Coral reefs throughout the world face an array of threats: nutrient pollution, starfish predation and deadly bleaching that follows warmer sea temperatures. Yet they provide between $10,000 and $100,000 in economic benefits to nearby communities, according to one estimate, including everything from coastal protection from storm surges to better fishing. More than 75 percent of the world's reefs lie in the Pacific Ocean, where a new analysis reveals that an average of roughly 600 square miles of the marine outcroppings disappeared annually between 1968 and 2004—since 1995 the rate of their destruction has doubled.

"Global coral loss began earlier and is far more rapid and geographically extensive than we anticipated," says marine biologist John Bruno of the University of North Carolina at Chapel Hill. Twenty years ago, many reefs boasted living coral on more than 60 percent of their surfaces, he adds, but "today very few reefs in the Indo-Pacific, only about 1 or 2 percent, have coral cover close to the historical baseline."

Bruno and U.N.C. conservation biologist Elizabeth Selig sifted through more than 6,000 surveys, ranging from detailed studies of Australia's Great Barrier Reef to simple photos of reefs taken by sport diving organizations like Reef Check, to assess the state of coral cover. "Living coral cover is a metric of reef habitat quality and quantity analogous to coverage of trees as a measure of tropical forest loss," Bruno says. "The ability to include data from other sources really helped us expand the scope of our study."

By 2003 coral cover had slid from 42 percent in the 1980s to an average of just 22 percent of each reef's surface area, the researchers report in PLoS One. The decline accelerated after 1995, with an average loss of more than 1,200 square miles, or 2 percent of the total of living reef per year—more than double the rate of rainforest loss in the 1990s. "Corals are dying worldwide for a number of reasons, particularly because of pest outbreaks (diseases and predators) but also due to climate change, nutrient pollution, destructive fishing practices and coastal development that can smother corals with sediment," Bruno says.

Reefs in other parts of the world face similar hurdles, but recent research has shown that even severely degraded corals can recover, given the opportunity. "Even on Caribbean reefs in Jamaica, which are highly degraded, we have seen reefs bounce back surprisingly quickly," Bruno notes.

But Bruno says that can only be achieved if greenhouse emissions are reduced, dynamite and arsenic fishing is eliminated and other steps are taken to halt reef killers. Given the economic—and ecological—benefits such reefs provide, that may be a small sacrifice to make.