California Wildfires and Climate Change

The rise in California’s wildfire activity in recent years has many asking, was that caused by climate change?

This is the question a group of researchers led by Park Williams sought to answer in a study published last summer in the journal Earth’s Future. In a word, yes, the study’s authors, who include CIRC researcher and wildfire expert John Abatzoglou, found compelling evidence that the increase in temperatures associated with human-caused climate change have indeed been a key contributor to the large increase in California’s wildfires.

More important still, the researchers were able to determine that the extent of the state’s recent fires increased exponentially with rising temperatures, meaning that each incremental rise in the mercury has tended to lead to larger and larger areas burned. As the authors point out, this exponential relationship between temperature and burned area is useful not only for understanding the state’s recent wildfire activity, it also points to a conclusion that many other studies have come to: California’s wildfire activity is very likely to increase as climate change continues.

Williams and colleagues came to their conclusion by carefully tracking wildfire activity in California from 1972 to 2018. During this 46-year period, our neighbors to the south experienced nearly 40,000 wildfires, with the annual area burned increasing five-fold. Of particular interest are the last two years of the study period, which share the dubious distinction of being the two most destructive wildfire years in California history.

The year 2017 saw both the Thomas Fire—at the time the largest individual wildfire in the state’s history, burning 114,078 hectares (440 square miles)—and the Tubbs Fire—which burned the largest number of structures, 5,636 to be exact, in the state’s history. The Tubbs Fire also led to the loss of 22 lives. At least 46 people lost their lives during the 2017 fire season. Just one year later, these records were broken.

The 2018 Mendocino Complex Fire reset the state’s record for area burned, burning 185,800 ha (717 sq. miles), making it the largest individual wildfire in the state’s history as of this writing. The year 2018 also saw the Camp Fire, best known for devastating the town of Paradise, killing at least 87 people, and destroying 18,804 structures. All told, the two years resulted in roughly 1.2 million ha (nearly 4,500 sq. miles) of area burned, burdening the State of California with $1.5 billion in fire suppression costs.

Williams and colleagues began their study by first recognizing the other factors that might have led to California’s increased wildfire activity. For starters, California has a long history of fire suppression, which has led to a buildup of fuels, especially in the state’s northern forests.

(The need to reduce fuel loads is further explored in our post “Western US Needs More Controlled Burns.”)

California, the authors note, is also ecologically diverse. The region is known as much for the oak savannas that make up its south and central coasts, as it is for the conifer forests that make up its northern and Sierra Nevada regions. California is also heavily populated. This means that while climate change might have provided much of the warm and dry conditions that have made wildfires more likely in the state, humans—not natural sources, such as lightning—are the major reason fires have ignited across the state in recent years, including many of the devastating fires in 2017 and 2018.

To account for California’s ecological diversity, Williams and colleagues divided the state into four regions: North Coast, Sierra Nevada, Central Coast, and South Coast. The state’s agricultural Central Valley as well as its desert regions were not considered in the study.

The largest increase in the number of California wildfires from 1972 to 2018 occurred along the North Coast and in the Sierra Nevada regions, which experienced a 630% and 618% increase in area burned respectively, according to the study. This is perhaps not surprising since the two regions contain most of the state’s forests. Taken as a whole, burned area in California’s forested regions saw an increase of 766% from 1972 to 2018, according to the study. By contrast, the less forested Central and South Coasts—and their much smaller fuel loads—did not see a significant change in area burned over the study period.

To track the role that warming temperatures played in creating the right conditions for California’s wildfires to ignite, the authors used a variable called vapor pressure deficit.

Vapor pressure deficit (VPD) is essentially a way to determine how dry the air is. In a nutshell, VPD represents how large of a difference there is between the moisture content of the air and how much moisture the air can hold when saturated. The higher the deficit, the drier the air. As you might imagine, VPD is closely connected to temperature. Here’s where the study gets especially interesting and where climate change’s impact comes into focus.

(For more on VPD and how it differs from relative humidity, see our post from last August “This Summer—A Welcome Reprieve.”)

To understand the relationship between VPD and fires, the authors looked at seasonal changes in VPD. The largest number of fires in California burned during the summer months, according to the study. This is perhaps not surprising. California, like much of the West Coast, tends to get most of its precipitation during the cool winter months, while its summers tend to be warm and dry.

Due in large part to this seasonal variability, temperature increases during the state’s already dry summers tend to lead to very dry air that shows up as significant VPD gains. This is essentially what Williams and colleagues found. California’s summer months—here defined as March to October—saw the largest percent gains in VPD.

In fact, if we zoom out a bit and examine a much larger period of time, we see a very clear relationship between VPD and temperature. From 1896 to 2018, maximum daily temperatures across the entire state of California during the summer months increased by 1.8 degrees Celsius (3 degrees Fahrenheit) with a corresponding 13% increase in VPD, according to the study. If we examine VPD during the very destructive summers of 2017 and 2018 we see that VPD numbers were sixth and ninth highest since records began in 1896.

Using a regression analysis, the researchers determined that the vast majority of the five-fold increase in area burned from 1972 to 2018 was connected to VPD percent gains over the same period. In other words, the temperature gains led to a drying out of the air that dried out vegetation, turning them into fuel that only needed the right ignition source and seasonal weather patterns to go up in flames. And because we know from previous studies that the rise in temperatures is connected to human-caused climate change, we can attribute the rise in wildfire activity to climate change. As Williams and colleagues put it: “The large increase in California’s annual forest‐fire area over the past several decades is very likely linked to anthropogenic warming.”

However, this analysis of the past is not the study’s most concerning take-home message. As hinted above, Williams and colleagues found the that relationship between VPD and fire activity was exponential, meaning each incremental increase in VPD led to larger and larger burn areas.

Here’s where the past can tell us something about the future.

As Williams and colleagues note, the current rise in fire activity in California has been driven by relatively modest temperature gains that nonetheless led to an increase in VPD—a 10 % increase since 1800, according to the study. And there’s the rub, VPD numbers statewide are projected to double by 2060, according to the study. In other words, California is in for some very destructive fire seasons in the decades ahead, especially, as the authors note, in the state’s forested North Coast and Sierra Nevada regions.


OSU_icon_key_01Key Findings:

  • Human-caused climate warming is very likely responsible for the large increase in California’s wildfires.
  • The relationship between the dryness of the air as measured as vapor pressure deficit (VPD) and fire activity is exponential, meaning each increase in VPD led to larger burn areas.

OSU_icon_pencil_Black RotatedCitation:Williams, A. Park, John T. Abatzoglou, Alexander Gershunov, Janin Guzman‐Morales, Daniel A. Bishop, Jennifer K. Balch, and Dennis P. Lettenmaier. “Observed impacts of anthropogenic climate change on wildfire in California.” Earth’s Future(2019).


Featured Image: “California Wildfires,”August 26, 2013. (Photo Credit: US Air Force, U.S. Government Works.)


Nathan Gilles is the managing editor of The Climate Circulator, and oversees CIRC’s social media accounts and website. When he’s not writing for CIRC, Nathan works as a freelance science writerOther posts by this Author. 

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