Satellite-based analysis of an unverified mass coral bleaching event on the Great Barrier Reef in 2021

Abstract The Great Barrier Reef (GBR), Australia, has experienced more frequent heat stress events since the region’s first recorded mass coral bleaching event in 1998. Seven confirmed mass coral bleaching events have occurred on the GBR, including five events since 2016. Due to the large scale of the GBR, comprehensive visual bleaching surveys are challenging, particularly as heat stress events accumulate more frequently in remote regions. One such event lacking extensive bleaching surveys occurred on the northern GBR in 2021, suggesting a need for alternative monitoring options. Satellite-based heat stress monitoring products recorded significant bleaching-level heat stress (4 °C-weeks) on 67% of the northern GBR in 2021, comparable in extent to previously confirmed events. Here, we demonstrate a strong correlation between satellite-based heat stress metrics and mass bleaching on the GBR, and indicate that the GBR has likely experienced at least eight mass bleaching events from 1998 to 2024, three of which were consecutive events affecting the northern GBR from 2020 to 2022. In-water surveys and time-series observations of individual corals at Lizard Island confirm partial and fully bleached corals (> 50% coral cover bleached) were common in shallow and deep habitats in 2021. From 1998 to 2015, there were two widespread bleaching events; this increased to one event every 1.5 years from 2016 to 2024, indicating that the GBR is approaching an annual bleaching scenario. As heat stress events become more common, bleaching thresholds from satellite-based monitoring products remain accurate and should continue to be used to predict and understand the potential impacts from thermal stress to reef ecosystems.