Global patterns and impacts of El Nino events on coral reefs: A meta-analysis

Impacts of global climate change on coral reefs are being amplified by pulse heat stress events, including El Nino, the warm phase of the El Nino Southern Oscillation (ENSO). Despite reports of extensive coral bleaching and up to 97% coral mortality induced by El Nino events, a quantitative synthesis of the nature, intensity, and drivers of El Nino and La Nina impacts on corals is lacking. Herein, we first present a global meta-analysis of studies quantifying the effects of El Nino/La Nina-warming on corals, surveying studies from both the primary literature and International Coral Reef Symposium (ICRS) Proceedings. Overall, the strongest signal for El Nino/La Nina-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 Nino/La Nina 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 Nino event instigated unprecedented global coral heat stress across the world's oceans. With El Nino 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.