Overfishing, ocean pollution and global warming are wreaking havoc on the delicate coral, home to more than a quarter of the world's marine life, and many reefs may have lost the ability to replenish themselves, according to a study published online in the journal Science on Thursday.
Kent Carpenter, director of the Global Marine Species Assessment project at Old Dominion University and lead author of the study, tracked 704 of the 845 known species of coral. He found that 241 of those are at risk of extinction, and they're dying at an alarmingly fast rate.
Carpenter's study adds to the already sobering body of evidence on the state of the world's coral.
"Because of the rate of decline of coral reefs, there's a problem with the corals' ability to replenish themselves," Carpenter said. "If some of the corals start to go extinct, then the ecosystem will cease to function."
The study is the first to rate a large body of coral according to criteria established by the International Union for the Conservation of Nature's Red List of Threatened Species, the world's most comprehensive threatened-species list.
John Bruno, a marine ecologist at the University of North Carolina at Chapel Hill, called the study a milestone for coral reef conservation efforts.
"It's not new in the sense that it's saying that corals are in trouble," Bruno said. "There's been lots of research saying that populations are declining. But it's new in showing that they are threatened by extinction."
Coastal pollution, overfishing, and logging and other land use all contribute to rapid coral decline.
"Overfishing is unbelievable in the ocean's reefs," Bruno said. "And there are a lot of local problems from sediment run-offs from farms and poor land use practices. Sediment make coral sicker."
Warming water temperatures compound the problem by disrupting the symbiotic relationship between coral and the algae that live inside it and are essential for its survival. As temperatures rise, the inner algae, or zooxanthella, are expelled in a process known as bleaching, making coral even more susceptible to die-offs and disease.
"Bleaching and disease events are becoming more frequent and more widespread," Carpenter said. "And most scientists agree that it's directly linked to climate change."
Carbon emissions are also changing the pH level of the world's oceans, according to recent research. Oceans have absorbed about 40 percent of the carbon dioxide emitted by humans, according to a study released in Science earlier this month. As a result, oceans are becoming more acidic. That further damages marine life and reef habitats, and it allows jellyfish and toxic algae to grow unchecked.
Caribbean reefs -- such as the Elkhorn and the Staghorn -- are in particularly bad shape, as are reefs in the Philippines, where Carpenter does most of his work, he said.
In 1998, marine biologists witnessed a massive coral bleaching event that sent marine diversity tumbling and resulted in a mass decline of the world's reefs. Yet some of them later recovered, Carpenter pointed out.
His concern now is that many reefs have lost the ability to recuperate, as they did in the past.
"The prime problem is the frequency," Carpenter said. "If the local threats combined with the global threats make die-offs more frequent, then coral won't be able to recuperate in time. Then they're going to be lost."
Coral loss doesn't just impact marine animals, he added, but the hundreds of millions of people living along coastlines. "If we do not curb our CO2 problem in our atmosphere and enact strong coral reef conservation measures, then we are going to lose our most important ecosystem."