Scientists typically use restrained language about nature observations, but words get dramatic when they report on undersea coral turning white – called bleaching – and sometimes dying as record heat takes a devastating toll. Parts of the Great Barrier Reef off nortrheast Australia have suffered the largest annual decline in coral cover since records began nearly 40 years ago, according to the Australian Institute of Marine Science, which has studied about 200 living reefs in the past year. It describes a "graveyard of corals" in a new report that says Australia's second-largest reef, Ningaloo on the western coast, also has experienced repeated bleaching. Both areas simultaneously turned white this year for the first time. Worldwide, more than 80% of reefs are affected at least somewhat (see video below).

Mass coral bleaching, also seen in the Atlantic Ocean and Indian Oceans, is caused by rising water temperatures driven mostly by the burning of fossil fuels. Fragile coral gets stressed and loses its vibrant colors when the water is too warm, especially in shallow areas. (Coral also may bleach because of extremely low tides, pollution or too much sun.)

Offshore reefs normally are bright because of microscopic algae (pronounced AL-gee), which are expelled when water gets too hot and stresses the coral. If the temperature stays high, the coral won't let algae back and will die. Thousands of marine animals depend on coral, including some species of turtles, fish, crabs, shrimp, jellyfish, sea birds and starfish. Reefs provide shelter, spawning grounds and protection from predators. Reefs also absorb the force of waves and storm surges, keeping coastal communities safer. Additionally, reef tourism brings in billions of dollars each year and supports thousands of jobs on islands and in shore communities.

Coral can recover if temperatures aren't too extreme. But only a global reduction in greenhouse gas emissions to limit ocean warming before too much longer will guarantee at least some coral survives, scientists predict. The Great Barrier Reef can bounce back, but there are limits to its resilience," warns Richard Leck of Australia's branch of the World Wide Fund for Nature. "It can't get repeatedly hammered like this. We are fast approaching a tipping point."

Scientist says: "The Great Barrier Reef is such a beautiful, iconic place, it's really, really worth fighting for. And if we can give it a chance, it's shown an inherent ability to recover." -- Mike Emslie, lead researcher for the Australian Institute of Marine Science

Photographer says: "Everything was just completely white. To see it now . . . it's just skeletons of what it was before. It's some of the worst coral bleaching I've ever seen." – Brooke Pyke, describing Ningaloo reef this year

Environmental group says: "If we really want to solve the coral bleaching problem, we must address climate change. That means global, comprehensive and immediate action to reduce emissions." – Lorin Hancock, World Wildlife Fund