A recently released study of 20 years of wildfires found humans caused the vast majority of wildfires.

While climate change has contributed to the lengthening of fire season and vulnerability of forested land, the ultimate cause of wildfires is most often a human being — both intentionally (arson) and carelessly.

The study, recently published in the “Proceedings of the National Academy of Sciences,” found that human activity or behavior was the cause of nearly 84 percent of 1.5 million fires between 1992 and 2012. Lightning caused 15 percent of wildfires during this period.

Further, human ignitions also lengthened the fire season in the spring and fall. The fire season for human-caused fires was about 154 days compared to 46 days for fires caused by lightning.

Slightly more than three-quarters of lightning-caused fires occurred in the summer, between June and August, and 12 percent occurred between September and November.

In contrast, human-caused fires were more evenly distributed over the year. Only 24 percent were started in the summer. In the spring, 38 percent of human-caused fires were started.

Examining the days when fires occurred, University of Colorado researchers easily identified the Fourth of July as the most common fire day. More than 7,750 fires were started on Independence Day.

Lightning most likely ignited fires on July 22, they said.

While humans were the greatest cause of fires throughout the country, the fires were not evenly distributed throughout the regions. The study’s authors found that human-caused fires compared to lightning fires were increasing in the eastern U.S., and Central and Southern California.

Human-started wildfires were caused by a variety of sources: equipment use, smoking, campfire, railroad, arson, debris burning, children, fireworks, power line, structures and miscellaneous fires.

About 20 percent of wildfires, including in California, start from arson, according to the U.S. Fire Administration. In 2015, Cal Fire reported that arson was the cause of 8 percent of wildfires. Other human behavior creating fires were campfires, debris burning, electrical equipment and playing with fire. Lightning was the cause of 11 percent of the 2015 wildfires.