As the planet warms, the frequency and severity of marine heatwaves (MHWs) are increasing, accelerating the degradation of coral reefs through mass bleaching events. Most bleaching prediction systems consist of a single global threshold based on progressive warming, but the characteristics of the extreme temperature events that impact corals may differ geographically. This thesis investigates the use of MHWs as a new approach for detecting coral bleaching. First, the Red Sea is used as a model system, as its corals are exceptionally heat-tolerant, yet bleaching events have increased in frequency. A specific MHW-based coral bleaching threshold is identified for the Red Sea, and we reveal that those conditions extended farther and occurred more often than bleaching events were reported. Furthermore, an emergent pattern of MHWs in the northern Red Sea was observed, until now thought to be a thermal refuge for corals. MHW-based coral bleaching thresholds are then identified for thirty-one Ecoregions in the global tropical oceans, using the largest coral bleaching database available. Marked variability was observed in how coral bleaching-associated MHWs manifest between Ecoregions, but globally MHWs have increased in spatial extent and duration over most coral reef zones. Furthermore, MHWs reach greater depths, jeopardising deep cooler waters that offer thermal refugia. These MHW-based bleaching thresholds are then applied to Coupled Model Project Intercomparison Project Phase 6 model outputs to project how global warming will affect coral bleaching over the next century. Even under the low emissions scenario, no bleaching-associated MHW properties are projected to return to historical levels within this century. Over 90% of coral zones are estimated to become impacted yearly by a MHW from 2040 under all emission scenarios, and heat stress is projected to increase up to 20-fold by the end of the century under the high emissions scenario. Finally, this thesis investigates the combined impacts of MHWs and biogeochemical extremes on Red Sea coral bleaching severity. We find that MHWs present a North to South gradient in their contribution to coral bleaching, yet nutrient limitation, acidification, deoxygenation and low salinity play clear roles in modulating bleaching severity throughout the Red Sea.
Lily holds a BSc in Biology from Imperial College London, and an MRes in Applied Marine Science from Plymouth University. She is now a PhD candidate in KAUST, working on investigating the impacts of extreme environmental conditions on coral reefs, using remotely-sensed data and model simulations.