Corals are bleaching in every corner of oceans, threatening their web of life

First around Fiji, then the Florida Keys, then Australia’s Great Barrier Reef, and now in the Indian Ocean. In the past year, anomalous ocean temperatures have left a trail of devastation for the world’s corals, bleaching entire reefs and threatening widespread coral mortality - and now, scientists with the National Oceanic and Atmospheric Administration and International Coral Reef Initiative say the world is currently experiencing its fourth global bleaching event, the second in the last decade.

At least 53 countries and local regions have experienced mass bleaching across the Atlantic, Pacific and Indian oceans. Bleaching - which occurs when stressed coral turn white after expelling symbiotic algae that provide food and color - must be confirmed within each ocean basin to be declared a global bleaching event.

Derek Manzello, an ecologist and head of NOAA’s Coral Reef Watch program, said the frequency and severity of bleaching events has increased since the early 1980s. That intensity and regularity has also ramped up in the last two decades.

“Now we’re just reaching a point in this warming cycle where these events are becoming so extreme and they’re just getting worse and worse and worse,” Manzello said.

According to the Global Bleaching Event Index, which is based on sea surface temperatures data, more than 54 percent of all the reef areas on the planet have experienced bleaching level heat-stress in the past 365 days. And that number is increasing by one percent each week, Manzello said. For context, during the worst global coral bleaching event on record between 2014 and 2017, the Global Bleaching Event Index peaked at 56 percent.

“Right now, we’re almost equal with the worst global bleaching event on record in terms of spatial extent,” Manzello said. It’s possible the overall percentage of reefs experiencing heat-stress will surpass the previous record in the next few weeks.

“This should be a global wake-up call. The fact that corals are bleaching in each ocean basin essentially simultaneously,” Manzello said. “And more than half the reefs on the planet have basically experienced bleaching level heat stress in the last year.”


A bleached coral doesn’t mean the coral is dead, but that the coral is starting to starve. If conditions return to more comfortable, favorable conditions, corals can recover. But corals, like any other species, can only survive such circumstances for so long before they die.

Many corals experienced blistering temperatures for prolonged periods of time, as temperatures spiked earlier and lingered longer. The Atlantic Ocean appears to be experiencing the brunt of the heat-stress, Manzello said.

“Heat stress has been very consistent across huge areas, in terms of this huge blanket of very, very hot water in the Atlantic Ocean,” Manzello said.

Scientists worldwide had been bracing for unprecedented ocean temperatures since marine heat waves sent temperatures soaring into triple digits in shallow waters off Florida’s coast and coral restoration groups scrambling to evacuate corals from the ocean.

Corals in the Florida Keys endured the hottest ocean temperatures on record, and the longest-lasting marine heat wave recorded in three decades during the summer of 2023.

For coral experts in the Florida Keys, into the Caribbean and down to the Great Barrier Reef, the sentiment was the same last summer when looking at the havoc wrought by warm water temperatures.

The harrowing sights bewildered even the most seasoned experts at the Coral Restoration Foundation, a nonprofit coral restoration group in the Florida Keys.

“This has been the most challenging year of my professional life, it’s just too much to handle,” Phanor Montoya-Maya, a restoration program manager at CRF, told The Washington Post in October, describing the year of bleaching off Florida. Montoya-Maya had lost a lot of corals due to various events throughout his 20 years in coral research. But, no event was as severe as this.

Beyond Florida’s shores, bleaching also gripped islands in the Caribbean.

“The bleaching was more devastating this year. It was really every coral on the reef that was bleached,” said Francesca Virdis, a chief operating officer at Reef Renewal Bonaire. The nonprofit organization operates on the island of Bonaire, an overseas territory of the Netherlands off the coast of Venezuela. “There were no species that were spared.”

Even corals that had a history of being more resilient against higher temperatures bleached, she said.

“As predicated, the situation is getting critical globally,” Virdis said. “It’s hard to find a silver lining or a positive note with everything happening.”

Scientists hope that with the projected transition from El Niño, characterized by warmer-than-normal surface waters in the tropical Pacific Ocean, to La Niña, characterized by cooler surface waters, there will be some relief to the corals. But it’s not clear how temperatures will respond in the Atlantic Ocean where temperatures have been the harshest.

“We may be getting to a place where temperatures are so high that large-scale mass bleaching events are occurring during any phase of ENSO, including La Niña,” Manzello said.