Amazon dry season will lengthen under future climate

The Amazon rainforest is a key component of the Earth system, regulating regional climate, sustaining high biodiversity and carbon stocks. However, it is threatened by climate and land‐use changes. In recent decades, the region has experienced intensified droughts and heatwaves, trends expected to worsen under future warming. In this study, we assess how the timing, length, and spatial distribution of the dry season across the Amazon is projected to change this century under different Shared Socioeconomic Pathways. Using a multi‐model ensemble of the Coupled Model Intercomparison Project (CMIP6) weighted according to their RMSE performance, we estimate monthly water balance based on precipitation and evapotranspiration from 2000 to 2100. We calculated dry season onset, end, and length from accumulated water deficits, providing a spatially explicit characterization of seasonal dynamics. We find a significant lengthening of the dry season in 35% of the basin under low‐emission scenarios (SSP1‐2.6) and up to 56% under high‐emission scenarios (SSP5‐8.5), with increases of up to 2 months in the southern and eastern Amazon by 2100. Such changes pose major risks for forest degradation, regional water availability, and climate feedback, potentially reducing the biome's role as a carbon sink. With such large changes even under low emission, our findings demonstrate the urgent need for spatially targeted climate adaptation strategies and mitigation policies that consider future changes in seasonal water availability. While such strategies are essential to enhance societal resilience, safeguarding forest ecosystems and preserving Amazonian hydrological functions ultimately depend on ambitious efforts to reduce greenhouse gas emissions. These actions are essential not only for local and regional sustainability but also for global climate stability.