Extreme Heat Toasted the Caribbean’s Corals

This article was originally published by Hakai Magazine.

In the Northern Hemisphere, the summer of 2023 was the hottest on record. In the Caribbean, coral reefs sat in sweltering water for months—stewing in a dangerous marine heat wave that started earlier, lasted longer, and climbed to higher temperatures than ever recorded in some locations. In some places, the water was more than 32  degrees Celsius—as toasty as a hot tub. Ever since the water started to warm, researchers and conservationists have been anxiously watching to see how the debilitating heat has affected the region’s corals.

For many Caribbean corals, last year’s heat proved too much to bear. The more time corals spend in hot water, the more likely they are to bleach, turning white as they expel the single-celled algae that live within their tissues. Without these symbiotic algae—and the energy they provide through photosynthesis—bleached corals starve. Survival becomes a struggle, and what was a healthy thicket of colorful coral can turn into a tangle of skeletons.

Corals can recover from bleaching. But while some Caribbean corals survived last year’s bleaching, and others were unaffected, multitudes perished. And for many corals, the harrowing experience isn’t even over.

Lorenzo Álvarez-Filip, a marine ecologist at the National Autonomous University of Mexico, says that, for a coral, recovering after bleaching is like recuperating from a long illness. It takes time. Yet even now, several months after the water has cooled to temperatures that no longer stress corals, researchers across the Caribbean are still finding bleached corals living in limbo.

[Read: How coral researchers are coping with the death of reefs]

In the Bahamas, where the shallowest reefs were hit particularly hard, Valeria Pizarro, a marine biologist at the Perry Institute for Marine Science, started to see some bleached corals recover in October and November 2023, gradually regaining patches of color as symbiotic algae recolonized their still-living tissues. But as recently as January 2024, she and her team were still finding bleached corals that had yet to regain their algal allies.

“Some days it’s just frustrating,” says Pizarro.

Last summer’s extreme heat also bleached and killed many of the corals within parts of the Mesoamerican Reef—the Western Hemisphere’s largest barrier-reef system, which stretches from the coast of Mexico’s Yucatan Peninsula south to Belize, Guatemala, and Honduras. At the Mexican end of the barrier reef, calm water near shore rose to some 3 degrees Celsius warmer than normal, causing widespread coral mortality. The same was true farther south, in a shallow lagoon of the barrier reef in northern Belize.

Although these shallow reefs suffered heavier losses, Álvarez-Filip says corals in the deeper reefs he surveyed also experienced widespread bleaching. Even 50 to 80 feet below the waves, “it was just bright white everywhere,” Álvarez-Filip says. “It was really hard to find a coral that was not bleached.”

Many of these corals in deeper water have been left partially dead and partially alive, says Álvarez-Filip. Because each coral is usually a colony, some clones—genetic copies of the parent coral—can die while others survive, which leaves the coral with dead patches. Although grim, it’s better than the outcome in the shallow lagoon he monitored, where many corals died completely.

Even amid such sweeping losses, however, not all Caribbean reefs were decimated by the heat.

On certain Bahamian reefs, Pizarro says, coral survival rates were much higher. There, some corals didn’t bleach at all, while others did but have already recovered. A sprawling archipelago of hundreds of islands, the Bahamas includes broad, turquoise shallows where water is likely to overheat. But it also includes locations where currents bring cooler water into the reefs, which may have helped protect the corals.

Another apparent sanctuary was Mexico’s Limones Reef, where large groups of branching elkhorn coral held on to their deep-orange color. According to temperature sensors within the reef, the water was a bit cooler than in other reefs—still warmer than normal, but not as deadly.

As winter once again turns to spring in the Northern Hemisphere, researchers in the Bahamas and Mexico will be looking into how corals in some locations were able to avoid bleaching, and investigating whether those animals owe their success entirely to cooler conditions, or whether they themselves are more able to cope with heat.

Mass coral bleaching was first observed in the early 1980s and has become more common, especially in years when tropical waters are heated by both climate change and El Niño, which is what happened in 2023. Although last year’s heat was more extreme than anything recorded before in parts of the Caribbean, it may be a harbinger of things to come: As the planet continues to warm, marine heat waves may become more common and more intense.

But amid all the loss, “there are some corals that have energy and are resistant,” Pizarro says. “We need to keep working for them.”