California wildfire season is longer, peaks earlier

In a new study from UC Irvine, experts report that California’s wildfire season has grown more intense with a longer duration every year this century. The researchers also found that the wildfire season is peaking earlier in the year. 

The longer fire season has been driven by climate change and human expansion into areas that were once uninhabited.

For their investigation, the experts analyzed wildfire statistics from the California Department of Forestry and Fire Protection wildfire from 2000 to 2019. They compared this information with historical data from 1920 to 1999.

The study revealed that the annual burn season has lengthened in the past two decades and that the yearly peak has shifted from August to July. 

The team also determined that the number of hotspots with severe fire risk has increased in recent years. This increase is tied to higher temperatures, drought, and an elevated risk of human-caused fires.

“CALFIRE data show that each new year of the 21st century has been a record breaker in terms of wildfire damage in California,” said study co-author Professor Tirtha Banerjee. 

“We also have seen that about 80 percent of the total number of the state’s wildfires over the past few decades have been small, measuring less that 500 acres. But when fires get large, their deadliness greatly increases.”

In order to gain a proper understanding of the growth of fire risk in California, it is important to put large and small incidents into separate buckets, explained Professor Banerjee. The experts discovered that just about 20 percent of the fires were responsible for 97 percent of the total area burned from 2000 to 2020.

“And more than nine-tenths of the casualties and property losses can be attributed to fires exceeding the 500-acre threshold,” said Professor Banerjee. He noted that over the past two decades, there has been a significant increase in extreme wildfires scorching more than 10,000 acres. 

According to lead-author Shu Li, one of the most alarming findings of the study is the substantial spatial growth of fire risk throughout the state. 

From 1920 to 1999, California’s only hotspot that was classified as having “very high wildfire density” was Los Angeles County. But over the past 20 years, these types of hotspots have greatly expanded to include Ventura, Riverside, San Diego, and San Bernardino Counties, and even some areas of northern California.

“Before 2000, there were almost no human-caused wildfires along California’s Pacific coastline, but now nearly every coastal county is experiencing increased risk, and the San Benito-Monterey Unit and the San Luis Obispo Unit have even become new hot spots,” said Li.

“The concurrence of human-caused climate change, which is drying out our forests and grasslands and creating longer stretches of hot weather, and a steady influx of people into remote areas is creating conditions for the perfect fire storm,” said Professor Banerjee. “But there is some good news in all of this; human-caused fire risk can be mitigated by better fire management practices by humans.”

Professor Banerjee said he hopes the study – and the near real-time analysis of fire risks in California’s natural environment it provides – can be used by government agencies and public policy officials to both prevent and combat costly blazes.

The study is published in the journal Scientific Reports.

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By Chrissy Sexton, Earth.com Staff Writer