Coral Bleaching: The Equatorial-Refugia Hypothesis.

The rising threat of marine heatwaves has led to numerous predictions that coral reefs, especially those near the Equator, will be severely degraded by the end of the current century. Yet, environmental conditions near the Equator may regionally moderate coral bleaching by reducing thermal stress during marine heatwaves. We deployed a Bayesian spatio-temporal model over Earth to examine which environmental conditions may characterize marine-heatwave refugia for coral reefs by testing the relationship between the severity of coral bleaching and a suite of temperature, hydrodynamic, topographic, atmospheric, and biological variables. The model considered the severity of coral bleaching as the proportion of bleached hard corals during 30,266 coral-reef surveys conducted at 8728 sites, at depths of up to 20 m, and located between 35° north and south of the Equator across 81 countries, from 2002 to 2020. Except for the eastern Pacific Ocean, the severity of coral bleaching during marine heatwaves was lower on equatorial reefs than on higher-latitude reefs, suggesting that marine-heatwave refugia for corals have been concentrated in the equatorial Coral Triangle region. Indeed, equatorial reefs in the Coral Triangle were, on average, exposed to the weakest marine heatwaves, potentially because they were shielded from extreme insolation by frequent cloud coverage in the Intertropical Convergence Zone. Coral bleaching may also be moderated during marine heatwaves on reefs that experience high wave energy, high current velocity, high cloud frequency, or turbidity. Coral bleaching was also less severe on reefs that historically endured frequent heatwaves than on reefs that were naïve to thermal stress. Based on modern and historical responses of coral reefs to acute thermal stress, we hypothesize that many equatorial reefs will continue to serve as marine-heatwave refugia for corals.