Catastrophic coral bleaching is killing the world’s reefs (and that’s bad news for humanity)

A new study suggests we are running out of time to save the world’s coral reefs, pointing to a decided increase in the death of coral due to global warming.

As part of the expansive investigation, the team of researchers measured the rate of coral bleaching at 100 reef locations throughout the year, for the period from 1980 to 2016. What they found was a dramatic shortening of the gap between these damaging bleaching events, signifying a dire future for coral ecosystems.

Bleaching happens when the normally endosymbiotic relationship between coral polyps and algae falls out of whack. A healthy coral reef is made up of algae providing energy for the polyps through photosynthesis, which in turn provide the algae with carbon dioxide and ammonium. If negative environmental conditions occur, the polyps expel the algae from its tissue. This lets the polyp keep its nutrients, and therefore survive for a short period, but if the algae do not return the coral will die from starvation.

There are a number of triggers for bleaching, but one of the most notable is an increase in water temperature due to global warming.

“The time between bleaching events at each location has diminished five-fold in the past three to four decades, from once every 25-30 years in the early 1980s to an average of just once every six years since 2010,” says lead author of the study, professor Terry Hughes, Director of the ARC Centre of Excellence for Coral Reef Studies (Coral CoE).

“Before the 1980s, mass bleaching of corals was unheard of, even during strong El Niño conditions, but 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.”

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(Above: A bleached coral, surrounded by healthy coral) 

The study, published in Science, outlines a “new era” where the interval between bleaching events is becoming too short for a full recovery of mature reefs. It characterises a period before the 1980s when bleaching only tended to occur locally, and a period in the 1980s and 1990s when mass bleaching was first recorded. While the average since 2010 is one bleaching event every six years, the transition to the Anthropocene increases the likelihood of annual bleaching in the coming decades.  

“Reefs have entered a distinctive human-dominated era: the Anthropocene,” says co-author, Dr C. Mark Eakin of the US National Oceanic & Atmospheric Administration. “The climate has warmed rapidly in the past 50 years, first making El Niños dangerous for corals, and now we’re seeing the emergence of bleaching in every hot summer.”

As an example, the Great Barrier Reef has bleaches four times since 1998, with unprecedented back-to-back events in 2016 and 2017. This is causing extremely high levels of damage to the reef, and it can take at least a decade to replace even the fastest growing of coral species.

Aside from looking pretty, coral reefs support the highest marine biodiversity in the world, as well as a great deal of human activity. According to the International Union for the Conservation of Nature, more than 500 million people worldwide depend on them for food, storm protection, jobs, and recreation. Taken altogether, their resources and services are worth an estimated 375 billion dollars each year.

“We hope our stark results will help spur on the stronger action needed to reduce greenhouse gases in Australia, the United States and elsewhere,” notes Hughes.

Image credit: ARC Centre of Excellence for Coral Reef Studies

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