Abstract
Tropical corals live close to their upper thermal limit making them vulnerable to unusually warm summer sea temperatures. The resulting thermal stress can lead to breakdown of the coral-algal symbiosis, essential for the functioning of reefs, and cause coral bleaching. Mass coral bleaching is a modern phenomenon associated with increases in reef temperatures due to recent global warming. Widespread bleaching has typically occurred during El Niño events. We examine the historical level of stress for 100 coral reef locations with robust bleaching histories. The level of thermal stress (based on a degree heating month index, DHMI) at these locations during the 2015–2016 El Niño was unprecedented over the period 1871–2017 and exceeded that of the strong 1997–1998 El Niño. The DHMI was also 5 times the level of thermal stress associated with the ‘pre-industrial’, 1877–1878, El Niño. Coral reefs have, therefore, already shown their vulnerability to the modest (~0.92 °C) global warming that has occurred to date. Estimates of future levels of thermal stress suggest that even the optimistic 1.5 °C Paris Agreement target is insufficient to prevent more frequent mass bleaching events for the world’s reefs. Effectively, reefs of the future will not be the same as those of the past.
Highlights
Tropical corals live close to their upper thermal limit making them vulnerable to unusually warm summer sea temperatures
Warming of the tropical oceans has been substantially less than the global average rate throughout much of the central and eastern tropical Pacific with some small areas (
Large-scale mass coral bleaching events have increased in frequency, extent and intensity since the latter decades of the 20th century[28,35]
Summary
Tropical corals live close to their upper thermal limit making them vulnerable to unusually warm summer sea temperatures. The major El Niño event of 2015–201626 resulted in significant warming of large areas of the tropical oceans and continued bleaching of substantial areas of reef, including Australia’s Great Barrier Reef in early 201627 Given this recent extensive coral bleaching associated with the major 2015–2016 El Niño event, which is occurring against a backdrop of global warming, this study aims to (1) document how warming of coral reef SST compares to global average warming, (2) determine the level of thermal stress experienced at 100 reef locations with robust bleaching histories[28] in 2015–2016, (3) place the recent thermal stress in an historical context, e.g. how does the level of thermal stress compare with the very strong ‘pre-industrial’ El Niño of 1877–187829,30? Given this recent extensive coral bleaching associated with the major 2015–2016 El Niño event, which is occurring against a backdrop of global warming, this study aims to (1) document how warming of coral reef SST compares to global average warming, (2) determine the level of thermal stress experienced at 100 reef locations with robust bleaching histories[28] in 2015–2016, (3) place the recent thermal stress in an historical context, e.g. how does the level of thermal stress compare with the very strong ‘pre-industrial’ El Niño of 1877–187829,30? and (4) what estimated level of thermal stress might be expected at these selected reef sites, given current warming rates, if global warming is limited to 1.5 °C, 2.0 °C as per the Paris Agreement[31] or 3 °C, the trajectory based on the Nationally Determined Contributions of individual nations under the UNFCC32 (NDCs)?
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