Writing in the Proceedings of the National Academy of Sciences, researchers from the University of Massachusetts Amherst say during 20 years, human-started fires have “tripled the length of the fire season and dominated an area seven times greater than that affected by lightning-caused fires.”

The study looked at the years 1992 to 2012 and found that of the 1.5 million fires that were large enough to require fire fighting, humans were responsible for 44 percent of the area burned.

Researchers say humans are responsible for expanding what they call the “fire niche,” which analyzes ignition sources, fuel mass and dryness to measure fire potential.

“Humans are expanding fires into more locations and environmental conditions than lightning is able to reach,” said study co-lead, Bethany Bradley at the University of Massachusetts Amherst. “Humans create sufficient ignition pressure for wetter fuels to burn. Human ignitions have expanded the fire niche into areas with historically low lightning strike density.”

Researchers say efforts to stem the rise in wildfires need to focus on how humans are increasing the fire niche. For example, they say the “wild land-urban interface,” where houses are close to natural areas is expected to double by 2030.

“It’s generally pretty well known that people start a lot of fires; everything from campfires to burning yard waste to accidental fires in homes and other structures,” Bradley said.

“But in the past, I used to think of ‘wildfire’ as a process that was primarily natural and driven by lightning. This analysis made me realize that human ignitions have an extraordinary impact on national fire regimes. From our analysis, we learned that human-started fires are amazingly common. We found that humans play a primary role in redistributing wildfires in space and over time.”

The study found that lightning-started fires happen mostly in the inter-mountain west and almost only in the summer months. Human-started fires, on the other hand, happen everywhere and occur in the spring, summer and fall.

“Economic and ecological costs of wildfire in the United States have risen substantially in recent decades,” researchers said.“While climate change has likely enabled a portion of the increase in wildfire activity, the direct role of people in increasing wildfire has been largely overlooked.”
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