US researchers believe worst event on record is ending but fear coral won’t recover in time before oceans warm again

The worst coral bleaching event in recorded history, which has hit every major coral region on Earth since 2014, appears to be coming to an end, with scientists now worrying how long reefs will have to recover before it happens again.

After analysing satellite and model data, and finding bleaching in the Indian ocean no longer appeared widespread, the US National Oceanographic and Atmospheric Administration (Noaa) has announced the event is no longer occurring on a global scale, and appears to be coming to an end.

Over an unprecedented period of three years, unusually warm water spread around the world, bleaching and killing coral.

Coral bleaches when the water is too warm for too long. The coral polyps get stressed and spit out the colourful algae that live in inside them, leaving them white. Since the algae provides the coral with 90% of its energy, the coral starves and – unless the temperatures quickly return to normal – dies.

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In Australia, the Great Barrier Reef suffered the worst bleaching event in recorded history in 2016 and then again in 2017. It was estimated about half of its coral was killed between the two events.

In the Indian Ocean, reefs were badly hit. One survey in the Maldives found all reefs there were affected, with between 60% to 90% of coral colonies bleached. Christmas Island had virtually all its coral bleached, and 85% of its coral died.

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In some other reefs, virtually all the coral was killed, with 98% of the coral around Jarvis dying, for example. Japanese reefs were badly hit, as were reefs in every other coral region.

“We know it has been the longest-lasting event and it has been the most widespread,” said Mark Eakin, coordinator of Noaa’s coral reef watch program. “And it probably has been the most damaging. In some places it definitely has been.”

Eakin said the data on total reef damage had not yet been analysed, so he could not say for sure whether it had been the worst but he said he would bet it had been.

The event started in 2014 when waters in the Pacific Ocean started to warm, in a pattern that resembled El Niño. The El Niño never fully kicked in, but the warming caused widespread bleaching.

In 2015 an El Niño did occur, which spread the bleaching even further, and the effects continued all the way until now.

Although the El Niño cycle tipped water temperatures over the edge, and triggered the bleaching, Eakin said there was no doubt the underlying cause of the bleaching was climate change. There have been two recorded global bleaching events previously, both of which occurred when strong El Niño events warmed oceans around the world – in 1998 and then 2010.

Eakin said the underlying warming was priming the ocean for coral bleaching, potentially with every El Niño.

“We didn’t even have an El Niño in 2014-15,” Eakin said, adding that a near-El Niño was enough to cause widespread bleaching then. “At this point I’d say any El Niño, even moderate ones, will probably result in widespread, if not global, bleaching.”

That view is backed up by studies with modelling that suggests the conditions causing the most recent global bleaching event would be average conditions within two decades.

Coral reefs need between 10 and 15 years to regain their coral cover, Eakin said. But that assumes they are not hit with too many local problems – such as pollution – or another bleaching event.

“The big fear is just simply that these events keep coming,” Eakin said.