After a year of ocean heat waves and unprecedented temperatures off the coast of Florida that alarmed conservationists last summer, the world is currently experiencing a “global coral reef bleaching event,” the fourth ever recorded and the second in the past decade, according to climate scientists at the National Oceanic and Atmospheric Administration (NOAA).
Coral reef bleaching occurs as warming temperatures stress the tiny organisms that form the foundation of complex reef ecosystems, and without relief from cooler waters, bright and colorful reefs turn a ghostly white. Bleaching increases the risk that reefs suffer disease, starvation and death.
If current trends continue, the world will soon witness the most severe global bleaching event ever recorded, according to Derek Manzello, an ecologist and coordinator of NOAA’s Coral Reef Watch program.
Bleaching occurs when stress from heat breaks down the symbiotic relationship that transfers nutrients from algae to coral. Over the past year, about 54 percent of the global ocean area that is home to coral reefs experienced heat stress at levels that cause bleaching, according to NOAA’s Global Bleaching Event Index. The index peaked at 56 percent between 2014 and 2017.
“That 54 percent number has been going up by about 1 percent per week now for a few months, so it’s likely that the record will be broken here in a few weeks,” Manzello said in an interview. “The forecast is that El Niño is coming to an end and will switch to La Niña, and once that happens, that number will start coming down.”
Global ocean warming has increased steadily since the 1970s. Since early 2023, scientists have confirmed the mass bleaching of coral reefs throughout the tropics, including coastal areas of Florida and Brazil; the eastern Tropical Pacific stretching from Mexico down to Colombia; Australia’s famous Great Barrier Reef; large swaths of the South Pacific islands; the Persian Gulf, Red Sea, and beyond, according to NOAA’s satellite monitoring system of surface water temperature.
Few areas of the world remain untouched. NOAA received confirmation from partner scientists of bleaching across shallower areas of the Indian Ocean ranging from the eastern coast of Kenya to the western coast of Indonesia. However, bleaching is more scattered in these areas compared to the tropical Atlantic Ocean, where 95 percent of reefed areas experienced bleaching-level heat over the past year. That was a “big surprise,” Manzello said.
While coral reefs only cover about 0.2 percent of the sea floor, the habitats are so diverse that they support 25 percent of marine species.
“Just in terms of the sheer magnitude of the heat in the Atlantic is very shocking, and the other thing is, a lot of places like Florida and Bahamas, it started five to six weeks earlier than, you know, the ‘normal’ sort of bleaching season,” Manzello said, adding that bleaching events are now more severe and frequent than in the 1980s.
About 90 percent of global warming is occurring in the world’s oceans, which cover 70 percent of the planet’s surface. Coral reefs around the world now serve as a gauge of climate disruption. While coral reefs only cover about 0.2 percent of the sea floor, the habitats are so diverse that they support 25 percent of marine species, according to the Global Coral Reef Monitoring Network.
Along with a multitude of marine wildlife, coral bleaching on a global scale also threatens local economies that depend on reefs for food security and tourism dollars, for example, and the total economic value of the world’s reefs is estimated to be $2.7 trillion per year. Manzello said bleaching events have become more extensive, frequent and severe since the 1980s.
“When these events are sufficiently severe or prolonged, they can cause coral mortality, which hurts the people who depend on the coral reefs for their livelihoods,” Manzello said.
There are multiple factors behind the “marine heat waves” scientists are tracking in the world’s oceans, but scientists say climate change and warmer waters brought by El Niño in the Pacific were major drivers of heat waves over the past year. Marine heat waves can last for months, even years.
While the current global bleaching event may have occurred without the burning of fossil fuels by humans, the baseline temperature of the world’s oceans is higher due to climate change, making bleaching more severe and widespread. As of February 2024, up to 38 percent of the global ocean was experiencing a marine heat wave, including along the U.S west coast and much of the tropical Eastern Atlantic and Caribbean, according to NOAA.
As of February 2024, up to 38 percent of the global ocean was experiencing a marine heat wave.
“This wouldn’t be possible without climate change and El Niño beyond that,” Manzello said.
Waters off the coast of Florida reached unprecedented temperatures over the past year, threatening both beloved reefs and the beach-focused tourism economy. The heat wave started earlier, lasted longer and was more severe than any prior heat wave recorded in the region, Manzello said.
The heat wave brought national attention to Florida’s iconic coral reefs and sent conservationists scrambling to mitigate the damage, including by physically moving reefs to deeper, cooler waters and constructing “sunshades” over the surface of the ocean. NOAA scientists said the effort brought better scientific understanding of the problem, but global action is needed to save the world’s reefs.
Britta Schaffelke, the manager of international partnerships at the Australian Institute for Marine Science, said more long-term data on the health of coral reefs is needed to put the global bleaching event in context for conservationists exploring potential human interventions.
“[The data] allow us to quantify coral mortality, track recovery, and identify areas that don’t recover naturally and may need further protection or a helping hand through innovative interventions,” Schaffelke said in a statement on Monday.
Pepe Clarke, the oceans practice leader at the World Wildlife Foundation, said humans must urgently stop burning fossil fuels or we will lose coral reefs, with devastating consequences for coastal communities worldwide.
“If we need a specific, visual, contemporary case of what’s at stake with every fraction of a degree warming, this is it,” Pepe said in a statement. “The scale and severity of the mass coral bleaching is clear evidence of the harm climate change is having right now.”
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Mike Ludwig is a staff reporter at Truthout based in New Orleans. He is also the writer and host of “Climate Front Lines,” a podcast about the people, places and ecosystems on the front lines of the climate crisis. Follow him on Twitter: @ludwig_mike.
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