Prof Justin Marshall, a marine biologist and neuroscientist, has witnessed the coral around Lizard Island on the Great Barrier Reef change from a healthy brown to white over 30 years as a result of bleaching. He says he has never seen it in a worse state. Coral bleaching is caused by the ocean warming over a long period, which in this case is believed to have been caused by the El Niño effect, on top of climate change over many years