California’s recent wildfires have been nearly unprecedented in terms of their destructiveness and size and the season in which they burned. The Thomas Fire, for example, has grown into one of the largest wildfires in the state’s history, devouring thousands of acres daily as it moves from Ventura to Santa Barbara at a time of year more prone to gray skies and cold rain than burning forests.
“The fact that one of California’s biggest wildfires is burning in December is mind-blowing,” said Jens Stevens, a postdoctoral researcher of forest ecology at the University of California, Berkeley.
Still, the year’s devastating fires aren’t entirely surprising to Stevens and other scientists and forest managers, many of whom expect more of the same as development in forested areas continues and the region’s climate grows warmer and drier.
Now, many experts are questioning how best to manage woodlands and protect society against wildfires in the future. This will be a tricky task since natural fire cycles differ dramatically from one forest type to another. Intentionally ignited prescribed burns could reduce the severity of wildfires in some ecosystems, especially high-country conifer forests. In others, though – like the coastal zones that burned so destructively this year – fires burn in different patterns. Here, solutions may be more complicated, largely because these areas have become densely populated.
“There are some real risks in lighting fires in these coastal chaparral systems,” said Eric Knapp, a United States Forest Service research ecologist. Because lightning strikes, which ignite many high-country fires, are rare in coastal zones, these lower areas naturally burn less frequently. This allows heavy buildup of dense underbrush, which makes fires here – whether wildfires or prescribed burns – extremely hot and potentially destructive.
To Stevens, the infernos that have now killed dozens of people are the almost inevitable culmination of one record-breaking weather event after another, starting with the drought.
“To have four extremely dry years like that was unprecedented for at least 500 years,” Stevens said. “Any of those years on its own wouldn’t have been so unusual, but we had four in a row.”
Knapp said most of the shrubs and trees of the coastal lowlands survived the drought.
“But under those kinds of conditions, they shed their branches” as a defense mechanism against water loss, he said. “After several years of that, there was a lot of dried wood in those forests. They were set up to burn.”
The historic dry spell preceded one of the wettest winters in decades, which was then followed by the hottest summer ever recorded in California, with several coastal cities experiencing all-time high temperatures. This alternating sequence of extreme moisture and heat – which researchers say is a clear mark of climate change – led to a buildup of fuels, including 129 million dead trees in the Sierra Nevada since 2010 and, in coastal areas, dead brush and knee-deep dried grass.
“All that dried grass allowed the fires to spread faster,” Stevens said.
Bill Stewart, a forestry specialist at U.C. Berkeley, feels the howling hot Santa Ana winds, combined with a dry autumn, are more to blame for the fires. Los Angeles has received almost no rain since the start of the water year on October 1 and most of Southern California is either abnormally dry or experiencing moderate drought.
“In my opinion, it doesn’t matter how much rain we had,” he said. “What matters is what sort of weather we have in the fall, and when the first rains come.”
Regardless of what factors drove the fires, experts are now discussing ways in which to reduce fire intensity in the future. One of the most effective tools, according to sources, may be prescribed burning. Applied during low-risk times of the year, the practice of intentionally setting fires in overgrown woodlands can effectively, and safely, clear out potentially explosive brush and dead wood.
“An acre burned under controlled conditions is an acre that doesn’t burn under wildfire conditions,” Knapp said.
Already, prescribed burning is used as a management tool, though perhaps not as much as it should be used. According to a study published in 2007 in the journal Forest Ecology and Management, 4.4 million acres of California used to burn naturally every year – mostly low-intensity fires that fed off ground-level shrubbery and fallen wood while also providing ecosystem services, like opening pinecones and cracking seed pods, and making the forest floor navigable for foraging wildlife. Today, thanks to aggressive fire suppression, wildfires consume much less land, allowing fuel to build up over decades. According to Knapp, wildfires have burned an average of 600,000 acres per year since 2002. Prescribed fires burn another 100,000 acres or less each year.
That makes a total of just one-sixth of the historical burned acreage. As a result, a given parcel of land today experiences much longer intervals between fires, massive fuel buildup and much hotter fires when they do occur.
But forestry experts say prescribed burning is most effective for managing high-country conifer forests, like those of the Sierra Nevada. Coastal areas dominated by oak, chaparral and other broadleaf trees and shrubs, they say, are trickier to manage.
“These coastal systems naturally have long returns between fires, usually between 30 and 50 years,” Knapp said.
This means understory fuels accumulate for decades between each fire, making the blazes, when they ignite, extremely hot and powerful – very different from the frequent blazes that naturally burn with low intensity in the Sierra Nevada. Knapp said the species living in coastal zones have actually evolved to depend on hot, powerful and infrequent fires. Frequent prescribed burns, he said, could have negative consequences for these plants and animals, producing lower-temperature flames that fail to stimulate germination processes, or even killing the native plants and allowing invasive ones to replace them.
Starting prescribed burns in such fuel-packed environments could also be extremely dangerous.
“It is difficult to conduct prescribed fire in chaparral since it tends to burn hot,” Stevens said.
Climate change will almost certainly make California much warmer in the future, according to Park Williams, a bioclimatologist at Columbia University’s Lamont-Doherty Earth Observatory. While it remains unclear whether climate change will bring more or less precipitation to the region, fire severity will probably increase. “Anthropogenic climate change will continue to chronically enhance the potential for Western U.S. forest fire activity,” Williams and coauthor John Abatzoglou wrote in a paper published in 2016 in the Proceedings of the National Academy of Sciences.
Moreover, the past year has amply demonstrated that drenching winter rainfall won’t necessarily stifle the outbreak of fires. As long as the summers become hot enough, fire risk, especially at lower elevations, will probably grow no matter how much it rains in the future. Steve Burns, a forest fire management officer with the U.S. Forest Service, said areas above 7,000ft were protected against burning in 2017 by the wet winter.
“It bought us some time in the mountains, because we had snowpack into August, but lower down it got really hot, and it dried out,” he said.
Stevens expects destructive fires to become more frequent in coastal areas, because of development trends and hotter, drier summers.
“The fires in these areas tend to be caused by human ignition,” he said. “That means the more people we have in this area, and the drier it gets, the more fires we’re going to have.”