Compound dry hot fire extreme events in the Pantanal

Abstract

In recent years, the Pantanal biome, the largest continuous wetland in the world, located within the Upper Paraguay River Basin (UPRB), has been severely affected by vegetation fires associated with the intensification of hydroclimatic extremes. The combination of severe droughts, heatwaves, and anthropogenic pressures culminated in 2020 in the most extreme fire event ever recorded in the region, when more than 30% of the biome was consumed by fire, generating substantial socio-economic and environmental impacts. More recently, in June 2024, at the onset of the fire season, the Pantanal registered a record burned area of approximately 440,000 hectares, about 50 times the historical average for the month. This episode reflected exceptional climatic conditions, with an estimated return period of 35 years, within a broader context in which 2024 became the warmest year ever recorded both globally and in Brazil. In the Pantanal, these anomalies manifested as extreme drought, persistent heatwaves, and meteorological conditions highly conducive to fire spread, resulting in an exceptionally prolonged fire season from June to October. Over recent decades, the Pantanal has experienced an increase in the frequency and intensity of hydroclimatic extremes, driven by interactions among regional climate change, alterations in the hydrological regime, and anthropogenic pressures. These processes have modulated fire dynamics, favoring the occurrence of large-scale fires. Within this context, this thesis adopts an integrated approach to investigate how hydroclimatic extremes, particularly severe droughts, heatwaves, and deficits in moisture transport, modulate the occurrence, severity, and expansion of fires in the Pantanal, while also advancing the development of methods for burned-area monitoring and mapping. The first research axis investigates the atmospheric mechanisms and the occurrence of compound dry–hot events that culminated in the extreme fires of 2024, the warmest year ever recorded globally. Through the integration of satellite data, climate indicators, and synoptic analyses, 2024 is identified as the most severe drought within the 1980–2024 period, characterized by exceptional precipitation and soil-moisture deficits, the absence of the flood pulse, and the occurrence of persistent heatwaves. These factors acted simultaneously and synergistically, generating highly fire-prone conditions and elevating fire-danger indices. The second research axis deepens understanding of large-scale atmospheric processes that modulate critical droughts in the biome by analysing moisture transport and its anomalies during years of extreme drought associated with high fire activity. The application of the Lagrangian model FLEXPART allowed the identification of changes in the relative contributions of terrestrial and oceanic moisture sources, revealing that persistent deficits in moisture transport play a crucial role in the onset, duration, and severity of droughts. These patterns help explain the interannual variability of fire activity and reinforce the influence of remote atmospheric processes on regional hydroclimatic dynamics. The third research axis develops and validates a harmonized burned-area product for the Pantanal covering the 2014– 2024 period, based on the integration of the Landsat-8/9 and Sentinel-2 constellations. The method incorporates automatic sampling guided by VIIRS detections and morphological filtering to generate consistent training samples, resulting in classification accuracy exceeding 99.6% and performance metrics above 0.99. The resulting annual and monthly maps document the spatial and temporal patterns of fire activity across the biome, providing a robust foundation for ecological, climatic, and environmental management studies. By integrating physical diagnostics, atmospheric-process analyses, and methodological advances in remote sensing, this thesis contributes to a comprehensive understanding of the relationship between hydroclimatic extremes and fire in the Pantanal. The results underscore the importance of incorporating the compound nature of these events into mitigation and adaptation policies, while also providing an analytical framework and a set of tools to improve operational monitoring and integrated fire management under a scenario of accelerated climate change