Coral reefs around the world have been subjected to unprecedented heat stress since early 2023. A new report finds heat-related coral bleaching has damaged corals in more than 80 countries, making it the most extensive bleaching event ever recorded, with no clear end in sight.
Between January 2023 and April 2025, heat stress impacted 84% of coral reefs worldwide, from the Mesoamerican Reef in the Caribbean to so-called supercorals in the Red Sea, an area previously believed to be resilient to damage caused by extreme temperatures.
“The fact that this most recent, global-scale coral bleaching event is still ongoing takes the world’s reefs into unchartered waters,” Britta Schaffelke, coordinator of the Global Coral Reef Monitoring Network, said in a statement.
Scientists officially confirmed the bleaching event in April 2024, making it the fourth such global event since 1998. It comes on the heels of the third global bleaching event that ended just six years earlier, in 2017, impacting 68% of coral reefs. The short recovery time between events makes it difficult for corals to adequately rebuild.
Coral bleaching happens when water temperatures become too warm and corals expel the colorful algae that live inside them, leaving reefs a ghostly white. Thousands of fish and other marine species are left without a safe habitat. These vibrant ecosystems support roughly a quarter of all marine life at some point in their life cycle.
“Bleaching is always eerie — as if a silent snowfall has descended on the reef — there is usually an absence of fluttering fish and an absence of the vibrant colors on the reef,” Melanie McField, a Smithsonian scientist who directs a coral monitoring program for the Mesoamerican Reef, said in a statement.
Oceans absorb roughly 90% of the excess heat from climate change driven by fossil fuel emissions. As a result, waters have been warming with each passing decade. In 2024, the world saw the highest average sea surface temperatures on record, 0.6° Celsius (1.1° Fahrenheit) above the 1981-2010 baseline.
“The magnitude and extent of the heat stress is shocking,” McField added. “Some reefs that had thus far escaped major heat stress and we thought to be somewhat resilient, succumbed to partial mortalities in 2024.”
For Peter Thomson, the U.N. secretary-general’s special envoy for the ocean, the path to protecting coral reefs is clear: curb emissions. “There are many reasons for the demise of coral reefs, but let’s not beat around the bush,” he said in a statement. “If we want coral reefs to survive, we must drastically reduce our emissions and keep global warming below 1.5 degrees Celsius [2.7°F].”
Banner image: Coral bleaching in Ningaloo, Australia, in February 2025. Image courtesy of Daniel Nicholson/Ocean Image Bank.
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