Ocean acidification is another threat to corals caused by global warming, along with rising sea levels, higher sea surface temperatures and coral bleaching, the scientists said.

Coral reefs are a "sentinel ecosystem," a sign that the environment is changing, said one of the experts, Billy Causey of the US National Marine Sanctuary Program.

"Although ocean acidification is affecting the health of our oceans, the same thing -- increased carbon dioxide in the atmosphere -- is going to in fact be affecting terrestrial environments also," Causey said by telephone from Hawaii.

Coral reefs offer economic and environmental benefits to millions of people, including coastal protection from waves and storms and as sources of food, pharmaceuticals, jobs and revenue, the declaration said.

But corals are increasingly threatened by warming sea surface temperatures as well as ocean acidification.

Oceans are getting more acidic because they have been absorbing some 525 billion tonnes of the greenhouse gas carbon dioxide over the last two centuries, about one-third of all human-generated carbon dioxide for that period.

The carbon dioxide combines with sea water to form carbonic acid.

Marine researchers have long recognized acidification in deep ocean water far from land, but a study published this year in the journal Science found this same damaging phenomenon on the Pacific North American continental shelf from Mexico to Canada, and quite likely elsewhere around the globe.

The water became so corrosive that it started dissolving the shells and skeletons of starfish, clams and corals.

Stabilizing carbon dioxide emissions was the Honolulu Declaration's top long-term recommendation. The key short-term recommendation was to nurture coral reefs that seem to have natural resilience against acidification.

This could be adopted immediately by managers of protected marine areas, Causey said.

The Honolulu Declaration will be presented to the United Nations and to other global, regional and national forums.