Why does the American West have so many wildfires?

This story was originally published in the New York Times on Aug. 1, 2022

In just one weekend, the McKinney Fire, fueled by strong winds and high temperatures, burned more than 55,000 acres in Northern California, becoming the state’s largest wildfire so far this year. The blaze is only the beginning of the West’s fire season, which traditionally peaks between mid-July and October.

All fires need fuel and a spark. In the West, fuel is plenty, with flammable pine needles, shrubs and grasses that can ignite easily. And while the region’s dry vegetation has always made it prone to fires, climate change is intensifying wildfires and lengthening fire season.

There’s not necessarily an increase in the number of fires in the West, “but the fires are bigger, more severe and last longer, so the total area burned per year is rapidly increasing,” said Morgan Tingley, an ecologist at UCLA, who studies the effects of wildfire on Western ecosystems.

Why does the West have so many catastrophic wildfires? There are four key factors.

Climate and landscape

The Western United States has what’s called a Mediterranean climate, which means that most of its rain comes during the winter months, which are wet and cool. It also means that summer, the region’s driest season, aligns with its hottest, creating suitable conditions for a fire.

The region’s vegetation — pine needles, dry grasses, shrubs — is more flammable than the moist maple or beech leaves found in the East coast’s deciduous forests.

In addition, the seasonal Santa Ana winds bring strong gusts of extremely dry wind from the Great Basin area into Southern California, usually during the fall. The winds dry out vegetation and fan wildfires, but they can also knock down power lines and carry embers, spreading fires farther.

After thousands of years, Western vegetation has adapted to frequent fire. Some pine trees have developed thick cones, known as serotinous cones, that are glued shut with a strong resin. To reproduce, these trees actually need fires, which melt the resin, allowing the cones to open up and release seeds.

Yet wildfires in the western United States have grown so extreme that some tree species, like the giant sequoia, that have evolved to coexist with fires for thousands of years are being killed in unprecedented numbers.

Climate change

Wildfires are inextricably linked with climate change, which is making the West hotter and drier.

California’s official government fire record dates back to 1932, but nine of the state’s 10 largest wildfires occurred in the past decade, including the August Complex fire, California’s largest ever, that burned more than 1 million acres of land in 2020.

The West has warmed 1.5 degrees Fahrenheit on average compared with a century ago. Higher temperatures and a lack of rainfall has killed small plants, dried out vegetation and caused deciduous trees to shed their leaves earlier in the season.

“These things compound, so you have good fuel lying on the ground, warm air and lack of precipitation,” Tingley said, making it “much more likely that any given place has the right conditions for a fire to start.”

Climate change is also lengthening the fire season, which now starts earlier in the year and lasts longer. On average, the fire season has become 2 1/2 months longer than it was in the 1970s.

Fire suppression

Before the modern settlement of the American West that began in the 1860s, forested lands burned naturally from lightning sparks or intentionally by native communities as a form of forest maintenance.

Beginning in the 20th century, though, the Western United States had a policy of aggressive fire suppression, which meant that fires were put out as quickly as possible. In 1935, the U.S. Forest Service instituted the “10 a.m. policy,” which aimed to contain any fire by 10 a.m. the day following its initial report.

This practice has led to increasingly dense forests and ample brush on the forest floor. As a result, forests end up being “tinderboxes” for more explosive fires, said Jennifer Marlon, a research scientist at the Yale School of the Environment and creator of the Global Paleofire database, a collection of fire history records.

“When you have fuels that are more densely packed, they will burn hotter and faster and more severely,” Marlon said.

Experts say that fire suppression has also changed the forest floor, making fires more severe. There are now more fire-intolerant shrubs and tree species, such as white firs, at lower elevations. White firs have needles running up their trunks that serve as ladders up to the canopy, creating crown fires that are the hardest to contain and most deadly for trees.

In recent years, firefighting has turned toward the use of “prescribed,” or controlled, burns, to treat fire-prone land by thinning its brush. Last year, the Forest Service used prescribed fires across a record 1.8 million acres of federal land. The agency hopes to ramp up operations nationwide in the coming years, but public backlash to the practice has grown. Opponents point to prescribed fires that sometimes get out of control, like those that burned through New Mexico earlier this year.

Human settlement

As the Western population has grown, so has the risk of sparking a wildfire.

Half of all wildfires are ignited by lightning strikes. The other half are ignited by humans, whether indirectly — felled power lines or sparks from a train as the wheels press against the rails — or directly, from tossed cigarettes, cars backfiring or campfires.

Wildfires ignited by human activity spread more than twice as fast and killed more trees than those ignited by lightning, according to research presented at a 2020 meeting of the American Geophysical Union. “Where humans live, we bring opportunities for fires to begin,” Tingley said.