Greater emissions reductions and delivering on the Paris climate agreement are now “the only opportunity” to save coral reefs the world over from decline, with local responses no longer sufficient, a report by Unesco has found.
The first global scientific assessment of the impacts of climate change on the 29 world heritage-listed coral reefs, published on Saturday, found that the frequency, intensity and duration of heat-stress events had worsened with increasing global warming, with massive consequences for the 29 world heritage sites.
Analysis of recent studies and newly-developed data from the US national ocean and atmospheric administration (NOAA) coral-reef watch showed that 13 of the 29 listed reefs had been exposed to levels of heat stress that cause coral bleaching, on average more than twice per decade from 1985 to 2013.
Bleaching had occurred more frequently in recent years than in decades prior, with coral mortality during the third global bleaching event from mid-2014 to mid-2017 “among the worst ever recorded”. Twenty-one listed sites had suffered severe and/or repeated heat stress in the last three years.
Compounding the devastating impact of bleaching – which can take coral communities at least 15 to 25 years to recover from – were more frequent and more severe extreme weather events, increasing ocean acidification, and pollution.
The Great Barrier Reef, “one of the world’s most iconic coral reef systems” and among four of the total 29 listed located in Australia, had been “seriously affected” by back-to-back severe bleaching events this year and last, despite considerable investment in efforts to build resilience.
Professor Terry Hughes, director of the Australian Research Council centre of excellence for coral reef studies in Townsville, provided an analysis of bleaching records for the report. “It basically makes the point that everywhere is bleaching,” he said. “It’s certainly not a phenomenon only on the Great Barrier Reef.”
Australia’s scientific community had appealed to the UN world heritage committee to list the Great Barrier Reef as “in danger” last year. Hughes said this was “not on the current agenda”, as the committee awaited the Great Barrier Reef Marine Park Authority’s third outlook report due in 2019.
The Unesco report found that local efforts to increase reefs’ resilience “remain necessary but are no longer sufficient” without complementary national and international efforts to limit the temperature increase to 1.5C above pre-industrial levels – the most ambitious target set by the Paris agreement, and understood to be the maximum possible to secure coral reefs’ long-term survival.
“We need all of the above,” said Jon Day, a former director with the Great Barrier marine park authority, now at James Cook University. “We can’t just assume local responses are enough, and they must be augmented by global’s efforts too.”
He said while the world heritage convention aimed to “transmit the world heritage values” of listed sites for future generations, a natural system would inevitably change with time. “The question is what is acceptable change, and the reported levels of coral bleaching and coral mortality can hardly be considered by anyone to be acceptable.”
The report found that, if emissions were to follow their current trajectory and not decline – similar to a “business-as-usual scenario” – 25 of the 29 world heritage reefs (68%) would suffer severe bleaching twice per decade by 2040, rapidly killing most corals present and preventing successful reproduction necessary for their recovery.
Reducing emissions so that they peak around 2040 and then decline would reduce that number of affected sites to 14 (48%), and allow an extra 12 years, on average, for them to recover.
Hughes said the prospects of coral reefs’ long-term survival was at a crossroads, with the worst-case scenario able to be avoided only “if we quickly adopt the 1.5C target”.
“1.5C or 2C degrees won’t be a particularly comfortable place for reefs – they will still see quite regular bleaching and they will be different to how they were 15 or 20 years ago – but they will be able to survive.”
He said he was optimistic about reefs’ prospects, given that the business-as-usual path was looking “increasingly unlikely” as cities and states the world over moved to exceed federal or commonwealth commitments to curbing emissions.
A draft decision prepared by Unesco, to be addressed by the world heritage committee at its meeting in Krakow in Poland from 2 to 12 July, had stated the report’s findings were of “utmost concern”.
But Day said it was “very disappointing” that the draft decision only committed to further studies at this stage: “How much evidence do they need?”
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