Coral Reef Bleaching Is Happening So Frequently That Full Recoveries Are Now Impossible


Robin Andrews

Science & Policy Writer

Heron Island, within the Great Barrier Reef, can ber seen here surrounded by Wistari coral reef. JPL/NASA

There’s no two ways about it: The world’s coral reefs are being annihilated by climate change. Adding insult to injury, a new study in Science has found that coral bleaching is happening way more often than it used to.

Around the time of the early 1980s, coral bleaching events occurred once every 25 to 30 years. An analysis of 100 coral reefs spread across the globe revealed that this recurrence rate is now once every 5.9 years in 2016.


The authors explain that “tropical reef systems are transitioning to a new era in which the interval between recurrent bouts of coral bleaching is too short for a full recovery of mature assemblages.”

Of the reefs sampled, 300 bleaching episodes were considered to be severe, which means that 30 percent or more corals were drained of their hues on a scale of tens or even hundreds of kilometers. Another 312 were denoted to be moderate, which is when 30 percent or less of corals have been bleached.

Of the 100 coral reefs, only six escaped severe bleaching.

“We have known for quite a long time now that bleaching events are becoming more frequent and intense and that a global coral reef crisis is underway,” study co-author Dr Verena Schoepf, a research fellow and program co-leader at the ARC Centre of Excellence for Coral Reef Studies, told IFLScience.


“In fact, this has been predicted quite a while ago but back then, scientists were still hoping that this may have been an overly pessimistic outlook.”

“So while our study has confirmed what we already knew in a general sense, crunching the actual numbers and seeing that reefs are running out of time to recover from recurrent bleaching – and that none of the 100 reefs that we examined have remained free of any bleaching since 1980 – was heartbreaking.”

The international research endeavor was led by the Australian Research Council Centre of Excellence for Coral Reef Studies at James Cook University, and it’s another unwelcome melancholy narrative about the state of our oceans. It follows hot on the heels of a separate study that found that our oceans are also being rapidly deoxygenated.

The full recovery of mature coral reefs, post-bleaching, is now not possible. q phla/Flickr; CC BY 2.0

Coral bleaching, in case you’re not aware, is a visual representation of the collapse of coral reefs.


Corals, those huddled marine invertebrates, create their calcium carbonate-rich castles around them for protection. At the same time, microscopic algae live within the corals’ tissues.

It’s a mutualistic relationship: The algae is protected by the coral, and the coral feeds on the leftovers from the algae when they photosynthesize. Aesthetically speaking, the algae give the coral reefs their incredible color.

That’s why, when water temperatures get out of an optimum range – too hot or too cold – the corals become stressed and their algae are expelled. During such a “bleaching” event, the colors drain from the corals, which are left vulnerable to attack, disease, or starvation. If they cannot recover, they die.

Climate change is the obvious culprit. As we pump increasing amounts of carbon dioxide into the skies, the world warms, and that includes the oceans. At the same time, plenty of excess carbon dioxide gets absorbed into the oceans too, and this generates carbonic acid, which eats away at the calcium carbonate exoskeletons.


“The impacts that we are already seeing today are the result of close to 1°C (1.8°F) increase in average global temperature,” Schoepf pointed out.

When asked about the Paris agreement’s more ambitious lower limit of 1.5°C (2.7°F), the response wasn't one imbued with much hope.

“While it is absolutely critical to limit further warming as much as possible, 1.5°C warming will nevertheless mean even more bleaching than we have seen to date,” she noted. 

That’s not all there is to the story though; El Nino and La Nina, collectively known as the El Nino-Southern Oscillation (ENSO), also plays a major role.


To put it simply, this periodic climate phenomenon has a planet-wide impact on atmospheric and oceanic systems. With regards to the Pacific Ocean, El Nino is the warm phase and La Nina is the cold phase.

These temperature fluctuations have always had an effect on coral reefs, but this new study highlights how times have changed dramatically. Today, the cold La Nina phase is now hotter than the warm El Nino phase back in the early 1980s.

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

Bleached Coral. OSU/Flickr; CC BY-SA 2.0

Coral reefs are more biodiverse than tropical rainforests, and 25 percent of the world’s fish biodiversity alone is associated with them. At the same time, they generate $30 billion in goods and services every single year.


Whether you’re more concerned about extinctions or the economy, it doesn’t matter. If reefs die out, we will be living in a poorer world, no matter how you measure it.


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