From shallow-water reefs in the Red Sea to graceful gorgonian species in the Caribbean and the rugged branching corals that form the structure of the Great Barrier Reef, the past year brought bleaching, decline and death to coral reefs around the world.

The National Oceanic and Atmospheric Administration and the International Coral Reef Initiative today confirmed the scope of recent reef damage, announcing that prolonged ocean heatwaves caused the fourth global bleaching episode, following similar events in 1998, 2010 and 2014-2017.

In the past 12 months, bleaching has been documented in both the Northern and Southern Hemisphere, and persists across the Atlantic, Pacific and Indian oceans, said Derek Manzello, coordinator of NOAA’s Coral Reef Watch.

Overheated oceans disrupt the symbiosis of corals and algae, which turns reefs white. If the water cools relatively quickly, some corals can recover, but increasingly, ocean temperatures are soaring so high that it’s killing corals outright.

“People generally don’t know that bleaching is only one very obvious manifestation of heat stress,” said Terry Hughes, a preeminent Australian coral researcher who has recently assumed a public watchdog role of his government’s efforts to downplay the impacts of Australia’s continued fossil fuel production, and to greenwash the damage warming is doing to the Great Barrier Reef.

“In both Florida last summer, and on the Great Barrier Reef during severe heat events, the corals don’t have time to bleach,” he said. “They just melt. One day they’re colorful and healthy and the next week they’re dead.”

More and more frequently, he said, water temperatures are rising more than 3 degrees Celsius above the normal summer maximums, causing acute heat stress that kills corals outright. Corals can also fluoresce, turning very colorful, often in shades of purple or pink, which is an “emergency response” in which they produce a protein with sunscreening properties.

“They’re basically saying, ‘I’m dying here, so I’m going to produce this protein, see if it helps me,’” he said. “But in our experience, if they get to a fluorescent stage, the temperatures are so high that most of them will die.”

Manzello said the current coral bleaching has been extremely severe and widespread in the Atlantic Ocean, across the wider Caribbean and in Florida.

“Brazil is going through it right now,” he said. “They’re experiencing record-setting heat stress that they’ve never had before.” Across the Atlantic, 98.5 percent of reef areas experienced bleaching-level heat stress in the last year, he added.

“On a global scale, about 54 percent of reef areas have experienced bleaching level heat stress in the last year,” he said. Bleaching conditions are increasing by 1 percent each week, so the affected area will likely soon surpass the record of 56.1 percent set during the 2014-2017 global bleaching disaster. 

New types of corals are also being affected, as are new geographic areas.

Along the Mesoamerican Reef, the second-largest barrier reef in the world, which extends down the eastern coasts of southern Mexico and Central America, soft gorgonian corals also bleached for the first time on record, according to Lorenzo Alvarez-Filip, a coral researcher with the Biodiversity and Reef Conservation Laboratory in Puerto Morelos, Mexico. The loss of reefs in 2023 is an ecological trauma that will leave lasting marks, he said.

The 2023 bleaching was destructive along the Mesoamerican Reef because of the long-lasting ocean heat wave that began in May and persisted for months, peaking with water temperatures more than 3.5 degrees Celsius above average in August, he said. 

Previous ocean heatwaves in the Caribbean were short-lived, peaking in September and then waning as the waters cooled in the autumn. 

“In previous years, 2010 and 2015, the corals started to bleach in October and then the water temperature goes down and the system recovers,” he said. 

But the scientists weren’t expecting an ocean heatwave that would last for three to four months.

“Seeing one colony die is painful,” he said. “But when you see it all along the reef, it’s a completely different scale. It’s like a graveyard.”

Soft corals also died in Florida for the first time on record, another warning sign that the oceans are becoming too warm even for the least vulnerable species, Manzello added.

“Populations of gorgonians and soft corals have been either stable or even increasing, so one of the ideas out there was, oh, these guys are going to be the winners,” he said

But last year, the water got so hot so fast that scientists observed an acute heat shock response in the soft corals.

“That was completely unexpected,” he said. “What ended up happening is, they got hit with so much heat so fast, they just kind of disintegrated. They started sloughing off their tissues. That was definitely one of the most shocking things to me last year.”

Corals are also being damaged by new diseases that are spreading in warming oceans, as well by polluted runoff from land and other human activities. And most recently, some near-shore corals in the Caribbean have also been directly killed by massive accumulations of sargassum seaweed that block sunlight needed for photosynthesis and simply smother the corals.

The Intergovernmental Panel on Climate Change projects that, if global warming exceeds 1.5 degrees Celsius above the pre-industrial average for a sustained period, close to 80 percent of existing reefs will die or be severely diminished.

As habitat for young fish, coral reefs are the basis of many ocean ecosystems and sustain important coastal fisheries. And they also have protected shorelines from some of the worst impacts of tropical storms and hurricanes.

Atop the damage from previous decades, last year’s bleaching in the western Caribbean has left the massive tourism developments along the Yucatán coast nearly completely exposed to storm surge from future hurricanes in the area, said Brigitta van Tussenbroek, who studies coastal ecology in Puerto Morelos.

“During big storms, the waves are 10 meters outside the reef, but the corals break the waves and they’re only 3 meters when they reach the shore,” she said. 

“But that barrier doesn’t exist anymore,” she said. “I’m really afraid of what will happen during the next big hurricane. I don’t see how these coastal developments will survive.”