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The Impact of Climate Change on California’s Wildfires

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Wildfires have recently become an increasingly concerning environmental issue in California as the summers become hotter and the fires burn longer, leading to lasting consequences on the environment and ecosystem.

A recent study published in the Science Advances journal by UCI’s Department of Earth System Science reveals that fires in the Sierra Nevada ecoregion, the geographic and ecological area in eastern California, have “increased considerably” over the past few decades.

Led by project specialist Aurora Gutierrez and co–author Dr. James Randerson, the study found that the reason for this spike in fires is the shift of vegetation composition and residential development — a result of 20th century fire suppression policies. Such changes led to “a new fire regime of infrequent, high-intensity fires” that arose during the mid-to-late 1900s. Combined with population growth and housing development that changes the pattern of land use, there has been an increase of “denser, more flammable vegetation with a more connected landscape for fire spread.”

Higher temperatures due to climate change — namely in hotter, drier summers — extend the duration of fire season, according to Gutierrez and Randerson. This in turn leads to heightened fire risk by drying fuels, increasing their flammability and spreading patterns. 

Current climate change patterns further modify the structure and function of the Sierra Nevada’s ecosystem, and the evolution of the ecoregion in response to global warming is unclear.

Gutierrez and Randerson accurately predicted an upward trend in fire frequency by examining its relationship with temperature: the “likelihood of fire occurrence increases nonlinearly with daily temperature … a 1°C increase [yielding] a 19[%] to 22% increase in [wildfire] risk.”  

Additionally, the study stated that “current estimates of past and future fire occurrence relationships may underestimate climate-driven changes in ignition and initial fire spread.” This suggests that focusing on efforts to mitigate extreme temperatures may have value in fire prevention, even outside of red-flag conditions.  

The effect of daily temperature on fire occurrence is closely correlated to the likelihood of the fire escaping human control, according to the study. Activities that may pose a limited fire risk under normal conditions can greatly exacerbate fire spread during hotter periods when vegetation is dry. 

Gutierrez and Randerson found that fires that ignited during these heat waves, even those lasting only a few days, may grow into larger wildland wildfires that are difficult to contain, despite temperature conditions returning to normal. Higher initial rate of fighter spread was found to increase the resilience of these disasters to subsequent periods of cooler, more humid conditions. 

In addition to the environmental impact, the study reported that there are numerous effects of these heat waves on humans since “increases in summer heat waves … will contribute to more hospital visits and higher mortality.” 

Although the temperature increase as of now is gradual, the resulting fires and their increased frequency reduce the quality of both air and life in the western U.S.

Lauren Le is a STEM Apprentice for the winter 2022 quarter. She can be reached at