The Great Barrier Reef is the world's largest coral reef. Australia's Great Natural Wonder, it consists of almost 3,000 individual reefs, stretching for almost 2,500 kilometres. But severe coral bleaching is damaging what is the world's largest living ecosystem. Divers took these photos last week, apparently showing bleaching in the reef's northern part. They sent them to the National Oceanic and Atmospheric Association, which warns that many reefs will bleach over the next three months. Bleaching happens when the water is too warm, forcing coral to expel living algae, causing it to calcify and turn white. Mildly bleached coral can recover if the temperature drops. Otherwise it may die. Global warming and the effects of El Niño climatic changes are blamed. It's the fourth consecutive year that severe bleaching of the reef has been witnessed.
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