Across most of California, human activities and infrastructure — like campfires, arson, electrical equipment, and power lines — start the vast majority of fires.

Tap to play or pause GIF Tap to play or pause GIF Peter Aldhous for BuzzFeed News / Via frap.fire.ca.gov Cal Fire’s records of areas burned by decade, 1950-2017.

Six people are dead as fire rages around the city of Redding, California. The iconic Yosemite Valley has been deserted, shrouded in smoke. After the devastating wildfires of 2017, which razed more than 10,000 buildings and killed 43 people, 2018 is shaping up to be another bad year for fires in the Golden State. It’s easy to simply blame climate change — and warmer, drier conditions are definitely a big factor in a trend toward larger fires in recent years. But experts studying California’s fires say that the state’s burgeoning population and its sprawl into areas that were once wild have helped create a perfect firestorm. “There is a clear human influence on fire patterns,” Alexandra Syphard, an ecologist with the Conservation Biology Institute, based in San Diego, told BuzzFeed News. Studies by Syphard and others show that the risks of wildfire increase when housing is built in rural areas that used to be able to burn without threatening lives and property. This rural sprawl has been massive in California, as the state’s population has almost quadrupled since 1950. But people haven’t just placed themselves in the line of fire: They’re also causing most of the blazes that wreak such havoc. Across most of California, human activities and infrastructure — including campfires, arson, electrical equipment and vehicles, and power lines — start the vast majority of fires. That is creating a big headache as the state’s leaders wrestle with how to protect people and property from the flames. “Even if we can reverse climate change, that’s going to take decades,” Lynne Tolmachoff, a spokesperson for the California Department of Forestry and Fire Protection, or Cal Fire, told BuzzFeed News. “This is something we have to deal with right now.” A BuzzFeed News visualization of California fires from 1950 to 2017 shows that larger fires have become more frequent and the fire season has expanded — particularly for human-caused fires.

Big fires have gotten more common. Peter Aldhous for BuzzFeed News / Via frap.fire.ca.gov Each fire is a dot that is scaled by the area that ultimately burned, centered on the date on which the alarm was sounded.

Most of California’s rain and snow falls in between October and March, which means that fire season peaks in the summer, as vegetation dies and dries out. In Southern California, the season extends into the fall, when Santa Ana winds, which blow from the dry interior toward the coast, whip up small fires into major conflagrations. As the state has dried and warmed, the fire season has started earlier and larger areas have burned. Similar changes have occurred across the western US.

But the pattern is different for natural and human-started fires. Tap to play or pause GIF Tap to play or pause GIF Peter Aldhous for BuzzFeed News / Via frap.fire.ca.gov California wildfires by year and alarm date, by cause.

Before people dominated the landscape, almost all wildfires were started by lightning. Cal Fire’s data shows that these natural fires have become larger in recent years — exactly what scientists would expect to see with climate change. The severe drought that parched the state between 2011 and 2017 made matters worse. In California’s dried-out forests, at least 129 million trees have died, many killed by bark beetles, creating a tinderbox of dead timber. But human-caused fires have done most to extend California’s fire season. They include the massive Thomas Fire, which raged around Santa Barbara in December 2017, destroying more than 1,000 buildings and killing 21 people. It is now the subject of lawsuits from residents who blame the electricity utility company Southern California Edison for the blaze.

California’s problems with human-caused fires set it apart from most of the West. Peter Aldhous for BuzzFeed News / Via fs.usda.gov Wildfires from 1992–2015, shown in a grid of half a degree longitude and latitude. Dots in each grid cell are scaled by the total area burned, and colored by the extent to which that area corresponds to human-caused fires. Yellow dots show areas dominated by human-caused fires; purple shows areas where natural fires dominate.

But if California can’t reduce the number of catastrophic fires, last year’s record season may become the new normal. Peter Aldhous for BuzzFeed News / Via frap.fire.ca.gov Total area burned by wildfires recorded by Cal Fire, 1950–2017.