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Extreme Heat Bleaches Coral, and Threat Is SeenThis year’s extreme heat is putting the world’s coral reefs under such severe stress that scientists fear widespread die-offs, endangering not only the richest ecosystems in the ocean but also fisheries that feed millions of people. From Thailand to Texas, corals are reacting to the heat stress by bleaching, or shedding their color and going into survival mode. Many have already died, and more are expected to do so in coming months. Computer forecasts of water temperature suggest that corals in the Caribbean may undergo drastic bleaching in the next few weeks. What is unfolding this year is only the second known global bleaching of coral reefs. Scientists are holding out hope that this year will not be as bad, over all, as 1998, the hottest year in the historical record, when an estimated 16 percent of the world’s shallow-water reefs died. But in some places, including Thailand, the situation is looking worse than in 1998. Scientists say the trouble with the reefs is linked to climate change. For years they have warned that corals, highly sensitive to excess heat, would serve as an early indicator of the ecological distress on the planet caused by the buildup of greenhouse gases. “I am significantly depressed by the whole situation,” said Clive Wilkinson, director of the Global Coral Reef Monitoring Network, an organization in Australia that is tracking this year’s disaster. According to the National Oceanic and Atmospheric Administration, the first eight months of 2010 matched 1998 as the hottest January to August period on record. High ocean temperatures are taxing the organisms most sensitive to them, the shallow-water corals that create some of the world’s most vibrant and colorful seascapes. Coral reefs occupy a tiny fraction of the ocean, but they harbor perhaps a quarter of all marine species, including a profusion of fish. Often called the rain forests of the sea, they are the foundation not only of important fishing industries but also of tourist economies worth billions. Drastic die-offs of coral were seen for the first time in 1983 in the eastern Pacific and the Caribbean, during a large-scale weather event known as El Ni