Human-caused climate change, rather than the naturally occurring El Nino climate pattern, has been the primary driver behind every global coral bleaching event over the last four decades, according to a new international study that warns the world’s coral reefs could disappear without rapid cuts in greenhouse gas emissions.
The study, led by Climate Central and published in the journal Oceanography, concludes that global mass coral bleaching events in 1998, 2010, 2014-17 and the ongoing 2018-2025 event would not have occurred without the warming caused by fossil fuel emissions.
Using a climate attribution approach, researchers separated the influence of human-induced warming from natural climate variability, including El Nino. They analysed daily sea surface temperatures using 13 climate model simulations and NOAA’s Coral Reef Watch bleaching model to compare observed ocean temperatures with a hypothetical world unaffected by human-caused climate change.
“Our study shows that without climate change, coral bleaching would be a rare and isolated event, and global mass coral bleaching simply would not occur,” said Andrew Pershing, chief program officer at Climate Central and the study’s lead author.
“Millions of people and entire countries rely on healthy coral reefs for livelihoods and food security. But our emissions of carbon pollution are causing increasingly widespread damage and death to these vital and vibrant ecosystems. Without immediate emissions reduction, coral reefs as we know them will disappear.”
The researchers found that during the 2018-2025 global bleaching event, bleaching would have been virtually absent in a world without human-induced warming.
Of the 71 coral reef regions, where bleaching has been documented, only one region, the central Kuroshio Current near Japan, would have experienced even moderate bleaching risk under natural climate conditions. The remaining 70 regions would have experienced little or no temperature-driven bleaching risk. Fifty-one of those regions would have recorded degree heating weeks below one, indicating minimal thermal stress. The findings overturn the long-held perception that El Nino is the principal cause of mass coral bleaching. While El Nino contributes to short-term ocean warming, the study concludes it merely amplifies an already elevated baseline created by decades of global warming.
Researchers found similar patterns for all previous global bleaching events. Even during the first mass bleaching event in 1998, which coincided with one of the strongest El Nino episodes on record, human-induced warming was responsible for pushing ocean temperatures beyond bleaching thresholds across almost every affected reef.
Global warming overtaking El Niño
The study also examined the relative influence of global warming and El Nino across 100 coral reef regions worldwide.
Researchers found that as early as 1989, the effect of rising global temperatures had already become stronger than El Nino in half the world’s coral reef regions.
By 1998, when the first global bleaching event occurred, global warming had become the dominant driver in 61 out of 74 regions where both climate change and El Nino significantly influenced bleaching.
Today, with global temperatures around 1.35°C above pre-industrial levels, warming exceeds the influence of El Niño in all but three coral reef regions. At current warming rates, climate change is projected to outweigh El Niño everywhere by 2028.
The authors conclude that climate change has been the principal driver of coral bleaching risk throughout the past 40 years, with warming now exceeding natural climate variability in 93% of coral reef regions analysed. All coral reef regions are expected to cross that threshold by 2040.
“Without human-induced climate change, reef-wide bleaching would be exceedingly rare and global bleaching events simply would not occur,” the researchers wrote, adding that El Nino alone is no longer sufficient to trigger widespread bleaching.
Oceans continue to set temperature records
The findings come as global oceans continue to experience unprecedented warming.
According to the latest Copernicus Marine Service mid-year bulletin, the first six months of 2026 were the second warmest on record, with an average global sea surface temperature of 20.94°C between January and June, just below the record set in 2024.
June 2026 itself became the warmest June ever recorded, with average global sea surface temperatures reaching 21.0°C.
Marine heatwaves expanded steadily through the first half of the year, affecting approximately 82 per cent of the global ocean by the end of June, second only to 2024, when about 83 pc of the oceans experienced marine heatwave conditions. The tropical Pacific matched its previous record for January-June average sea surface temperature at 26.91°C, while June 2026 set a new monthly record of 27.26°C. Scientists said the exceptionally warm conditions are consistent with the development of a new El Niño, but stressed that background warming from climate change is intensifying these natural cycles. Persistent marine heatwave hotspots were observed across the tropical Pacific, subtropical North Atlantic, waters west of Europe and the eastern Pacific off Peru, Chile and California.
Bleaching risk expected to continue
Applying the same attribution method, researchers also projected bleaching risk during the 2026-27 El Nino event now underway. While El Niño alone may produce limited bleaching in a few locations, human-driven warming is expected to trigger widespread bleaching across much of the world’s coral reefs, particularly in the southern Caribbean, the Pacific coasts of Central and South America and along eastern Asia.
The study warns that continued warming is outpacing corals’ ability to adapt. Although some reefs have shown temporary resilience after previous bleaching events, adaptation through acclimatisation, genetic change or shifts in coral species is unlikely to keep pace with rapidly rising ocean temperatures.
Calls for a ‘Blue COP’
The new findings have also intensified calls to place oceans at the centre of global climate negotiations.
More than 150 scientists, policymakers and conservation leaders, including the World Resources Institute, UNESCO’s Intergovernmental Oceanographic Commission, Woods Hole Oceanographic Institution and renowned oceanographer Sylvia Earle have issued an open letter urging hosts of COP31 to make the upcoming UN climate summit a “Blue COP.” The signatories argue that the ocean absorbs more than 90 pc of excess heat generated by climate change and captures around one-quarter of global carbon dioxide emissions. They estimate that scaling up ocean-based climate solutions, including restoring blue carbon ecosystems, expanding offshore renewable energy,
decarbonising shipping and promoting sustainable fisheries, could deliver up to 35 pc of the emissions reductions required to keep global warming below 1.5°C under the Paris Agreement.
