Study of 100 tropical reef locations finds time between bleaching events has shrunk and is too short for full recovery

This article is more than 2 years old

This article is more than 2 years old

Repeated large-scale coral bleaching events are the new normal thanks to global warming, a team of international scientists has found.

In a study published in the journal Science, the researchers revealed a “dramatic shortening” of the time between bleaching events was “threatening the future existence of these iconic ecosystems and the livelihoods of many millions of people”.

The study examined 100 tropical reef locations across the world, analysing existing data on coral bleaching events as well as new field research conducted on the Great Barrier Reef after the longest and worst case of bleaching caused by climate change killed almost 25% of the coral.

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“Before the 1980s, mass bleaching of corals was unheard of, even during strong El Niño conditions,” said lead author Prof Terry Hughes, director of the ARC Centre of Excellence for Coral Reef Studies. “Now repeated bouts of regional-scale bleaching and mass mortality of corals has become the new normal around the world as temperatures continue to rise.”

The study found that time between bleaching events had diminished five-fold in the past 30 to 40 years, and was now too short to allow for a full recovery and was approaching unsustainable levels.



While mass bleaching events used to occur about once every 27 years, by 2016 the median time between them had shrunk to 5.9 years. Only six of the 100 sites had escaped bleaching.

“Our analysis indicates that we are already approaching a scenario in which every hot summer, with or without an El Niño event, has the potential to cause bleaching and mortality at a regional scale,” the paper said.

Globally, the annual risk of severe and moderate bleaching had increased by almost 4% a year since the 1980s, from an expected 8% of locations to 31% in 2016.

The Western Atlantic remained at highest risk but Australasia and the Middle East saw the strongest increases in risk of bleaching.

Hughes said he hoped the “stark results” would prompt stronger action on reducing greenhouse gases. In May scientists warned that the central goal of the Australian government’s protection plan was no longer feasible because of the dramatic impact of climate change.



Friday’s paper also determined the link between El Niño and mass bleaching events has diminished as global warming continues.

Prior to the 1980s mass coral bleaching on a regional scale was “exceedingly rare or absent” and occurred in localised areas stretching tens of kilometres, not the hundreds of kilometres affected in recent times, the paper said.

These local bleaching events were largely caused by small-scale stressors like unusually hot or cold weather, freshwater inundation or sedimentation.

Then global warming increased the thermal stress of strong El Niño events, the paper said, widening the impact of individual bleaching events. Now, they are occurring at any time.

“Back in the 80s it was only during El Niño events that waters became hot enough to damage corals and induce them to bleach,” co-author Andrew Baird, a professor at James Cook University, told Guardian Australia.

“But now it’s 30, 40 years later and we’re seeing those temperatures in normal years.”

Baird said it was difficult to know if the current conditions were reversible but “the window to address it is diminishing”.

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“It’s impossible to know if this is the end of coral reefs but it might be,” he said. “We really need to get on top of climate change as soon as possible.”

There have been several large-scale and devastating mass bleaching events in recent years. The 2015-16 event affected 75% of the reefs studied by the researchers, who said it was comparable to the then unprecedented mass bleaching of 1997-98, when 74% were affected.

“Interestingly one of the first papers that effectively drew attention to the issue – back in 1999 – suggested that by 2016, 2017, 2020, we would be seeing bleaching annually,” Baird said. “That’s pretty close to what’s happening unfortunately.

“Some of these earlier works were quite prescient in their prediction and unfortunately we didn’t pay enough attention back then.”

The study follows a discovery late last year that 3% of the Great Barrier Reef could facilitate recovery after bleaching – a finding the researchers at the time suggested was akin to a life-support system but small enough not to be taken for granted.

On Friday scientists announced that a major outbreak of coral-eating crown of thorns starfish had been found munching the Great Barrier Reef in December, prompting the Australian government to begin culling the spiky marine animals.

The predator starfish feeds on corals by spreading its stomach over them and using digestive enzymes to liquefy tissue.

Facebook Twitter Pinterest Coral-eating crown of thorns starfish are destroying Australia’s Great Barrier Reef. Photograph: Kathrina Fabricius/AFP/Getty Images

“Each starfish eats about its body diameter a night, and so over time that mounts up very significantly,” Hugh Sweatman, a senior research scientist at the Australian Institute of Marine Science told ABC radio. “A lot of coral will be lost,” he said.

The crown of thorns were found in plague proportions in the Swains reefs, at the southern edge of the Great Barrier Reef, by researchers from the Great Barrier Reef Marine Park Authority.

The authority already killed some starfish at Swains reefs in December and said it would mount another mission in January.

There have been four major crown of thorns outbreaks since the 1960s in the Great Barrier Reef but it recovered each time because there were always healthy populations of herbivorous fish. The outbreaks are usually triggered by extra nutrients in the water but the reason for the current outbreak was unclear, Sweatman said.

Additional reporting by Reuters

