A survey of 704 species of coral—tiny polyps with hard shells, some of which form spectacular underwater reefs—has found that nearly 33 percent of them face a greater threat of becoming extinct as the globe warms. The main culprits, according to the study published today in Science: bleaching—when corals expel the algae that normally feeds them and gives them color—as well as disease outbreaks in coral weakened by warming sea-surface temperatures.

"If we cannot manage the [carbon dioxide] in the atmosphere, there's a very good possibility that bleaching events and disease events will be occurring with greater frequency and, if that occurs, there is a good chance that some species are not going to be able to replenish themselves fast enough," says marine biologist Kent Carpenter of Old Dominion University in Norfolk, Va., who led the research. "Add ocean acidification [also caused by rising CO2 levels in the atmosphere], which is even more insidious than ocean warming, and you've got a real dire picture."

Researchers assessed the health of coral species worldwide by measuring declines in their abundance on the reefs and ocean beds they call home and then used criteria developed by the International Union for Conservation of Nature (IUCN) to determine the risk of extinction. Previous studies have found declines of as much as 80 percent in the number of coral living within particular reefs.

"Corals are the backbone of the ecosystem," Carpenter notes, and reefs harbor roughly one quarter of all known marine species—from fish to algae. "What is going to happen to that huge biodiversity that is dependent on coral reefs? We don't know, but our consensus is that it would most probably lead to a massive loss of biodiversity in the oceans."

A similar assessment of coral health in U.S. waters released this week by the National Oceanic and Atmospheric Administration (NOAA) concluded that roughly half of the coral species in these waters are struggling and continue to decline.

"Predictions are that within 50 to 100 years not only will we see decline in growth rates for corals and other shell-dependent species—they may actually begin to dissolve," says marine biologist Jenny Waddell of the NOAA Center for Coastal Monitoring and Assessment, who helped prepare the report. Climate change "is somewhat of an x-factor. We don't really know how resilient corals are."

The U.S. government did, however, take the unprecedented step in 2006 of listing two species of coral—Elkhorn (Acropora palmata) and Staghorn (Acropora cervicornis)—as threatened under the Endangered Species Act. This means that both Caribbean reef-builders face a significant risk of extinction within the next 30 years. Plans for how to deal with that threat and protect the two species are still being finalized, according to Weddell.

But not all marine biologists agree that corals are in dire straits. "Clearly lions and tigers are threatened by extinction when there are currently only a few thousand of them left. But is a coral species, whose population was reduced from maybe a billion to 300 million or even a few hundred thousand, really threatened by extinction? Personally, I very much doubt it," says marine biologist John Bruno of the University of North Carolina at Chapel Hill. "But I think the ecological function of many reef-building corals is threatened by quite drastic losses in their abundances."

Instead of focusing on saving individual coral species, Bruno argues, the overall health of the oceans could be protected by managing the protection of coral reefs to maximize their overall abundance—which would then also have the effect of maximizing the numbers of all the species that rely on them for food or habitat.

The goods news is that coral reefs can recover within decades, according to Bruno and Waddell, a process that has already started to occur at some reefs in the Caribbean and Pacific. But only if they are free of man-made pressures such as water pollution, overfishing and climate change.

And if the tiny polyps continue to be pummeled by these factors? The outlook is grim, Carpenter warns. "Whether or not [coral species] actually do go extinct depends on whether corals continue to have more frequent bleaching events and disease events because of increased sea-surface temperatures," he says. "If these events continue to become more frequent, there's a real possibility."