Humans to blame for 84 percent of wildfires, longer wildfire season

People have triggered five out of six wildfires in the USA 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 say human activities play an even bigger role.

In an analysis of 20 years of wildfire records led by researchers at UMass Amherst and the University of Colorado-Boulder, they found human-started fires accounted for 84 percent of all wildfires, exhibiting "a remarkable influence" on modern U.S. wildfire regimes.

As well as extending the areas in which wildfires occur, the researchers' information - drawn from the US Forest Service Fire Program Analysis Fire Occurrence Database - indicates that human-induced fires happen throughout the year, in contrast to lightning-caused blazes, which tend to be concentrated in a period of a few weeks during summer.

University of Colorado fire ecologist Jennifer Balch found that human-sparked fires increased the fire season from 46 days to 154 days. All the findings done by the investigation team co-led by Bethany Bradley from the University of MA were published in the journal Proceedings of the National Academy of Sciences. Bradley explains, "Humans are expanding fires into more locations and environmental conditions than lightning is able to reach". They found that people caused an average of 40,000 wildfires yearly throughout the spring, fall and winter, igniting more than 35 times the amount of fires started by lightning. Humans were responsible for nearly 1.25 million wildfires, while lightning was only responsible for the remaining 250,000. "Human ignitions have expanded the fire niche into areas with historically low lightning strike density". While fire experts have long blamed people more than lightning, this study digs into the details of how and where the people problem is and how it interacts with global warming to make matters worse. Humans live everywhere in the USA, all year round. 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.

"We saw significant increases in the numbers of large, human-started fires over time, especially in the spring", said Bradley. In northwestern states - including Idaho, Washington, Oregon, and Montana - humans started about half of all wildfires, says University of Idaho researcher John Abatzoglou. In parts of the south and the west, human-ignited fires are more common in September, October, and November than in other seasons.

The researchers found that fires can be caused by various human-related reasons: equipment use, smoking, campfire, railroad, arson, debris burning, children, fireworks, power line, structure, and miscellaneous reasons.

Researchers say wildfire mitigation efforts would be well-served to focus on limiting the human-led expansion of the fire niche. They did not include prescribed burns or agricultural fires.

Not only were the vast majority of them caused by humans, the anthropogenic blazes accounted for 44% of the land burnt.

  • Carolyn Briggs