Alexandra D. Syphard Jon Keeley 2025 <div class="title">Background</div><p>Understanding the relative contribution of climate and human factors to wildfires is critical for managing risk across California’s diverse ecosystems, in the United States (US).</p><div class="title">Aims</div><p>We propose a model that distinguishes between proximate and ultimate drivers of fire regimes and apply it to a century of fire and climate data to assess regional variation in causal mechanisms.</p><div class="title">Methods</div><p>We analyzed fire statistics (1910–2021) alongside climate and weather data, stratifying the state by 10 ecoregions.</p><div class="title">Key results</div><p>Northern forests had the strongest correlation with the proximate factor fuel aridity, ultimately due to climate. Fire rotation intervals exceeded 100&nbsp;years, implicating woody fuel accumulation as an additional factor. Lightning ignitions occurred in decadal bursts, with dense strike events potentially overwhelming fire-fighting resources. Lower elevation/latitude foothill ecoregions experienced highest fire activity following wet winters and springs, implicating control by herbaceous fuel loads and a negative effect of global warming on future fires. Human ignitions dominate in these ecoregions, and population growth contributes to expansion of powerlines, a major ignition source.</p><div class="title">Conclusions</div><p>While climate change may increase fire activity in forested ecoregions, its role is less pronounced in non-forested ecoregions, where human ignition sources are the dominant factor.</p><div class="title">Implications</div><p>Different areas within ecoregions may require different management actions that reflect the specific proximate and ultimate factors at play.</p> application/pdf 10.1071/WF25166 en CSIRO Causal analysis of fire regime drivers in California article