Humans — not lightning — trigger most wildfires in the United States. That’s according to a study published February 27, 2017 in the Proceedings of the National Academy of Sciences. The study reports that, between 1992 and 2012, 84 percent of the blazes that firefighters were called to in the United States were started by humans. Some common ways that people start wildfires, according to a statement from NASA’s Observatory, include:
… discarding cigarettes, leaving campfires unattended, and losing control of prescribed burns or crop fires. Sparks from railroads and power lines, as well as arson, also routinely cause wildfires.
The study scientists analyzed reports of 1.6 million wildfires from a U.S. Forest Service, and discovered that almost all (80 percent or more) of the fires in central and southern California, the eastern United States, and the temperate rainforests of the Pacific Northwest were caused by humans. In contrast, lightning started the largest percentage of fires in the forests of the Rocky Mountains and the Southwest. In Florida, which is moist but has a great deal of lightning, between 60 and 80 percent of wildfires were caused by people.
The researchers also found that human-ignited fires tripled the length of the wildfire season. Lightning-ignited fires were clustered in the summer, the researchers said, but human-ignited fires occurred in the spring, fall, and winter as well, which are times when forests tend to be moist. During these seasons, people added more than 840,000 fires—a 35-fold increase over the number of lightning-started fires.
But, according to the study:
Despite the high number of incidents, human-ignited wildfires accounted for just 44 percent of the total area burned because many of them occurred in relatively wet areas and near population centers, where firefighters likely could quickly extinguish the fires before they spread.
The researchers also compared the wildfire reports to other satellite-based measurements of fire activity and found that both human-ignited and lightning-ignited wildfires have grown larger and more severe since 1992.
The new study does not suggest that 84 percent of all fires in the United States are caused by humans — just wildfires. Other research has shown most of the active fires that satellites detect in the United States are prescribed fires and crop fires lit intentionally by land managers and farmers.
Bottom line: A February 2017 study in the Proceedings of the National Academy of Sciences reports that 84% of U.S. wildfires between 1992 and 2012 were started by humans.
Eleanor Imster has helped write and edit EarthSky since 1995. She was an integral part of the award-winning EarthSky radio series almost since it began until it ended in 2013. Today, as Lead Editor at EarthSky.org, she helps present the science and nature stories and photos you enjoy. She also serves as one of the voices of EarthSky on social media platforms including Facebook, Twitter and G+. She and her husband live in Tennessee and have two grown sons.