Hydroclimatic intensification as a compound driver of extreme wildfires

The catastrophic January 2025 Los Angeles fires highlight the growing importance of compound hazards arising from hydroclimatic intensification under climate change. This study investigates how sequences of anomalously wet and dry conditions interact to create high-impact wildfire events, framing extreme fires as outcomes of cascading and compounding hydroclimatic processes rather than isolated meteorological extremes.

We identify a recurrent hydroclimatic rebound mechanism, in which prolonged moist anomalies (lasting 6–27 months) promote vegetation growth and fuel accumulation, followed by a rapid drying in the months preceding ignition. This drying phase, characterised by negative Standardized Precipitation Evapotranspiration Index anomalies, increasing vapour pressure deficit, and declining soil and fuel moisture, transforms accumulated biomass into highly flammable fuel. The combination of these sequential processes constitutes a compound hazard, where antecedent wetness and subsequent drought jointly amplify wildfire risk.

This mechanism is particularly pronounced in fuel-limited Mediterranean and desert Californian biomes, including events such as the Palisades and Eaton fires, where hydroclimatic rebound emerges as a key driver of extreme fire behaviour. In contrast, forested mountain regions exhibit a different compound structure, with fire activity primarily governed by prolonged drought and short-term fire weather conditions, and weaker influence from antecedent moistening.

Our findings demonstrate that hydroclimatic intensification operates through long-memory processes that propagate across the atmosphere–soil–vegetation continuum over timescales of up to two years. These results extend existing drought propagation frameworks by explicitly linking them to fire danger and highlighting biome-dependent pathways of risk amplification.

Incorporating these compound and lagged hydroclimatic signals into fire prediction systems significantly improves the representation of fuel conditions and enhances the predictability of extreme wildfire events. This underscores the need to move beyond traditional fire weather indices and adopt a compound hazard perspective that integrates both short-term meteorological drivers and long-term hydroclimatic variability. Such an approach is essential for anticipating and managing increasingly extreme wildfire regimes in a warming climate.