Humans—not lightning—trigger most wildfires in the United States. According to a study published in February 2017 in theProceedings of the National Academy of Sciences, 84 percent of the blazes that firefighters were called to fight between 1992 and 2012 were ignited by people. Some common ways that people start fires 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.
University of Colorado scientist Jennifer Balch and several colleagues came to their conclusion after analyzing reports of 1.6 million wildfires from a comprehensive’s fire occurrence databasemaintained by the U.S. Forest Service. As shown by the map above, almost all (80 percent or more) of the fires in theMediterranean ecosystemsof central and southern California, the temperate forests of the eastern United States, and the temperate rainforests of the Pacific Northwest are 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. Though lightning-ignited fires were clustered in the summer, human-ignited fires occurred in the spring, fall, and winter as well, 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.
In the eastern United States, fire activity became more extensive in the spring; in the West, human-ignited fires tended to extend the fire season in fall and winter. 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 forest health. For instance, by comparing to a Landsat-based record of burn scars—known as the Monitoring Trends in Burn Severity (MTBS) project—they showed that both human-ignited and lightning-ignited wildfires have grown larger and more severe since 1992. For lightning fires, burning increased in mountainous parts of the West—not because there was more lightning, but because a warmer climate dried out forests. Among human-caused fires, there was an increase in large fires in the spring months in the Great Plains, possibly because climate change has prompted agricultural activities earlier in the year, Balch said.
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