Ocean warming has triggered three major bleaching episodes on the Great Barrier Reef over the past two decades. On page 373, Hughes et al. present an analysis of the extent and severity of these events (T. P. Hughes et al. Nature 543, 373–377; 2017).
Corals become bleached when stresses such as high temperature kill the symbiotic algae that lend corals their striking colours (pictured: Great Barrier Reef coral with algae intact) and supply them with energy in the form of sugars. Prolonged bleaching can result in coral mortality.
In 2016, a massive wave of bleaching struck the reef after an El Niño event brought abnormally warm waters to the region. Hughes et al. mapped this bleaching across the entire length of the reef through the use of aerial surveys and underwater measurements.
The authors found that the 2016 event was much greater in scope and severity than previous bleaching events in 1998 and 2002. In 1998, 45% of individual reefs surveyed escaped bleaching. This figure dropped to 42% in 2002 and to just 9% in 2016. Compared with the two earlier episodes, extreme bleaching events — in which more than 60% of corals in a given area lost their colour — were more than four times as frequent in 2016.
Hughes et al. show that the observed distribution of bleaching can be explained by the spatial patterns of ocean warming. Local management of fishing pressure and water quality provided little protection, suggesting that a curb in global warming will be needed to secure a viable future for the world's most spectacular reef system.