California is on fire again. CalFire, one of the agencies charged with putting those fires out, is tracking upward of two dozen conflagrations up and down the state at the moment—Detwiller, Grade, Bridge, Wall, Alamo, Garza, on and on—ranging in size from a couple hundred acres to nearly 50,000.

And it’s not just the Golden State. Across the North American West, from Wanblee, South Dakota, to the Rogue River-Siskiyou National Forest in Oregon; from the Coronado National Forest near the Mexican border in Arizona to Fort Fraser in British Columbia and even farther north, grasses, chaparral, and forest are all ablaze. The continent is deep into the seasonal cycle of wet-winter-grows-plants/dry-summer-turns-them-to-fuel/they burn.

It’s tempting to see each fire season as worse than the last, and to further see that as evidence of the kind of apocalypse that a changing climate will visit on civilization. If it ain’t rising water at the coasts it’s a “firenado” in the hills. But researchers have identified an even more pernicious problem: us. Specifically, where we build houses.

Whether the sheer number of fires in a season or the amount of land they burn has increased over years (or decades or centuries) is hard to tell. It depends on the dataset. “When you smear out fires over a continental scale, from the edge of the boreal forest to the tropics, you’re hiding a lot of regional variation,” says Mark Finney, a researcher at the US Forest Service’s Missoula Fire Sciences Laboratory. In fact, Finney says, Native Americans burned a lot more territory before Europeans arrived than after.

A changing, warmer climate does make wildfires more likely. And human beings have attempted to manage them in myriad ways over the last century or so. They’ve cut down trees that seemed likely to burn, set intentional fires to try to mimic the periodic fires ecosystems rely on, and even favored some kinds of plants—“fuel,” in the parlance of fire management, which should give you some sense of where fire managers’ heads have been about the nature of nature.

Even that’s a more complicated story than I just made it sound, though. As a paper in Science in June of 2017 put it, savanna ecosystems need frequent fires to stay healthy, but different kinds of forests have different kinds of needs. Those authors say that as human uses of land have altered landscapes and ecosystems—more agriculture, more cities—the total amount of area burned on Earth went down by about 24 percent over the past 18 years. They think that’s because with a more built-on environment comes more fire suppression to protect the buildings, farms, domestic animals, and everything else that comes with “civilization.”

It's not the number of fires or how bad they are, but where they are.

The key to how we think about fire’s severity, then, is not the number of fires, or even their magnitude, but where they are—and what’s near them. Wildfires tend to occur at what’s called the Wildland-Urban Interface. That’s where houses or other human-built stuff butt up against nature, what a less sophisticated academic might once have called the frontier. Humans set most wildfires—95 percent of them, according to CalFire. That’s a real problem when, as of 2004 in the continental US, the WUI was almost 278,000 square miles and contained 44.8 million housing units. In other words, 39 percent of all the houses in the country are in the WUI. People built 10 million new housing units in WUIs in the decade leading to 2010.

This situation is worst in California. According to a report from the Center for Insurance Policy and Research, 2 million homes in the state are in wildfire-prone areas—14.5 percent of all the houses in California. (Texas and Colorado fill the number two and three slots, respectively.) The Western promise of open land has a combustible side. “What has happened over time is that development has become less dense in the US,” says Volker Radeloff, a forestry professor at the University of Wisconsin, Madison and lead author on that 2005 WUI study. “People like to move to a 5-acre ranch, and that creates this volatile mix of houses and flammable vegetation.”