This photo released by the Great Barrier Reef Marine Park Authority shows a bleached section of Australia's famous coral reef. Scientists warned the coral could be killed by global warming in decades, but it has regenerated itself in record time. Despite this, a report in September warned that the Great Barrier Reef is still in serious jeopardy. Coral has symbiotic unicellular algae, which gives it its vibrant color. In times of stress, often caused by environmental factors, the coral lose their color and reveal the white of their calcium carbonate skeletons, an event known as bleaching.