Coral bleaching endangering reefs worldwide

Coral bleaching is among the most visual indicators of climate change. The image on the left was taken in American Samoa in December 2014, the one on the right in February 2015. In a few short months, this precious coral reef was devastated by warming ocean waters.

Corals live based on symbiosis with algae that grows within their tissue - but as oceans warm, corals expel this algae, which causes coral "bleaching." Scientists say bleaching of corals - like at this famous reef at Kaneohe Bay in Hawaii - is happening at an alarming rate.

The XL Catlin Seaview Survey set to work earlier this year, documenting the bleaching of coral reefs around the world. Shot with high-tech camera equipment by divers and underwater robots, some of the images have been uploaded to Google Street View.

The plant material living inside healthy corals - like these at Airport Reef in American Samoa, before the bleaching started - provide food for the whole organism.

Oceans absorb more than 90 percent of the earth's temperature increase, making corals a key indicator of climate change. Coral reefs also support vast biodiversity, including around 25 percent of all marine species.

A close-up view shows the white skeleton left exposed when a coral expels the algae living inside its tissue. This tolls a death knell for corals - if warmer conditions persist, the reefs could die off for good.

Once the plant matter has been expelled, the coral starves and eventually dies.

That's bad news for the myriad of marine life supported by coral reefs - like this long-nose file fish, which is completely reliant on healthy corals for food.

A fire coral, shown before and after bleaching. Scientists say it takes decades for coral reefs to cover from this kind of damage - if they ever do.

This year's coral bleaching phenomenon has been intensified by climate change and El Nino, scientists say - who add that the worst may be yet to come. Accurate mapping of coral reefs promotes understanding of global bleaching events, which could help toward developing responses to save the world's endangered reefs.