Climatic and Hydrological Changes Drive Contemporary Fire Regime Transitions in the World's Largest Wetland.

Tropical wetlands have historically been resilient to fire, but ongoing climate and hydrological changes are undermining this stability. Here, we provide robust multi-decadal evidence (1985-2024) of structural changes in the contemporary fire regime of the Pantanal, the world's largest wetland, associated with hydroclimatic disruption. We investigated changes in burned area, mean fire size, and fire seasonality using hydrological, climatic, and fuel-condition predictors. To identify the major drivers of fire dynamics, we applied trend analyses and Elastic Net models across multiple temporal and spatial scales. Although no significant monotonic trend was detected for annual burned area across the entire time series, change point analysis identified 2014 as the onset of a significant upward trend. Mean fire size exhibited a significant long-term upward trend across the full period, which intensified after 2018, suggesting larger fire sizes in recent years. Across models, reduced precipitation, shorter seasonal wetland flood pulses, and higher atmospheric and fuel dryness emerged as the strongest predictors of burned area and fire size. These changes have expanded the duration of the fire season, particularly in the northern and central Pantanal, by delaying the onset of rains and shortening the flood pulse period. Our findings suggest a weakening of the hydroclimatic coupling that has regulated ignition and fuel availability in the Pantanal. Broader environmental changes in the upper basin and regional climate may be reinforcing this transition. These results highlight the Pantanal's increasing vulnerability to drought-driven fires and underscore the urgent need for integrated water and fire management strategies to preserve the resilience of the Pantanal and other tropical wetlands under intensifying global change pressures.