Process-Based Attribution of the 2025 Iberian Wildfire Season Using a Storyline Framework

An enduring heatwave over the Iberian Peninsula in July and August 2025 led to exceptionally extensive wildfires, resulting in the fifth-largest burned area in Spain since 2001 and the fourth-largest in Portugal. Hot and dry fire weather conditions are a key driver of large wildfires in the Mediterranean, and are intensifying rapidly under anthropogenic climate change. However, strong interannual variability of burned area and changes in multiple non-climatic drivers (e.g., land management) complicate the attribution of individual fire seasons.

Methods for attributing climate impacts to anthropogenic forcing are commonly divided into statistical and storyline approaches. Statistical methods quantify changes in the probability of exceeding predefined thresholds across climate states with different forcing levels, whereas storyline approaches examine how a specific historical event might have unfolded in the absence of anthropogenic climate change. Such counterfactual storylines can be generated with Earth system models (ESMs) constrained by observed historical conditions, enabling a process-based interpretation of climate impacts. However, this type of storyline method has not been applied to the attribution of complex ecosystem processes such as fires.

Here, we use the 2025 Iberian wildfire season as a case study to evaluate our nudged circulation storyline simulation with the Community Earth System Model 2 (CESM2) and compare it to statistical attribution. The ESM-based storyline enables a process-based quantification of thermodynamic influences on fire weather and of biological factors controlling fuel load. However, the approaches differ on the role of thermodynamic climate change in intensifying the 2025 fire season. Statistical attribution suggests a large thermodynamic contribution but indicates that events of comparable intensity are not exceptional under present-day climate. In contrast, the storyline approach identifies the 2025 circulation anomaly as unprecedented in magnitude. We show that this discrepancy arises from decadal Mediterranean circulation trends, which are implicitly absorbed into the thermodynamic response in the statistical attribution framework.

Our results demonstrate the utility of a storyline framework in the causal attribution of complex processes such as fires, and highlight the need for caution when applying attribution methods in regions characterized by strong dynamical trends.