Climate change, not El Niño, caused coral bleaching on a global scale

Global coral bleaching events would be impossible without climate change caused by greenhouse gas emissions from human activities. That is the conclusion of a new study conducted by researchers from the Climate Central group and published on Thursday (23) in the journal Oceanography.

Contrary to previous beliefs, the scientists concluded that the occurrence of El Niño is not sufficient to cause mass bleaching on a global scale — something that has happened four times in human history, all within the last three decades: 1998, 2010, 2014-2017, and 2018-2025.

During the most recent event, the analysis indicates that almost no bleaching would have occurred, even locally, without global warming. Of the 71 regions where the phenomenon was observed during the period, only 1 would have presented a moderate risk of bleaching in a world without climate change.

The latest global event severely affected the Caribbean, Central America, and South America. Brazilian corals were heavily impacted.

"Furthermore, all previous events, including the original one in 1998, bore the hallmarks of human-caused climate change," the article states.

"Our study shows that, without climate change, coral bleaching would be a rare and isolated event, and mass bleaching on a global scale simply would not occur," says Andrew Pershing, director at Climate Central and lead author of the study.

Climate attribution

All global bleaching events coincided, partially or entirely, with periods of El Niño (characterized by the seasonal warming of surface waters in the equatorial Pacific), which makes it difficult to isolate the influence of climate change — driven primarily by the burning of fossil fuels and deforestation.

To reach their findings, the researchers used climate attribution models, as the field of study that determines the influence of the climate crisis on extreme weather events and their consequences is known.

Two versions of the ocean were compared: the real one, with the current 1.35°C of human-caused warming, and a simulated version that mimics the planet's temperature before the Industrial Revolution and estimates what conditions would be like without greenhouse gas emissions.

However, since corals have adapted throughout their evolutionary process to the conditions of the different ocean locations they inhabit, using an absolute temperature figure to analyze bleaching was not sufficient.

The two climate versions were therefore analyzed using the official NOAA indicator — the United States agency for climate and oceans — for predicting coral bleaching. The method analyzes marine warming above the historical maximum average over prolonged periods, in 12-week intervals. The calculation was applied to one hundred coral reef regions around the world, between 1982 and 2025.

A second statistical model compared, region by region, the relative weight of global warming and El Niño-driven temperature variation on bleaching risk.

"According to our analysis, major El Niño events amplify the risk arising from long-term warming, but the temperature variations caused by El Niño would not, on their own, be intense enough to trigger widespread bleaching," the authors state.

Advancing ocean warming

Researchers also highlight the growing trend in the number of regions at risk of bleaching since the 1980s. "This suggests that many regions are now facing stressful temperatures multiple times each decade," they write.

In 2023, due to record heat recorded in the world's oceans, NOAA updated the scale used to measure risk to corals. Until then, the maximum alert level was 2 (characterized by severe bleaching and likely significant mortality). Now, the methodology goes up to alert level 5, at which there is risk of near-complete mortality of coral reefs.

"I believe people underestimate the value of healthy reefs. They are home to an enormous number of species, provide food for many communities, generate income through tourism, and help protect coastlines against storms," says Pershing. "Millions of people and entire countries depend on healthy coral reefs for their livelihoods and food security."

In June, the oceans broke the heat record for the month. The researcher states that we are "almost certainly" facing another global bleaching event, given the current scenario of global warming compounded by El Niño.

The color of corals comes from colored microalgae (zooxanthellae) that live in their tissues. Water that is too warm for too long causes them to produce a substance toxic to the coral, which expels them, leaving its calcium carbonate skeleton exposed. A bleached coral is not dead — but it is weakened and susceptible to disease.

Pershing explains that local actions such as reducing water pollution and ending predatory fishing are important, because they help preserve pockets of healthy reefs, which allow for reconstruction in the short and long term.

"However, the pressure on reefs caused by climate change is growing and will continue to grow until carbon pollution is stopped," he warns. "Without an immediate reduction in emissions, coral reefs as we know them will disappear."