Research Spotlight: Ignitions and Wind Speed are Strongest Drivers of Area Burned in Santa Ana Wind Fires

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Over the past decade, California wildfires have continued to break records for area burned, with nine out of the state’s largest wildfires on record occurring since 2010. Climate change and more frequent human-caused ignitions have been suggested as reasons for the increase in area burned in California. However, these fires vary widely in ignition type, location, and fire behavior, with some fires driven largely by high fuel loads and others by high winds, and the causes of increases in area burned may consequently also vary. In Southern California, the most destructive fires are driven by Santa Ana winds, extreme seasonal winds that occur in the autumn and winter. The goal of the new study was to disentangle the potential drivers of area burned specifically for Southern California wildfires burning under Santa Ana wind conditions.

Using historical weather data, the researchers identified Santa Ana events from Southern California between 1948 and 2018. Then, using climate and fire records, they determined how well three types of variables explained the total area burned during these events: 1) weather associated with the Santa Ana wind event, 2) climate during the months and years prior to the event, and 3) the number of ignitions during the Santa Ana wind event.

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The analysis found that most climate and weather-related variables were not strong or consistent drivers of area burned over the 71-year time period studied. Higher temperatures during or in the months prior to Santa Ana Wind events were generally not associated with area burned, and precipitation was a significant factor only during some months and time periods. In contrast, wind speeds and number of ignitions were strongly and consistently associated with area burned.

The data highlight the fact that regardless of extreme winds and weather conditions, wildfires cannot occur without an ignition. Of the 643 Santa Ana wind events examined in the study, more than 75% had no fires, and even during extreme winds there was a greater than 50% chance of no burning. During Santa Ana wind events, 100% of the fires were caused by humans, either intentionally or accidentally. The type of ignitions changed over the study period: from 1948 to 1983 campfires were the leading cause, while from 1984 to 2018 arson and powerline failures dominated. In the last two decades, powerline failures have been the dominant cause of Santa Ana wind-driven fires in Southern California.

The results underscore the importance of managing ignitions during Santa Ana wind events. The study will inform land management planning for wildfire and decision making related to powerline infrastructure.

Management Implications

• Number of ignitions is a major factor in determining how much area burns during Santa Ana wind events, since more ignitions increase the odds that an ignition will escape initial attack during a wind event
• Powerline failures have been the dominant cause of ignitions during Santa Ana wind events over the last two decades
• Temperature and precipitation-related factors, including fuel-moisture, are not consistently associated with area burned in Santa Ana-wind driven fires, and only weakly associated with area burned in December
• October is the peak month for Santa Ana wind fires
• The Santa Ana wind fire season no longer includes September as an important month but there is now greater importance in December and January

This Brief Refers To:

J. E. Keeley, J. Guzman-Morales, A. Gershunov, A. D. Syphard, D. Cayan, D. W. Pierce, M. Flannigan, T. J. Brown, Ignitions explain more than temperature or precipitation in driving Santa Ana wind fires. Sci. Adv. 7, eabh2262 (2021).

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