Record marine heatwaves in 2024 drove mass coral bleaching across both subtropical and temperate coastal waters of Japan. This severe event challenges the long-standing assumption that cooler temperate seas serve as safe refuges for coral communities escaping tropical warming. Corals bleached as ocean waters reached four-decade heat highs and accumulated extreme thermal stress across a wide latitudinal gradient.
Surging ocean temperatures caused thermal stress to accumulate in shallow coastal waters for weeks. Much like a continuous fever overwhelming a living body, the prolonged heat pushed coral colonies past their physical limits. Expanding Acropora coral populations that recently moved northward suffered near-complete bleaching under the high heat. Bleaching rates still varied by species and location, leaving one community at Tateyama unbleached despite high heat measurements.
Researchers tracked thermal stress and coral bleaching responses across a latitudinal gradient spanning twenty-four to thirty-five degrees north. Heat stress exceeded eight degree heating weeks at every site and reached seventeen to nineteen degree heating weeks at high latitudes. The extreme heat caused fifty-five to ninety-five percent of corals to bleach at most temperate sites, including the first documented event at Tsushima.
The researchers report that the pace of ocean warming can outstrip the capacity of corals to adapt or shift their geographic distributions. The data also show that degree heating weeks alone cannot fully predict bleaching in temperate areas where local thermal thresholds differ.