A Multifactor Risk Analysis of the Record 2016 Great Barrier Reef Bleaching
Introduction. The 2016 global coral bleaching event was severe: 93% of the northern, 700km stretch, of the Australian Great Barrier Reef (GBR) coral was bleached and by June, >60% of this coral was killed in association with heat stress (Hughes et al. 2017; Fig. ES28.1). Bleaching occurred during an extended period of unprecedented global-average heat (Mann et al. 2016). Coral heat stress is accumulated through the magnitude and duration of temperatures above a threshold. A short-duration, high-magnitude event, or a long-duration low-magnitude event can cause bleaching, which is also moderated by thermal stress in preceding seasons (Ainsworth et al. 2016). While heat stress impacts bleaching risk, coral reef health and resilience also integrates multiple environmental and biotic factors. Resilience is predetermined in part by ecosystem health that depends on historical disturbance events, the present day water quality (e.g., exposure to runoff) and the functional redundancy of the resident reef biota (Bellwood et al. 2003). These factors are also affected by anthropogenic activities (e.g., greenhouse gas emissions, overfishing, pollution), local weather, and large-scale modes of variability (ENSO). The 2016 ENSO episode was among the most severe recorded and may have impacted GBR bleaching. The integration of multiple interrelated factors by reef ecosystems represents important considerations for attribution of bleaching. This study explores these complexities through a multifactor analysis of climatic and environmental influences contributing to the 2016 GBR bleaching.