More than 84 per cent of the world’s coral reefs have been affected by extreme heat stress in what has now become the most widespread coral bleaching event on record, scientists confirmed on Tuesday.
The mass bleaching, which began in early 2023, has impacted reefs in at least 83 countries and territories, including biodiversity hotspots like Australia’s Great Barrier Reef, the Caribbean, the Indian Ocean, and reefs across the South Pacific and Southeast Asia.
This is the fourth global coral bleaching event ever recorded, and the second in the last decade, according to the US National Oceanic and Atmospheric Administration (NOAA).
“As the oceans continue to warm, coral bleaching is becoming more frequent, more intense, and more widespread,” Derek Manzello, coordinator of the Coral Reef Watch programme at the US National Oceanic and Atmospheric Administration (NOAA), said.
A heat stress map released by NOAA’s Coral Reef Watch shows large parts of the Pacific and Caribbean oceans in dark red and purple – the highest alert levels for coral bleaching.
The agency recently expanded its warning system to account for how extreme the heat stress has become, with the new levels indicating widespread bleaching and a high risk of coral death.
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Bleaching occurs when ocean temperatures rise above normal for prolonged periods, forcing corals to expel the algae they rely on for food and colour. Left without this algae, corals turn white and become more vulnerable to disease and death.
The current event, confirmed by NOAA and the International Coral Reef Initiative (ICRI), has already surpassed the previous record bleaching event from 2014–2017, which affected 68.2 per cent of reefs worldwide. This marks the fourth recorded global bleaching event, and the second in less than a decade.
The damage is being driven by sustained marine heatwaves, compounded by the climate crisis and El Niño conditions. Scientists say some reefs, such as those off the coast of Mexico and in the southern Great Barrier Reef, have already suffered coral mortality rates above 40 to 90 per cent in certain zones.
However, scientists fear, the ongoing bleaching event may never end.
“We may never see the heat stress that causes bleaching dropping below the threshold that triggers a global event,” Mark Eakin, executive secretary for the International Coral Reef Society and retired coral monitoring chief for the US National Oceanic and Atmospheric Administration, said, according to The Associated Press.
“We’re looking at something that’s completely changing the face of our planet and the ability of our oceans to sustain lives and livelihoods,” Mr Eakin said.
Coral reefs, often called the “rainforests of the sea”, support about 25 per cent of all marine species. They also provide critical services for humans – supporting fisheries, protecting coastlines from erosion and storms, and fuelling tourism industries.
Yet the escalating threat to reefs highlights a broader failure to curb greenhouse gas emissions, which are heating up the planet and driving heat on land and ocean to record levels. In 2023, global sea surface temperatures outside the polar regions reached their highest annual average in recorded history at 20.87 degrees Celsius.
“Climate model predictions for coral reefs have been suggesting for years that bleaching impacts would increase in frequency and magnitude as the ocean warms,” said Jennifer Koss, director of NOAA’s Coral Reef Conservation Program (CRCP).
The group’s update comes as the US president Donald Trump is aggressively increasing production of fossil fuels which are the primary drivers of global heat, despite the rest of the world rapidly increasing clean energy production.
Florida’s coral reefs were among the earliest and hardest hit during this global bleaching event. NOAA described the 2023 marine heatwave in the region as the most severe on record – starting earlier, lasting longer, and reaching more extreme temperatures than any previous event. In response, the agency says it undertook emergency interventions, including relocating coral nurseries to deeper, cooler waters and deploying sunshades to shield vulnerable reefs, as part of its Mission: Iconic Reefs programme.
Efforts are underway to potentially repopulate the corals in the future and find ways to make them more resilient. Last year, researchers also uncovered coral colonies in the Great Barrier Reef that can withstand higher temperatures.
But scientists stress that such interventions are not long-term solutions. “The best way to protect coral reefs is to address the root cause of climate change. And that means reducing the human emissions that are mostly from burning of fossil fuels – everything else is looking more like a Band-Aid rather than a solution,” Mr Eakin said.
Scientists also warn there isn’t enough time.
“I think people really need to recognise what they’re doing – inaction is the kiss of death for coral reefs,” said Melanie McField, co-chair of the Caribbean Steering Committee for the Global Coral Reef Monitoring Network, a network of scientists that monitors reefs throughout the world.
