Massive bleaching event may be permanently changing the Great Barrier Reef

Enlarge / Although these corals are colored, they've been bleached, in that they have lost their photosynthetic symbiotes. (credit: ARC Centre of Excellence for Coral Reef Studies / Gergely Torda)

The intense El Niño event that started in 2015 drove global air temperatures to new records, helped by the long trend of human-driven warming. But the air wasn't the only thing affected. El Niño is fundamentally about Pacific Ocean temperatures, and those were exceptionally hot as well. One of the unfortunate results of this was a massive bleaching of the corals of the Great Barrier Reef.

While the damage to corals looked dramatic at the time, appearances aren't the same as data, and they don't give a comprehensive view of the damage, much less the corals' ability to recover from the bleaching. Now, a large Australian-US team of researchers has provided a comprehensive overview of the damage to and recovery of the Great Barrier Reef. The results are grim, showing that mass coral die offs started at lower temperatures than we had expected. The overview also shows that the entire composition of sections of the Great Barrier Reef have changed and are unlikely to recover any time soon.

Bleached to death

The corals that build reefs are actually a collaboration between animals (the coral proper) and single-celled algae that form a symbiotic relationship with corals, providing them with nourishment. At high temperatures, this relationship breaks down and causes the corals to lose their photosynthetic guest. The reefs turn white, giving bleaching its name. And, if recovery doesn't happen quickly enough, the corals will starve, causing a mass die off. Complicating matters, different species of coral will bleach at different temperatures and recover at different rates.