World's largest bleaching event on record has harmed 84 percent of coral reefs

Boston Globe

23-04-2025

NOAA says the latest global event began on Jan. 1, 2023, and mass bleaching has now been observed across at least 83 countries and territories, threatening marine life from Fiji to the Florida Keys to Australia's Great Barrier Reef.
'The ongoing global coral bleaching event is the biggest to date,' NOAA scientists said.
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Bleaching occurs when heat upends the coral's metabolism, causing it to turn white as it expels the symbiotic algae that provide it with nutrients and color. Bleaching does not mean the coral has died, but prolonged bleaching — which scientists say is made more frequent and severe by rising sea temperatures — can kill it.
'Bleaching becomes more severe if either the heat stress episodes are more intense or if they last for a longer time,' Joerg Wiedenmann, a marine biologist who heads the Coral Reef Laboratory at the University of Southampton in England, said in an email Wednesday.
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The Earth's rising temperatures have caused bleaching events to occur more often and last longer, he said. The changing and mixing of ocean currents can also influence regional water temperatures, he added, further increasing the risk of bleaching.
Since the early 1980s, scientists say, coral bleaching events have increased in frequency and severity as the burning of fossil fuels drives up the planet's temperature. NOAA documented the first global coral bleaching event in 1998 and the second in 2010. During the previous global event, between 2014 and 2017, 68 percent of the world's coral reefs experienced bleaching-level heat stress.
'More frequent bleaching events leave little time for corals to recover, resulting in the long-term decline of coral reefs,' Nicola Foster, a marine biologist at the University of Plymouth in England, said in an email.
Mass bleaching has frequently coincided with episodes of El Niño, a global climate pattern that usually increases temperatures. That was the case in 1998, 2016, and again last year, which was the warmest year on record.
Oceans have been steadily absorbing rising temperatures for years, but in 2023 and 2024, temperatures broke records with spikes that have alarmed scientists. Seas are also becoming more acidic as they warm, which can dissolve corals' skeletons and make it harder for them to grow.
In 2023, a blistering marine heat wave off Florida's coast caused parts of the continental United States' only living coral barrier reef to bleach dramatically, stunning experts with its rapid onset. Hot temperatures caused the coral tissue to melt from its skeleton so quickly that the color did not even have a chance to drain.
The bleaching was also so devastating that it prompted scientists at NOAA to introduce a new bleaching alert scale — adding three categories to account for higher coral mortality rates and bleaching levels. The new scale stretches from Level 1, signifying significant bleaching, to Level 5 - five times the amount of Level 1 — which means the coral is approaching mortality.
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Coral bleaching is considered to begin at four degree-heating weeks — meaning four weeks in which temperatures are 1 degree Celsius (1.8 degrees Fahrenheit) above the average summertime maximum.
Last year, a fatal heat wave hit southern sections of Australia's 1,400-mile Great Barrier Reef — threatening it with major fatalities for the first time. In 2016, a mass bleaching event hit 93 percent of the reef, killing nearly 50 percent of the coral in some northern sections.
Beyond the loss of a thriving underwater ecosystem, coral death can degrade reef structure and leave communities vulnerable to flooding hazards, according to the US Geological Survey. In Florida, for example, coral reefs act as a natural buffer to storm energy that can shield coastlines from destructive forces.
According to Wiedenmann, coral reefs support a third of all marine biodiversity either directly or indirectly. 'If the corals die, this support structure that provides food and homes is lost. Consequently, many species will suffer, as well,' he said.
According to a 2020 study, rising sea surface temperatures and acidic waters could eliminate nearly all surviving coral reef habitats by 2100.
'Unless we address the causes of climate change and reduce our emissions from burning fossil fuels, the future of coral reefs, and the millions of lives and livelihoods they support, is very much under threat,' Foster said.
Still, several scientists said corals' resilience offers some reason for hope.
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Derek Manzello, an ecologist who coordinates NOAA's Coral Reef Watch program, said in an email that the 'chronic, omnipresent stress' on corals is pushing researchers and conservationists to consider new approaches to minimize bleaching.
'This by no means implies that it is time to give up hope, or 'throw in the towel' on coral reefs,' he said, 'as they generate billions of dollars in ecosystem services to the US economy every year.'
'The ancestors of today's corals survived the impact of the asteroid that wiped out the dinosaurs on land and a lot of creatures in the sea,' Wiedenmann said. 'So, if we manage to decrease ocean warming, there is always a chance for corals to recover.'