Coral Bleaching: A Global Analysis of Bright Spots
Sully, Shannon Elizabeth
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Thermal-stress events associated with climate change cause coral bleaching and mortality that threatens coral reefs globally. Yet coral bleaching and coral cover patterns vary spatially and temporally, and few studies have identified bright spots where corals have the potential to survive through climate change. This study synthesizes over 10,000 field observations globally of coral bleaching and coral cover from 1997to 2017 and uses a suite of environmental and temperature metrics to analyze coral bleaching and coral cover patterns. The objectives of this dissertation are to(1) identify the environmental variables that constitute bright spots for coral reefs, and (2) identify where these bright spots and potential refugia are around the world. The study found that coral bleaching was most common in localities experiencing high intensity and high frequency thermal-stress anomalies. Coral bleaching was significantly less common in localities with a high variance in sea surface temperature (SST) anomalies, and in areas with turbidity levels between 0.080 and 0.127 Kd490. Geographically, the highest probability of coral bleaching occurred at tropical mid-latitude sites (15–20 ̊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 corals that survived have adapted, such that the remaining coral populations now have a higher thermal threshold for bleaching. Coral cover was greatest in areas with a high maximum historical SST. Coral cover was lowest at: (i) high latitudes, (ii) where cyclone frequencies and human populations are high, (iii) sites with a high mean SST, and (iv) sites that have experienced recent thermal stress (i.e., high degree-heating weeks). The study also projected coral cover into the years 2050 and 2100 using the Bayesian models developed on modern reefs and projected temperatures from global climate models. Coral cover was projected to decrease less on reefs with high turbidity than on reefs with low turbidity under both Representative Concentration Pathways (RCP) 4.5 and RCP8.5.We project future dark spots for coral reefs in eastern Africa and northwestern Madagascar, the Marshall Islands, Micronesia, Northern Mariana Islands, west Sumatra, and the northern Caribbean. We project future bright spots for coral reefs in New Caledonia, Papua New Guinea, the Sulu Sea, Northern Sulawesi and eastern Indonesia, the Greater Sunda Islands, Southern Red Sea, Lakshadweep Islands, and French Polynesia.