March 29, 2016

The Great Barrier Reef, off Australia’s northeastern coast, is the largest coral reef system on Earth.
Credit: © Gary Bell, Getty Images

Recent studies reveal that the Great Barrier Reef, the world’s largest system of coral reefs, is suffering from severe and record-setting bleaching. Bleaching, as the term suggests, is a whitening coral disease caused by the loss of colorful single-celled algae called zooxanthellae. Most coral depend on zooxanthellae for food and the creation of their limestone skeletons. Coral cannot survive for long without zooxanthellae. The main cause of bleaching is heat stress caused by rising water temperatures produced by global warming. This year’s El Nino, a periodic warming current in the Pacific Ocean, has added to the heat stress. The Great Barrier Reef is made up of more than 3,000 individual reefs that extend about 1,400 miles (2,300 kilometers) along the northeast coast of Australia.

Great Barrier Reef
Credit: WORLD BOOK map

Australia’s Coral Reef Studies group filmed the northern half of the Great Barrier Reef over a period of six days. The group—based at James Cook University in northeast Australia—found that 95 percent of 500 reefs studied showed evidence of severe bleaching, a drastic and record-setting percentage. Professor Terry Hughes, who led the film survey, fears the current bleaching event could kill up to 50 percent of the system’s reefs. The group will soon have results on the lower half of the reef, and the Great Barrier Reef Marine Park Authority will conduct in-water surveys to more exactly determine the extent of the bleaching. This year’s bleaching is the third such event since 1998, when the disease killed 16 percent of the world’s coral reefs. A 2005 bleaching wiped out half the coral reefs in American waters of the Caribbean Sea.

If water temperatures cool, zooxanthellae will return and corals will survive and resume their normal colors. This process can take decades, however, and bleaching stress decreases coral’s future ability to grow and reproduce. It also increases the likelihood of later—and increasingly worse—bleaching events.