Widespread coral bleaching across subtropical and temperate Japan under record marine heat stress

The frequency and intensity of mass coral bleaching events are increasing due to rising seawater temperatures associated with climate change and the intensification of marine heatwaves. Between 2023 and 2024, extreme and prolonged sea surface temperature (SST) anomalies occurred globally, resulting in the hottest summer on record and the most severe coral bleaching event observed to date. Here, we evaluated SST and coral bleaching prevalence at two subtropic reef sites (Ishigaki and Okinawa Island), and six non-reef temperate sites extending to 35 °N along the Japanese coast influenced by the Kuroshio and Tsushima warm Currents. The Degree Heating Week (DHW) index reached record-high values at all sites, with a maximum of 19.10, and the maximum monthly mean (MMM) temperature was the highest in the past 43 years at all sites except Ishigaki. In addition to mass bleaching at subtropical sites, we report, for the first time, widespread mass coral bleaching at temperate sites, including the recent poleward shifting scleractinian corals. Extensive bleaching followed by coral mortality occurred in shallow subtropical reefs, whereas most corals at temperate sites recovered after bleaching. Although temperate regions have been proposed as potential coral refugia under climate change, the widespread bleaching observed in 2024 suggests that the pace of climate change may now exceeded the capacity of corals to shift their distributions, rendering their long-term stability increasing uncertain.