More than 25 percent of the world’s coral reefs have been destroyed by global warming, pollution, and destructive fishing techniques, and most of the rest could be dead in 20 years if serious action isn’t taken to address these problems, according to scientists assembled in Indonesia this week at a major coral reef conference. Global warming is the most serious of the threats facing reefs; rising water temperatures cause the coral to expel microscopic organisms that are crucial to their health, a phenomenon known as coral bleaching. Some reefs have been hit particularly hard by water temperature increases over the past two years. For example, up to 90 percent of coral reefs have been killed around the Maldives and Seychelles islands in the Indian Ocean. Loss of the world’s reefs would not only be a major blow to the environment and biodiversity, but also to the 500 million people around the world who depend to some extent on coral reef systems for their food or livelihood.