Matthew Philips writes in Newsweek’s July 2-9, 2007 issue

Coral reefs are often called the rain forests of the sea. And like their terrestrial counterparts, they’re in big trouble. Since 1980, an estimated 20 percent of the world’s coral reefs have been destroyed—a number that could well triple by the end of the century. Though causes of the demise vary and are still being studied, warming ocean temperatures are a leading factor. Even in tropical climates, home to much of the world’s coral, an increase of just 1 degree Celsius can throw a delicate reef system totally out of whack. The result is often “bleaching” —a process that disrupts the essential symbiotic relationship between the tiny polyps that live inside the hard calcium coral and the tinier algae that live inside the polyp tissue and give healthy reefs their vibrant colors. When temperatures get too hot, the coral polyps become stressed and expel the algae, causing the coral to bleach and eventually die, leaving behind a barren, calcium skeleton devoid of color and life (above, right).
Coral can recover within a year or two if conditions return to normal, but scientists worry about the increased frequency of such events. In 1998 and 2002, the world’s largest coral system—the 1,400-mile Great Barrier Reef off the coast of Australia—suffered major bleaching due to El Niño-related warming. Though much of it recovered, 5 percent of the system was permanently damaged. “The concern is that as global temperatures rise, bleaching will be more frequent and the coral won’t be able to recover,” says marine biogeographer John Guinotte.