NATION NOWGreat Barrier ReefAdd TopicMass coral bleaching hits the Great Barrier ReefA handout photo made available by World Wide Fund for Nature (WWF) Australia on March 10, 2017, shows the Great Barrier Reef suffering mass coral bleaching for a second year in a row, at Vlassoff Cay, near Cairns, Australia, March 6, 2017. According to WWF Australia, the Great Barrier Reef has now been hit by four mass bleaching events: 1998, 2002, 2016, and 2017.WWF/BIOPIXEL HANDOUT, EPAThis combination of two photos taken in March and May 2016 released by The Ocean Agency/XL Catlin Seaview Survey shows bleached coral, left, and the same coral which has died in Lizard Island on Australia's Great Barrier Reef. Coral reefs, unique underwater ecosystems that sustain a quarter of the world's marine species and half a billion people, are dying on an unprecedented scale. Scientists are racing to prevent a complete wipeout within decades.Richard Vevers And Christophe Bailhache, APThe Great Barrier Reef is suffering a mass coral bleaching for a second year in a row, at Vlassoff Cay, near Cairns, Australia, March 6, 2017.WWF/BIOPIXEL HANDOUT, EPAAccording to WWF Australia, the Great Barrier Reef has now been hit by four mass bleaching events: 1998, 2002, 2016, and 2017.WWF/BIOPIXEL HANDOUT, EPAFeatured Weekly Ad