Towards a new climate regime: heatwaves proliferate and reshape seasonality in the world's largest tropical wetland

The Pantanal, the world's largest continuous floodplain, exhibits high sensitivity to thermal and hydrological variations, making it particularly vulnerable to the combined effects of climate change and anthropogenic pressures. The simultaneous occurrence of droughts and heatwaves intensifies vegetation flammability and has been a determining factor in the amplification of fires. Given this scenario, this study aimed to quantify and characterize future changes in the frequency, duration, and seasonality of heatwaves in the Pantanal for the period 2030–2060, analyzing projections under the SSP1-1.9, SSP2-4.5, and SSP5-8.5 emission scenarios, compared to the climatological normal of 1991–2020. For this purpose, four CMIP6 models were validated against the ERA5 reanalysis, with the MRI-ESM2-0 model selected for its superior performance in representing the daily maximum temperature in the biome area. The results show a significant increase in the occurrence of these events in all scenarios, with an average increase of + 126% to + 181% in the number of days above the extreme heat threshold. Heatwaves become more prolonged and concentrated in spring and autumn, indicating a restructuring of the biome's thermal seasonality. These findings suggest that the Pantanal is moving towards a new climate regime, characterized by intensified thermal stress and the expansion of the “hot and dry season,” which will have direct implications for biodiversity and fire management.