California's 2020 Wildfire Season

These forests experienced a naturally high frequency of fire before fire exclusion policies in the late 19th and early 20th centuries restricted burns, including from lightning ignitions and Native American cultural burning. Fire exclusion led to a huge increase in forest density and fuels, driving an explosion in large, destructive fires.

“In these ecosystems, reducing burned area is a cause of the current catastrophic trends, not a solution to them,” the authors said.

Key 2020 Wildfire Statistics
The authors summarized 2020’s burning conditions, burned area and fire sizes, fire weather, fuel moisture, fatalities, property damage, suppression cost data, vegetation types, fire history and other variables from public data sources.

They found that fires in 2020 followed an accelerating, decades-long trend of increased burn area, severity, size and costs to society and the economy.

The fires killed 33 people in 2020 with overall economic losses of more than $19 billion and firefighting costs approaching $2.1 billion. The years 2020 and 2021 together burned more area than the previous seven years combined, and only slightly less than the total burned between 1980 and 1999.

Between 2015 and 2020, total insured economic losses were more than $50 billion, and over 50,000 structures — mostly homes — were destroyed.

Air Quality and Wildfires
More than half of Californians experienced unhealthy, and sometimes hazardous, air quality index levels for a month or more in 2020, the study reports. The state’s worst five days of air pollution in history all occurred in 2020, according to CalFire.

Wildfire-driven air quality in August and September likely also caused 1,200 to 3,000 “excess” deaths among people age 65 or older.

The fires emitted nearly 112 million metric tons of carbon and 1.2 million tons of fine particulate matter (PM 2.5). That’s 120 times more fine particulate matter than California’s vehicles produced in the same year.

‘We Can Do Something About This’
The study also assessed what drove fire severity patterns in 2020. Fire severity is a measure of the impact of burning on an ecosystem, measured in this study by losses in vegetation biomass.

Overall, fire severity was best explained by fuel load-related variables, with dryness and wind also playing key roles. Longer-term drought — including its effects on tree mortality before 2020 — was less important overall except in a handful of fires.

Nearly 60% of the fires were caused by humans through arson, vehicles, power lines, campfires or unknown causes, while lightning sparked the rest. However, the lightning-sparked fires burned more than five times the area burned by human-caused fires.

“Extreme weather conditions are certainly playing a role, but climate change isn’t driving all the change we’re seeing,” said Safford. “Fuel loads played a major role in driving fire severity patterns in forested landscapes in 2020, like in other years. High fuel loads are due mostly to human management decisions over the last century or more, and we can do something about this issue.”

The authors said a recent agreement between the state and U.S. Forest Service that seeks to increase fuel-reduction activities is promising, as is the recent California Wildfire and Forest Resilience Action Plan. But, they add, “there is no time to lose.”