Wildfires in California have increased significantly in size and severity since 1980, and a new research paper points to climate change along with other contributing factors. 

Experts from 15 universities, research institutes and government agencies published a paper in May in the International Journal of Wildland Fire that summarizes the latest research on wildfires. In Drivers of California’s changing wildfires: a state-of-the-knowledge synthesis, by Glen MacDonald, Tamara Wall B, Carolyn A. F. Enquist, et al., the authors examine how California’s wildfires have changed since 1980, and the effects of climate change, land management, and land cover conversion on the fires.

How California wildfires have changed

The researchers found an upward trend in annual area burned since 1980, with 18 of the 20 largest fires in recorded California history having occurred since 2003. A dozen of these blazes happened between 2017 and 2021. Notably, the number of fires annually did not increase, but instead there have been “fewer, but larger, individual fires.” 

In addition, the severity of California forest fires has also increased over recent decades. Severity is gauged by the degree to which fires have altered soils and destroyed “overstory trees” that make up the uppermost canopy layer in a forest.

Trends in both area burned and fire severity in California are similar to the rest of the West, but California has “experienced the most pronounced impacts” of these fires over the past 40 years. Since 2002, California wildfires have destroyed more than 60,000 structures and resulted in more than 300 fatalities. In 2018 alone, the estimated economic cost of devastating fires was  more than $148 billion dollars.  

Changes to land cover is an important factor in analyzing the state’s wildfires. Forest cover has decreased by almost 7% due to fire, while shrub, herbaceous, and bare ground cover increased by approximately 3% each. In addition, housing development has resulted in shrublands being replaced by “weedy, non-native grasses,” which are more flammable and contribute to fires spreading much faster. 

The impact of climate change

The report authors found that climate change has “played a central role in the changing nature of California wildfires” by making the state’s climate warmer and, importantly, more arid. 

Higher temperatures promote aridity by decreasing the relative humidity of the atmosphere and increasing the speed of evaporation from land surfaces and the sublimation from snow (e.g., snow changing into water vapor in the air instead of melting into water). Warmer temperatures also increase the vapor pressure deficit (VPD) — the difference between how much moisture is in the air versus how much it can hold.

Greater aridity and VPD increases the flammability of vegetation and lengthens the fire season by increasing the number of days with high fire weather risks. 

In addition, climate warming has also shortened the state’s winter precipitation season over the past six decades and accelerated snowmelt in the Sierra Nevada. The earlier snowmelt results in “drier live and dead vegetation,” which increases the flammability of the landscape for a longer period.

“We’re getting hotter, drier air that makes fires easier to start,” said the paper’s lead author Glen MacDonald, a UCLA distinguished professor of geography in a press release. “It makes the fuel much drier and the fires spread faster. They’re more intense, and they're more difficult to fight.”

Land management in California

Wildfires are a natural part of forest ecosystems, the report authors explain. Surprisingly, “during the pre-colonisation period, the average annual area burned probably often equaled or exceeded the 1.6 million ha (4 million acre) mark reached in 2020.” 

Historically, wildfires reduced forest density by clearing out younger and fire-intolerant trees but leaving the older tree overstory and fire-tolerant species that are able to “continue growing despite fire damage.” Indigenous peoples helped steward the forests by clearing brush, harvesting foods, maintaining paths, and using fire.

However, according to the authors, “the reduction of the use of fire by Indigenous peoples following colonization, the introduction of grazing livestock, aggressive fire suppression in the 20th century, and early timber harvest practices (which removed large trees and increased surface fuels) resulted in more dense forests, more fire-intolerant vegetation species, and increased fuel loading in some California forests, leading to increasingly severe fires in these ecosystems.”

Forest management practices have changed in recent decades, the authors report. For example, since 1980, some fires in remote forested areas “have been allowed to burn to support ecosystem health.” The experts advise that forest and fire management will need to continue to evolve, especially since the changes to California's land cover require different management practices than forests.  

How fires are ignited in California

More than 90% of recorded fires in most California counties were started by human-caused ignition from 1919–2016. At the end of the 20th century, some human ignition sources like cigarette smoking declined, but human development “spread further into fire-prone areas, leading to more ignitions from cars, power lines and other infrastructure.”

While people start fires near population centers and transportation corridors, lightning ignitions are more common in higher-elevation mountain ranges and deserts. Human activity is attributed (or partially attributed) to eight of the 20 largest wildfires recorded in California, while nine have been at least partially attributed to lightning. At least 12 of the 20 most destructive California fires when it comes to structures and lives lost were human-ignited.

The future of wildfire prevention and management in California

The paper’s authors explain that while some politicians have claimed governmental overprotection of forests has caused more severe fires, “the reality is more nuanced.” The experts advise that managing wildfires in the future will require approaches that address environmental complexity. 

For example, while increased logging and clearing trees may help in some areas, it may make things worse in other locations where opening the tree canopy would allow sunlight to dry vegetation that increases the risk of wildfires. In remote areas, letting fires burn is “often the best approach,” MacDonald said in the press release, while “prescribed burns and vegetation thinning can help protect areas where communities and property are at risk.”

The authors agree that “solely investing in fire suppressions” — more firefighters on the ground and in aircraft — is unlikely to succeed. Collaboration across communities and local, state and federal agencies is essential. 

“It’s all these things working together to create one giant, synergistic, complex problem,” said paper co-author Carolyn Enquist, acting federal director of the United States Geological Survey’s Southwest Climate Adaptation Science Center. “No one agency, entity or government level is going to be able to tackle this on its own.”