After flames spread over tens or hundreds of thousands of acres, it can take quite a bit of time for the fire to finally go out. How long, exactly?
The length of any individual fire can vary quite a bit, Stephen Pyne, a fire historian at Arizona State University, explained in an interview. But more telling numbers can be seen in a decades-long, growing trend. On average, individual fires today burn for a significantly longer time than they used to.
Research conducted by fire scientist Anthony Westerling shows that between 1973 and 1982, fires burned for an average of six days. Between 2003 and 2012, this number skyrocketed to nearly seven and half weeks (52 days). This builds upon previous research published a decade earlier, which found an increase in average fire duration to more than five weeks.
“So long as there is fuel and it doesn’t rain or snow, a fire can burn, and burn, and burn,” Pyne said. (He cited the exceptionally long-burning Chinchaga Fire, which raged from May through October in 1950.)
These numbers show when a fire has been “controlled,” a somewhat blurry definition generally meaning that firefighters have contained the fire’s perimeter, and the likelihood of flames or embers escaping is quite small, said Pyne. It doesn’t mean the fires are completely out.
This is a critically important indicator for the millions living in the urban-rural fire interface, for example, Redding, California.
But deep inside the woods, the flames can continue.
“It’s not like a house fire — it can burn and smolder for a long time,” said Pyne. “When you’re talking about something that’s 300,000 acres, are you going to be able to go in with shovels on your back and put out every ember?”
That modern fires are burning longer than they were decades ago makes sense, as we live in a warmer, more fire-friendly climate today. This means longer fire seasons overall, said Valerie Trouet, a fire researcher at the University of Arizona. The fire season starts earlier in spring and extends through late fall, for a total of more than 80 additional days of fire season, compared with 1970s levels.
“With higher temperatures, the period which is hot and dry enough for fires to get started is getting much longer,” Trouet said in an interview. “So we see fires all year round,” she added, noting California’s 2017 Thomas Fire, which burned in the winter.
These fires are also more vicious than those we saw decades ago.
“Fires themselves are also getting harder to fight,” said Trouet. “With so much fuel on the landscape, they’re just higher intensity fires. They’re risky and difficult to control, which contributes to them burning for longer.”
Though the 2018 fire season has been terrible, it certainly isn’t an exceptional, stand-alone year. It’s part of a growing trend.
“For several years we’ve been seeing record-breaking individual fire seasons and areas burned,” Jennifer Marlon, a research scientist at Yale University’s School of Forestry and Environmental Studies, said in an interview.
“That’s quite striking,” noted Marlon. “We’ve been keeping records for quite a long time. It highlights how unusual the current activity is.”
In 2015, more than 10 million acres burned in the U.S., the highest number since reliable record-keeping began in 1983. Two years later, in 2017, another 10 million acres burned. In exceptionally parched California, the state recently broke two all-time fire records in just an eight-month span.
And in 2018, fire season isn’t nearly over. Meaningful rains aren’t expected in California for months. Swathes of forest, grass, and brush are just sitting there, parched by a series of record or near-record heat waves.
“There are so many large fires going on,” said Marlon. “Yet, there’s still plenty of forest out there to burn.”