Millennial-scale fire and climate dynamics in the world's largest tropical wetland show emerging fire threat to flooded ecosystems

The Pantanal, the world's largest tropical wetland and a globally significant centre of biodiversity, has been increasingly threatened by fire. The extreme fire season of 2019–20, which caused enormous environmental damage, was linked to drought. However, predicting the future of fire in this region is challenging because of complex interactions among topography, seasonal flooding, and diverse vegetation. Here, we investigate climate-driven changes to fire regimes across five distinctive vegetation types by integrating millennial-scale histories of fire and climate with high-resolution monitoring and remote sensing data from the last two decades. We show that the impacts of climate change on fire activity varies depending on vegetation. In savannahs, fire occurrence is highly correlated to biomass availability, while seasonally dry tropical forest burns only under extreme drought conditions. The seasonally flooded vegetation mosaic is observed to be highly flammable under low rainfall and protracted dry seasons. This study highlights that new fire threats to low-lying flooded vegetation are emerging as human impact and climate change reshape fire regimes in the Pantanal. • Pantanal fire is driven by complex interactions of vegetation and flooding. • Historical datasets show variations in flooding and rainfall over 3500 years. • Spatial analysis of fire over 20 years shows burning in flooded ecosystems. • Palaeodata show climate-driven burning in ecosystems classified as fire-sensitive.