One of the Seven Natural Wonders of the World, Australia's Great Barrier Reef, is undergoing "the worst, mass bleaching event in its history," reports NPR, adding to the coral damage already experienced in Hawaii and Indonesia, Mashable notes.

The news comes from Australia's National Coral Bleaching Task Force after aerial surveys of the more than 500 coral reefs that stretch from Cairns to Papua New Guinea.

“This has been the saddest research trip of my life,” Professor Terry Hughes of James Cook University in Queensland states in a March 29, 2016, press release, as reported by Mashable. “Almost without exception, every reef we flew across showed consistently high levels of bleaching, from the reef slope right up onto the top of the reef. We flew for 4,000 kilometers in the most pristine parts of the Great Barrier Reef, and saw only four reefs that had no bleaching. The severity is much greater than in earlier bleaching events in 2002 or 1998.”
The World Heritage Site's luminous beauty woos scientists and tourists alike, but coral bleaching means something is out of whack.

Think of coral bleaching this way: Coral and algae have a relationship, the coral feeds off the algae, which makes the coral pretty colorful. But if the coral gets stressed out, the algae splits, and the coral is weakened (no food) and looks bad. (Don't we all get this way after a breakup?) NPR underscores that algae "disappear when the reefs are exposed to stressful climatic conditions, such as temperatures even a few degrees higher than normal." 

There seem to be two key factors involved, Mashable writes: warm ocean temperatures tied to the El Niño event going on in the tropical Pacific Ocean; and the human factor: global-warming practices that boost ocean temperatures and increase ocean acidity levels, which affect the skeletons of coral.

While scientists say the bleached coral can heal (it can take a decade or more), some scientists worry that 50 percent of the affected coral is gone for good. Though it is summer in the Southern Hemisphere and there's the El Niño factor, one scientist believes the cause of the warm water temperatures is clear, as NPR reports: 

"What we're seeing now is unequivocally to do with climate change," Professor Justin Marshall, a reef scientist from the University of Queensland, explains to the ABC (Australia). "We're seeing climate change play out across our reefs."