Most researchers suspect the current crisis is caused by higher sea temperatures. As the water heats up, the coral polyps that build the reef with their skeletal remains spit out the microscopic algae that help feed the coral and give the reef its golden, red and yellow hues. The phenomenon is called "bleaching" because it leaves the coral with white blotches. Without its algal partner, the coral becomes weak and stops reproducing. After several weeks, it may die.

Many marine biologists believe that the coral bleaching is a sign that the delicate reef ecosystems are suffering from stress. Some suggest that the bleaching episodes are the first biological effect of global warming, and that like canaries in a coal mine, the reefs are a danger signal.

Until the worldwide bleaching episode of 1987, the periodic phenomenon was "virtually ignored," [Ernest Williams] said. He and his colleagues found that there was also widespread bleaching in 1979-80 and 1982-83. There have been isolated reports of bleaching since 1911, though only in recent years have the episodes been so widespread. In addition to higher temperatures, coral reefs may be harmed by pollution, changes in salinity and silt deposits.