The Thermal Stress History of South Atlantic Reefs Reveals Increasing Intensity, Duration, Frequency, and Likely Undocumented Bleaching Episodes.

The primary consequence of global warming for reefs is coral bleaching, often leading to extensive coral mortality. Although bleaching is well-documented globally, the thermal stress and bleaching experienced by the unique South Atlantic reefs remain largely unknown due to insufficient monitoring on both spatial and temporal scales. Therefore, this work aimed to reconstruct past thermal stress episodes across South Atlantic reefs, and assessed whether episodes are becoming more intense, longer-lasting, and more frequent. We retrieved daily 5 km-resolution Degree Heating Week (DHW) data from the U.S. National Oceanic and Atmospheric Administration Coral Reef Watch server for 33 reef sites spanning the last 40 years. For each thermal stress episode, we assessed the intensity (maximum DHW value), duration (number of continuous days under stress), and frequency (number of days between episodes). Generalized linear models were fitted to intensity, duration, and frequency data to evaluate the influence of latitude and the time x region interaction as predictors. We recorded multiple thermal stress episodes, increasing from 2010 onwards, ranging from 10 episodes between 1985-89 and 75 between 2020-24. Intensity and duration increased over time across the entire South Atlantic. Frequency also increased across the Southwestern Atlantic coast and oceanic islands, but not for Africa. Episodes at higher latitudes were more intense, prolonged, and frequent. The validity of the thermal stress history reconstruction was groundtruthed using information from the Abrolhos Bank, the only consistently monitored reef site in the South Atlantic-DHW data accurately matched the observed bleaching episodes at this site. With this, our dataset shows that multiple bleaching episodes likely occurred in the South Atlantic, but went undocumented in the field. Therefore, the information currently available for the South Atlantic likely underestimates the extent of bleaching occurring in the area, which is experiencing increases in intensity, duration, and frequency of thermal stress.