Australian ocean specialists see a warning sign offshore for the second year in a row. Massive stretches of formerly colorful sea coral, a living ecosystem just below the surface, have turned pale. That draws concern beyond the South Pacific continent because coral bleaching is linked to global warming. Rising sea temperatures cause the expulsion of algae that grow inside coral, turning the reefs white and eliminating their main energy source.

Australia's top environmental official calls climate change the "number one threat" to the Great Barrier Reef, which has the world's largest array of coral. With around 400 coral types and 1,500 fish species, it's a UNESCO World Heritage site and a major source of tourism and fishing revenue. Bleaching hit the same area in 2016, as well as reefs off Hawaii, Florida, Caribbean Islands and American Samoa. Australian scientists hoped 2017 would offer the delicate area a reprieve, but a recent aerial survey shows the opposite. Two thirds of the reef's corals are now bleached in a zone stretching for 900 miles.

A month-long temperature rise of just one or two degrees above the maximum average is enough to push corals out of their comfort zone. An estimated 275 million people globally directly rely on reefs for livelihood and food, according to the United Nations, which says they serve as nurseries for around one-fourth of the world's fish.

If temperatures drop, the vital algae can return to bleached coral. But if seas remain too warm, eventually the coral will die -- removing the natural habitat for many marine life species. "The significance of bleaching this year is that it's back-to-back, so there’s been zero time for recovery," says marine biologist Terry Hughes of James Cook University in Queensland, Australia, who led the new aerial survey.

Marine biologist says: "When it's so hot for this extended period of time, the corals don't just bleach. They cook and they die very quickly." -- James Kerry, Australian researcher

Retired official says: "We've been denying it for so long, and now we’re starting to accept it. But we're spending insufficient amounts addressing the problem." — Jon Day, a director of the Great Barrier Reef Marine Park Authority from 1998-2014

Environmental activist says: ”The [Great Barrier] Reef is now in danger thanks to our government's inaction on climate change. The government must take action on the root cause of coral bleaching -- and that is climate change, fueled by mining and burning fossil fuels like coal." -- Sebastien Blavier, Greenpeace in Australia