A global analysis of coral bleaching over the past two decades

TLDR

Climate‑driven thermal stress causes coral bleaching and mortality worldwide, yet bleaching patterns vary across space and time. The study synthesizes bleaching observations from 3,351 sites in 81 countries (1998‑2017) to analyze patterns using environmental covariates and temperature metrics. The authors applied these covariates and temperature metrics to assess bleaching patterns across the compiled dataset. Bleaching was most common in areas with high intensity and frequency of thermal‑stress anomalies, less common where sea‑surface temperature variance was high, peaked at tropical mid‑latitudes (15–20° N/S), and the onset temperature has risen by ~0.5 °C in the last decade, suggesting shifts in coral thermal thresholds.

Abstract

Thermal-stress events associated with climate change cause coral bleaching and mortality that threatens coral reefs globally. Yet coral bleaching patterns vary spatially and temporally. Here we synthesize field observations of coral bleaching at 3351 sites in 81 countries from 1998 to 2017 and use a suite of environmental covariates and temperature metrics to analyze bleaching patterns. Coral bleaching was most common in localities experiencing high intensity and high frequency thermal-stress anomalies. However, coral bleaching was significantly less common in localities with a high variance in sea-surface temperature (SST) anomalies. Geographically, the highest probability of coral bleaching occurred at tropical mid-latitude sites (15-20 degrees north and south of the Equator), despite similar thermal stress levels at equatorial sites. In the last decade, the onset of coral bleaching has occurred at significantly higher SSTs (∼0.5 °C) than in the previous decade, suggesting that thermally susceptible genotypes may have declined and/or adapted such that the remaining coral populations now have a higher thermal threshold for bleaching.