Since 2014, fires have burned through nearly forty percent of Butte County, according to a New York Times analysis of wildfire perimeters. An even larger share has burned in two neighboring counties, Plumas and Tehama, and in counties farther to the west, including in the heart of wine country.

Fires, of course, don’t know or stick to county lines. But calculating the share of counties affected by wildfires can provide insight into the growing wildfire risk statewide and across the American West.

The area that burned in Butte and Plumas Counties is more than four times as large as the area that had burned in the previous decade, the Times analysis shows, and the area burned in Tehama is more than five times as large. Over the past decade, most California counties have seen double the area burned compared with the area burned in the previous decade.

It’s not necessarily the case that more large fires are burning now than in previous decades, but the ones that do ignite are charring through much more land, according to Tirtha Banerjee, a professor and wildfire researcher at the University of California, Irvine. “What that says to me is that fires are getting more intense and more severe, and behaving in more unexpected ways,” he said.

A warming climate has fueled bigger and hotter wildfires, with increasingly intense spells of heat and drought turning forests into tinderboxes. The fire season arrives earlier in the year and lasts longer.

In California, decades of fire suppression policies have exacerbated the issue, leaving behind overgrown thickets of vegetation. Much of the area in the Park fire’s path, for example, hadn’t been burned for decades or longer, said Taylor Nilsson, the director of Butte County’s Fire Safe Council. That allowed large amounts of dense vegetation to accumulate, providing ample fuel for the fire.

Climate change and forest management are not the only risk factors. There is inevitably a bit of luck involved: High wind speeds can enable fires to spread farther and more rapidly.

All fires also require a spark in order to ignite. The movement of people into fire-prone areas near forests, grasslands and shrublands has bent that element of luck, making it more likely that a fire will spark.

While lightning caused several recent wildfires of historic proportions, human activity is the source for a vast majority of ignitions in the U.S. Of the 20 largest wildfires in California, seven were caused directly by people, and three by damaged power lines.

California’s wildfire history is punctuated by both “good” and “bad” fire seasons, but the overall size of burned areas has trended upward. In recent decades, quieter fire seasons have been followed by explosive and destructive ones. Often, a small number of extraordinarily large fires account for much of the area burned in a year.

This year, the number of acres burned by wildfires has more than doubled from the previous year. Two years of wet winters in 2022 and 2023 likely contributed to vegetation growth and the buildup of fuel, said Alex Hall, the director of the Center for Climate Science at the University of California, Los Angeles. Intense heat in the weeks before the Park fire sparked — most days in July in Chico climbed over 100 degrees Fahrenheit — greatly accelerated the drying process.

There are still several months left in this year’s fire season. On Aug. 1, the National Interagency Fire Center, which helps to coordinate federal fire response, issued new warnings about fire risk for this season, saying that it expects much of California and the Western United States to be under significant threat through at least the end of September.