Heat Stress Is Hitting Caribbean Reefs Earlier Than Ever This Year

As the world’s coral reefs suffer a fourth global bleaching event, heat stress in the Caribbean is accumulating even earlier than it did in 2023, the previous record year for the region, according to data made public on Thursday by the National Oceanic and Atmospheric Administration.

“I hate that I have to keep using that word ‘unprecedented,’ said Derek Manzello, coordinator of the agency’s Coral Reef Watch Program. “But, again, we are seeing unprecedented patterns again this year.”

Scientists hope that relief will set in as the natural climate pattern known as El Niño, which is associated with warmer ocean temperatures, fades. Officials said conditions were quickly changing to a neutral state, with a cooler La Niña forecast for this summer or fall.

But right now, temperatures in the Caribbean off Panama, Costa Rica, Nicaragua and Colombia are hitting levels that previously haven’t been seen until weeks later, an ominous signal after the heat that ravaged reefs across the region last year. Scientists say they are still determining the scale of the death from last year’s bleaching. A study of the reefs off Huatulco in Oaxaca, Mexico, found coral mortality ranging from 50 percent to 93 percent, depending on the reef area.

Bleaching occurs when stressors cause coral to lose the symbiotic algae that help to nourish them. While coral can survive bleaching spells, exposure to prolonged or repeated bleaching can kill the tiny animals. Even those that recover initially may succumb to disease in the following year or two.

While all kinds of stressors can cause coral bleaching, including pollution or changes in salinity, the four global bleaching events on record, which started in 1998, have all been caused by warm ocean temperatures. Researchers have estimated that the world has lost half of its coral cover since 1950.

Coral reefs are sometimes called the rainforests of the sea because of the outsize biodiversity they support. A quarter of marine species rely on reefs at some point in their life cycle, and they also protect coasts from storms.

The fourth global event has been growing since early 2023, with bleaching in at least 62 countries and territories. Last summer in the Florida Keys, scientists raced to preserve samples of endangered coral. In recent months, the Great Barrier Reef off Australia was pummeled. Researchers in Brazil are currently documenting the worst bleaching in that country’s history, Dr. Manzello said.

The fourth global bleaching event is already the most widespread in any 365-day period, with 60 percent of reef areas subjected to bleaching-level heat stress. In the Atlantic Ocean, that number is 99.7 percent, officials said.

Last month was the planet’s hottest April ever measured, making it 11 months in a row that the average global temperature has beaten all past records for the time of year.

Climate change is “the cornerstone” of the extreme ocean heat, Dr. Manzello said, but the current spike is even more pronounced than what scientists had expected from human-induced global warming. The natural El Niño climate pattern added to the heat. Scientists also wonder whether additional heating came from the ripple effects of a reduction in shipping pollution, or the eruption of an underwater volcano in 2022.

Raymond Zhong contributed reporting.