Abstract
Impacts of global climate change on coral reefs are being amplified by pulse heat stress events, including El Niño, the warm phase of the El Niño Southern Oscillation (ENSO). Despite reports of extensive coral bleaching and up to 97% coral mortality induced by El Niño events, a quantitative synthesis of the nature, intensity, and drivers of El Niño and La Niña impacts on corals is lacking. Herein, we first present a global meta-analysis of studies quantifying the effects of El Niño/La Niña-warming on corals, surveying studies from both the primary literature and International Coral Reef Symposium (ICRS) Proceedings. Overall, the strongest signal for El Niño/La Niña-associated coral bleaching was long-term mean temperature; bleaching decreased with decreasing long-term mean temperature (n = 20 studies). Additionally, coral cover losses during El Niño/La Niña were shaped by localized maximum heat stress and long-term mean temperature (n = 28 studies). Second, we present a method for quantifying coral heat stress which, for any coral reef location in the world, allows extraction of remotely-sensed degree heating weeks (DHW) for any date (since 1982), quantification of the maximum DHW, and the time lag since the maximum DHW. Using this method, we show that the 2015/16 El Niño event instigated unprecedented global coral heat stress across the world's oceans. With El Niño events expected to increase in frequency and severity this century, it is imperative that we gain a clear understanding of how these thermal stress anomalies impact different coral species and coral reef regions. We therefore finish with recommendations for future coral bleaching studies that will foster improved syntheses, as well as predictive and adaptive capacity to extreme warming events.
Highlights
Climate change poses an imminent threat to the persistence of the world’s coral reefs
6 studies reported on coral bleaching associated with El Niño Southern Oscillation (ENSO), and 30 studies reported on coral cover losses associated with El Niño/La Niña (Fig 1)
The additional benefits that coral reefs provide are extensive, including protection against wave action, provision of fish habitat [87], recreation and tourism, and aesthetic and cultural benefits [88,89,90]. The resilience of these benefits is incrementally being eroded, as local stressors decrease baseline resilience [91,92] and climate change disables coral bleaching protection by shifting ocean warming trajectories on reefs from “protective”, to “lethal” [93]
Summary
Climate change poses an imminent threat to the persistence of the world’s coral reefs. Anthropogenic ocean warming is fundamentally altering marine ecosystems [1], exacerbating chronic local stressors such as overfishing, eutrophication, and coastal pollution, and threatening the resilience of marine ecosystems [2,3]. El Niño impacts on coral reefs stress metrics are available at https://github.com/ daniclaar/CoralStress
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