Ranking of extreme drought events in the Amazon Basin between 1980 and 2024

The Amazon Basin (AB), the largest hydrographic basin in the world, spans across seven countries in South America. It constitutes a highly intricate system, rich in natural resources, and is marked by substantial biological heterogeneity. The AB plays a pivotal role in the regulation of environmental processes, serving a key component of the global hydrological cycle and climate systems. Understanding the increasing frequency, intensity and spatial extent of extreme drought events in this region is vital for safeguarding the regional ecosystem. This study aims to classify extreme drought events in the AB using the Standardized Precipitation-Evapotranspiration Index (SPEI), derived from ERA5 reanalysis data, covering the period from 1980 to 2024. To assess both agricultural and hydrological droughts, this research incorporates the accumulation periods of 6 and 12 months (SPEI-6 and SPEI-12). The ranking methodology accounts for various SPEI time scales, the extent of the affected area, and the average SPEI intensity within that area. The results highlight that the 2023/24 drought episode was the most intense ever recorded in the AB, with over 90% (80%) of the region affected for the month of January for SPEI-6 (SPEI-12), surpassing known past mega-events, such as the 2005, 2010 and 2015/16 episodes. These extreme conditions were observed across all timespans. Specifically, for January 2024 under the SPEI-6 and for September 2024 under the SPEI-12, more than half of the AB was categorized as experiencing exceptional drought, as established by the 1st percentile of the SPEI distribution. Furthermore, the results underscore the persistence of consecutive periods of drought, especially since the beginning of 2020. With the climate projections indicating continued warming in the region, increased evapotranspiration and lower rates of rainfall are expected, potentially leading to even drier periods. This marks the significance of studies focused on understanding the development and impacts of droughts, as they play a critical role in the mitigation of future environmental risks.