Effect of Reduced Summer Cloud Shading on Evaporative Demand and Wildfire in Coastal Southern California

• States that in much of coastal southern California, the frequency of summer clouds has declined rapidly in recent decades due to warming from urbanization and greenhouse gases
• Finds that since the early 1970s, cloud observations from airfields in coastal southern California (CSCA) indicate reductions of ~25–50% in warm‐season frequency of daytime stratus clouds at many sites, including fire‐prone wildland‐urban interface (WUI) zones
• Estimates that reduced warm‐season cloud shading since the 1970s significantly enhanced daytime solar radiation and evaporative demand throughout much of CSCA, particularly in greater Los Angeles
• In other words, cloud reductions have significantly reduced cloud shading and increased evaporative demand, particularly in greater Los Angeles and northern San Diego, such that a relatively cloudy summer today is similar to a relatively clear summer in the 1970s
• Correlation with burned area and live fuel moisture implicates stratus cloud shading as an important driver of warm‐season wildfire activity in CSCA

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