As Mass Coral Bleaching Occurs More Frequently, Hopes For Recovery Fade, Study Finds

"Before the 1980s, mass bleaching of corals was unheard of."
A turtle swims over a reef destroyed by bleaching.
A turtle swims over a reef destroyed by bleaching.

Mass coral bleaching events are happening far more frequently than in the past, giving some of the world’s most majestic and fragile ecosystems almost no time to recover, according to a new paper published Thursday in the journal Science. 

A team of researchers analyzed rates of coral bleaching around the tropics over the past four decades, events that most often occur when seawater becomes too warm for coral to remain healthy. Scientists discovered that in the 1980s, severe coral bleaching events occurred only about once every 25 to 30 years. But, in the six years after 2010, the rate between bleachings plummeted, and mass events now occur about once every six years.

“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,” Terry Hughes, the study’s lead author and the director of the ARC Centre of Excellence for Coral Reef Studies in Australia, said in a statement.

The paper warns that climate change could eventually lead to such events every year.

While reefs that bleach ― so named because once-colorful corals turn white ― aren’t immediately killed, many of the creatures die in the intervening days and weeks. A mass bleaching event on the Great Barrier Reef in 2016 left up to two-thirds of parts of the reef dead, and a back-to-back event in 2017 left some scientists in shock.

A goby perches on bleached coral at the height of the 2016 bleaching event at Lizard Island in Australia's Great Barrier Reef
A goby perches on bleached coral at the height of the 2016 bleaching event at Lizard Island in Australia's Great Barrier Reef.

Some corals, however, are able to recover if ocean temperatures return to normal and they’re given enough time to build up their health. But it usually takes 10 to 15 years for the fastest-growing species to recover, the paper states, and “far longer for the full complement” on a reef.

The shorter time-frame between mass bleachings reported in the paper makes such recovery increasingly difficult, the authors write, and our rapidly warming world doesn’t seem likely to give corals a respite.

Scientists have long said the world has already warmed 1 degree Celsius above pre-industrial levels, or about 1.8 degrees Fahrenheit. And if greenhouse gas emissions ― the prime driver of global warming and, in turn, coral bleaching ― remain unchecked, warming will skyrocket.

“Reefs have entered a distinctive human-dominated era ― the Anthropocene,” Mark Eakin, a co-author of the paper and a reef scientist at the U.S. National Oceanic and Atmospheric Administration, said in a statement. “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.”

The paper concludes on a somber note and urges global action to address climate change.

“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 authors write. “The future condition of reefs, and the ecosystem services they provide to people, will depend critically on the trajectory of global emissions.”

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