"Climate change really is damaging coral reefs worldwide," said Rich Aronson, senior marine scientist at the Dauphin Island Sea Lab in Alabama. "There is no escaping this conclusion."

Although most people probably think of coral reefs primarily as tourist attractions for divers and the source of those lovely tropical fish in restaurant aquariums, the economic importance of reefs is substantial.

"Coral reefs represent some of the most biologically diverse ecosystems on Earth, providing critical habitat to approximately 25 percent of marine species," according to "Coral Reefs and Global Climate Change," a report produced by the Pew Center on Global Climate Change.

"In addition, these ecosystems provide economic benefits through tourism and fisheries," the Pew report said. It cited a recent estimate that valued the annual net economic benefits of the world's coral reefs at $30 billion.

Coral reefs also appear to be among the most sensitive ecosystems to increasing global temperatures. A number of recent studies have found the reefs are being affected by several threats.

For example, an examination in 2002 of some 1,500 reefs by the group Reef Check found significant evidence of human impact. The study also found a 1997-98 bleaching event -- which usually is caused by rising water temperature -- reduced live coral cover by 10 percent worldwide.

Reef-building corals are colonial animals that work in a symbiotic relationship with single-celled algae called zooxanthellae. The corals feed on the zooxanthellae and the algae nest in the skeletons of dead coral and feed on their waste products.

"In tropical waters," Aronson said, "corals are living at the upper limit of their temperatures. At higher summer temperature, they puke out the zooxanthellae, and change color."

Since colorful pigments associated with reefs come from these algae, their loss causes the bleaching.

With global climate change ratcheting up temperatures, these bleaching events have become more frequent and more severe.

The Pew Report found widespread bleaching events were unknown before 1980. But "(the) atmosphere and the ocean have warmed since the end of the 19th century and will continue to warm into the foreseeable future, largely as a result of increasing greenhouse gas concentrations. El Ni�o-Southern Oscillation events have increased in frequency and intensity over the last few decades. This combination (of warming and intense El Ni�o events) have resulted in a dramatic increase in coral bleaching."

The 1997-98 mass bleaching event included extensive regions where 90 percent of the corals died.

Some corals would be able to migrate northward in the face changing ocean temperatures, but not many.

"It's not just how much warming, it's the rate of warming we're seeing today ... 25 percent of the world's reefs are damaged beyond repair," said Joan Kleypas, a marine biologist with the National Center for Atmospheric Research in Boulder. Reefs also need shallow water to grow and the amount of shallow water is greater in the tropics.

Climate change is not the only explanation for the reef crisis, however. Overfishing and pollution also have been fingered as culprits.

In the current issue of the Journal of Experimental Marine Biology and Ecology, Brian Lapointe, a marine scientist at Harbor Branch Oceanic Institution in Fort Pierce, Fla., wrote that in South Florida, at least, pollution from sewage and agricultural runoff is the main culprit in reef decline.

Seaweed is spreading over reefs, turning them from vibrant organisms into dull mounds of green, Lapointe wrote. This can smother the coral, and prevent fish and other organisms from finding the food and shelter that they need.

There are two competing hypotheses about the spread of seaweed: one that overfishing has reduced the fish grazing on it, and one that nutrients from pollutants have increased the growth of the seaweed.

Lapointe has found much evidence to support the theory that nutrients, mainly nitrogen, are the key controlling factor in seaweed spread.

"Study results supported Lapointe's past research that revealed a critical threshold for nitrogen of about 14 parts per billion, above which damaging seaweed spread is supported and below which it is generally prevented. By comparison, raw sewage is about 40,000 parts per billion nitrogen, while pristine oceanic waters would be about 1 part per billion," according to HBOI.

The Pew report did not dispute the importance of pollutants and overfishing in the declining health of coral ecosystems.

"Human activities including development in coastal areas, overfishing and pollution have contributed to a global loss of over 10 percent of these valuable ecosystems. An additional 15 percent have been lost due to warming of the surface ocean, and climate change will further contribute to coral reef degradation in the decades ahead," the report said.

"It is not reasonable to expect coral reefs to support export fisheries," said Terry Done of the Australian Institute of Marine Science. "Harvesting the grazing fishes is a catastrophe."

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