Caribbean reefs have half as much hard coral now as they did in 1980, a study has found.

The 48% decrease in coral cover has been driven by climate breakdown, specifically marine heatwaves. They affect the microalgae that feed coral, making them toxic and forcing the coral to expel them.

In 2023-24, the region’s coral experienced “the most destructive thermal stress ever recorded”, said Dr Jérémy Wicquart of the Global Coral Reef Monitoring Network, one of the study’s editors. It caused a 16.9% decrease in cover year-on-year.

Forty years ago, a diver would have seen a colourful, vibrant ecosystem that nurtured hundreds of marine species such as spiny lobsters, queen conchs, parrotfish, turtles and sharks. Coral reefs cover less than 1% of the world’s seafloor, but support at least 25% of marine species.

But when Wicquart went diving last year after a conference in Puerto Morelos, Mexico, he was met with bone-white, dying coral.

“All the corals were bleached. All white corals. I was very affected by that. It’s totally different when you see it in the field to when you see it on a graph.”

The bleaching effect is caused by the loss of essential microalgae, which nourish hard coral and give it colour. Without that microalgae the hard coral can recover, but is more often at risk of dying. The destruction is exacerbated by cyclones which buffet the coral, turning it to rubble on the ocean floor.

Coral reefs in the Caribbean generate $6.2bn (£4.6bn) a year through fisheries and tourism, with reef tourism accounting for 10% of the Caribbean’s GDP.

But where there was once coral, macroalgae is thriving. Without competition from coral, its coverage has increased by 85% since 1980. Its growth is encouraged by human activities in the area – overfishing their herbivorous predators in particular.

The study, which was compiled by more than 300 scientists in 44 countries and territories, also highlights conservation successes, which provide hope for the future. The southern Gulf of Mexico has faced severe heat stress since 2023, and yet there researchers discovered old, resilient, coral colonies, free from disease and containing critically endangered coral species.

As a result, the Mexican government created a new marine protection area in the Gulf which connects two existing national parks. Combined, they form a continuous corridor of reef habitats that allow corals and reef species to thrive.

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“The science is unequivocal, but it also points to hope. When pressures are reduced and resources are sustained, Caribbean reefs rebound,” said Sinikinesh Beyene Jimma, chief of marine and coastal ecosystems at the UN environment programme.

The Caribbean’s coral reefs are unique because of the dense human population that surrounds them. The number of people living within 20km of coral reefs has increased by 27.6% since 2000.

This means the reefs are more susceptible to local human activity, on top of human-induced climate change. It also means people have the agency to restore reefs with the right management techniques.

“If you act on climate change, you will reduce thermal stress to coral reefs, and the impact of cyclones. The second major solution is to reduce local threats. You can improve water quality by putting in place better wastewater management. You can restrict mass tourism, and implement marine protected areas. These are ways to improve the coral reef at a local scale,” said Wicquart.