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 heatwave that started earlier, lasted longer, and climbed to higher temperatures than any seen in the region before. In some places, the water was over 32 °C—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 had been 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.
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 of 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 have yet to regain their algal allies.
Marine ecologist Lorenzo Álvarez-Filip says that in addition to hard corals, many soft corals bleached as well. Soft corals are usually more resistant to higher temperatures, he says, demonstrating just how bad the marine heatwave was. Photo by Lorenzo Álvarez-Filip
“Some days it’s just frustrating,” says Pizarro.
Last summer’s extreme heat also bleached and killed nearly all 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 3 to 4 °C warmer than normal, causing widespread coral mortality. The same was true farther south, in a shallow lagoon of the barrier reef in northern Belize.
While these shallow reefs suffered heavier losses, Álvarez-Filip says corals in the deeper reefs he surveyed also experienced widespread bleaching. Even 15 to 25 meters 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 a colony, some clones 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 that did 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 onto their deep orange color. According to temperature sensors within Limones 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 the animals owe their success entirely to cooler conditions, or if the corals themselves are more able to cope with heat.
Mass coral bleaching first appeared in the early 1980s and has become increasingly common, especially in years when tropical waters are heated by both climate change and El Niño, which is what happened in 2023. While last year’s heat was more extreme than anything seen before in the Caribbean, it may be a harbinger of things to come: as the planet continues to warm, marine heatwaves are becoming more common and more intense.
But even among such heavy losses, “there are some corals that have energy and are resistant,” Pizarro says. “We need to keep working for them.”
