Suppression-induced fuel accumulation can amplify extreme fire seasons in Iberia

Abstract Recent extreme fire seasons in Europe highlight the need to understand what drives this escalation and how fuel accumulation, weather variability and suppression interact to shape it. We analyse May–October burned area across mainland Iberia during 2003–2025 and identify a bimodal distribution, with ordinary seasons separated from four extreme years exceeding 4,000~km$^2$. Seasonal totals in these extreme years are dominated by a small number of large, spatially clustered fire events, motivating an examination of how extreme seasons emerge when suppression capacity is exceeded. A conceptual stochastic model combining fuel accumulation, weather variability and suppression reproduces this bimodal structure, with suppression acting as a perturbation to the balance between fuel accumulation and fire-driven fuel removal. Storyline simulations indicate that strong suppression produces low-fire seasons but allows fuel to accumulate, setting the stage for occasional extreme seasons when containment capacity is exceeded. In a scenario with doubled fuel accumulation, potentially associated with landscape abandonment and greater fuel continuity, the simulated frequency of high-fire seasons increases from 12\% to 36\%, while the 90th percentile of seasonal burned area rises from 4,684 to 10,092~km$^2$. Stronger suppression can substantially reduces the frequency of high-fire seasons but has a smaller effect on their severity. These results indicate that fuel accumulation, reinforced by strong suppression practices, shapes extreme fire regimes. Fuel management and suppression must therefore be considered together to limit both the occurrence and severity of extreme fire seasons.