While many Californians pray for rain heavy enough to slow the spread of the 6,078 fires that have burned 977,932 acres in the state this summer, firefighters and climatologists acknowledge that the heavy winter rains are a big part of what’s causing this fire season scorched three times as much total area as in 2023.
After Northern California’s brutal summer of fires, including the massive Park fire that is now the fourth-largest wildfire in state history, Southern California has exploded with fires this month. The wildfire in San Bernardino County northeast of Los Angeles has grown to 35,000 acres in the week since it ignited, threatening tens of thousands of homes and forcing the evacuation of thousands of residents.
While there were 5,053 fires that burned 253,755 hectares by September 11 in 2023. By that date this year, about a thousand additional wildfires had collectively burned more than 3.85 times more hectares. Much of the increase can be attributed to what climatologists call “weather whiplash.”
Over the past four years, California’s weather has swung from drought between 2020 and 2022 to two excessively wet years in 2023 and early 2024. That moisture has fueled a surge in growth of what are known as fine fuels — grasses, small shrubs , moss, and twigs that grow quickly and catch fire easily.
Firefighters call them one-hour fuels because in just one hour, under dry, sunny conditions, they can dry to the point of catching fire.
Grassland ecosystems are more susceptible than trees to the weather lashings of recent years.
“A forest doesn’t appear or disappear or grow or die just based on one wet year versus another,” said Daniel Swain, a climate scientist at the University of California, Los Angeles. “But when you have a grassland ecosystem, it can be quite responsive to changes from year to year.”
While dry years make a grassland look like a parched lawn, wet years can produce waist- or head-high grasses. When hot and dry conditions return, that heavy fuel load grass cures quickly, ignites easily and burns hot and fast with flames racing across the landscape much faster than a wildfire to spread the flames to trees or structures.
The previous two years’ wet winters and atmospheric rivers fueled grass growth, but then the record-breaking summer heat cured vast expanses of grassland to fuel.
“We added more fuel to the fire, and then we dried it in the oven, essentially at that record heat,” Swain said.
The Park Fire, which burned 429,603 acres of California this summer, is a prime example of how grass can drive some of the West’s largest and most destructive wildfires. The fire ignited in remote grassland and shrubland and most of the area it burned was in fine fuel ecosystems. With a strong wind pushing them, the flames raced toward denser vegetation and forests, where the heavier loads of biomass can feed much more energy into a fire. While many ground fires burn slowly and low on forest floors, those with enough fuel can easily send flames up the branches of 100-foot-tall trees to ignite crown fires that race through forest canopies.
“It’s the worst of both worlds. We get the abundant grasses and we also get fairly dry forests,” Swain said. “You can achieve that if you have a wet winter, but then a record warm summer and fall to follow, which is what we’re seeing now.”
Fast-moving grass fires can also ignite wood fence rails, decks, landscaping and siding to spread the fire through communities. Many homeowners who live in areas surrounded by grass don’t realize their properties can be threatened by a wildfire just as homes in forests are, but experts recommend that they prepare their property just as much as people whose homes are in the woods is.
The danger comes from the speed of the fire. Grasslands ignite quickly – often near communities – and, when pushed by strong winds, the resulting fire will spread quickly with the potential to use structures as new fuel.
In 2021, between Christmas and New Year’s Day, a rare winter grass fire exploded into the most destructive fire in Colorado history – in just two days. The Marshall Fire burned 1,084 homes and seven commercial buildings.
About 80 percent of home loss from wildfires occurs in grasslands, said Ralph Bloemers, director of Fire Safe Communities for Green Oregon.
“Fire is just. Fire is inevitable,” said Bloemers. “The problem is the vulnerability of the communities we have built in the fire plain, not the fire, because we are not going to eliminate the fire from a Western fire-prone, fire-adapted landscape. It is a natural reality.”
A study Bloemers co-author emphasizes improving resilience in at-risk communities. Changing structures and landscaping around communities can make them less likely to burn in a wildfire, and can reduce the potential for ignitions in conditions where a fire can be difficult to control.
“Fire is just. Fire is inevitable. The problem is the vulnerability of the communities we built in the fire plain, not the fire.”
Forest dwellers have long been advised to build with non-combustible roofs that are less likely to catch fire, to remove bark mulch, brush and woodpiles away from their homes, and to reduce the density of flammable vegetation near their homes. While many residents whose homes are surrounded only by grassland and scrubland may believe that their homes are less at risk, Bloemers says that they must be equally diligent in making their property wildfire resistant. Studies show that most home loss nationwide occurs in fast-moving fires, and in grassland and scrubland ecosystems. People should be even more vigilant to prevent a fire in dry grasses that can be ignited easily, for example by avoiding the use of machinery that can create sparks in parched grasslands.
Aside from community preparedness, the public “needs to be aware that humans and human activities are responsible for the vast majority of vegetation fires—I’m talking north of 90 percent of [Cal Fire’s] fires are started by people and our activities,” said Isaac Sanchez, deputy chief of communications for Cal Fire.
A 2023 study found that wildfire-related structural loss was not just a function of acres burned. Instead, 76 percent of all structural loss in the West comes from unplanned human-related ignitions. In contrast to the historically devastating Marshall Fire, which burned 6,080 acres, the Park Fire destroyed 375 fewer structures while burning more than 70 times as many acres. Thousands of personnel reached 99 percent containment of the fire on September 11.
As destructive fires fueled by all fuel types become more common, “we simply cannot afford to be the careless firefighters we are early in the season and push our suppression response,” Bloemers said.
More work is needed to reduce the loss associated with the intensity of wildfires and future years of “weather whiplash,” he said.
