Marine heatwaves and cold-spells in global coral reef zones

Persistent ocean temperature extremes, known as marine heatwaves (MHWs) and marine cold-spells (MCSs), can be devastating for warm-water coral reefs. However, the difference between MHWs and MCSs in the mean and trend across global coral reef zones in the present and future climates remains unclear. In this study, we comparatively assessed these changes using satellite sea surface temperatures during 1982-2021 and the Coupled Model Intercomparison Project Phase 6 climate model projections during 2022-2070. We found distinct differences in the mean and trend between MHW and MCS metrics across global coral reef zones, with a rapid increase in the total days and average duration of MHWs, whereas the MCSs exhibited an opposite trend. Moreover, the increasing trend in the average duration of MHWs and decreasing trend in MCS frequency both induced a nearly permanent MHW state in future projections. Upper-ocean warming was the main driver of the increase in MHWs and the decrease in MCSs. Under greenhouse warming, the area ratio between the MHW and MCS would rapidly increase from 1982 to 2070, reaching nearly 28 times under the Shared Socio-economic Pathway (SSP) 245 and 109 times under SSP585 by the year 2070. As El Nin similar to o-Southern Oscillation (ENSO) events are critical for the occurrence of MHWs and coral bleaching in tropical oceans, our results emphasize the necessity for understanding the projected changes in extreme ENSO events.