The fourth mass coral bleaching event started in early 2023 and ended in 2025, according to experts from NOAA. But while the event is over, the recovery process is still underway, and a strengthening El Niño is going to be a major hurdle for the planet’s coral reefs.

Over the span of two years, the biggest coral bleaching event to date impacted 84% of the planet’s coral reef area, reaching at least 83 countries and territories.

The end of the event was officially declared in early June of this year, roughly a year after its actual conclusion, as experts watched for signs of widespread bleaching in the Southern Hemisphere’s summer months from December through February.

Now reefs have entered a recovery period, which typically takes at least 9 to 12 years, according to some studies.

But now, a potentially record-breaking roadblock is growing in the way of that recovery: El Niño.

El Niño spells trouble for coral reefs

An El Niño was officially confirmed in June, and the chance of the strongest El Niño on record is growing; a super El Niño is all but certain.

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Every strong El Niño since 1998 has been accompanied by a global bleaching event, and the fourth global bleaching event on record (in 2010) followed a moderate El Niño from 2009 to 2010.

Because the strengthening El Niño is interrupting early in the recovery phase for the planet’s coral reefs, they are especially vulnerable to abnormal conditions.

The most common cause of coral bleaching is warm ocean temperatures, and it only takes warming by 1 degree Celsius to trigger an event as the algae that feed coral leave as the corals become stressed.

El Niño is, at its very definition, a warming of water in the central and eastern equatorial Pacific Ocean. 

While it’s impossible to say with 100% certainty what impacts will come from any given El Niño, some of the common effects on global weather patterns become more likely the stronger the El Niño becomes.

One impact we’re highly confident of from the upcoming El Niño, whether it reaches “super” or record-breaking status or not, is an increase in global temperatures during the El Niño. As global temperatures rise from El Niño, we are more likely to see coral bleaching heat stress impact our oceans’ most diverse ecosystems.

Coral reefs across much of the north Pacific Ocean, Florida and the Caribbean are at a high risk of coral bleaching heat stress for the months of July through October of 2026, and the El Niño is expected to continue, even strengthen, past that point into 2027.

(MORE: Marine heat wave resistant reefs: selective coral breeding for better heat tolerance)

Coral bleaching events are occurring five times more often than they were just a few decades ago, and our oceans are going to continue to get hotter amid a warming climate.

But researchers are working to find different ways to protect the ecosystems that are not only vital to the health of our oceans - they are also vital to the survival of many humans.

Sara Tonks is a content meteorologist with weather.com and has a bachelor’s and a master’s degree from Georgia Tech in Earth and Atmospheric Sciences along with a master’s degree from Unity Environmental University in Marine Science.