Coral Reefs May Suffer Worldwide Bleaching and Decline by 2056

Coral reefs may be in more trouble than we thought. A new study that uses the latest emissions scenarios and climate models suggests that there may be worldwide coral bleaching by 2056.

Coral bleaching occurs when ocean waters become too warm for the coral to handle. When that happens, light becomes toxic for the algae that live within the coral structures. Known as zooxanthellae, this algae helps coral conduct photosynthesis and gives the coral its bright colors. When temperatures warm, though, a coral's zooxanthellae are expelled into the surrounding waters. If temperatures cool quickly, then corals can usually reacquire their zooxanthellae. Yet if temperatures continue to remain at heated levels, the coral eventually starves and dies, leaving a graveyard of calcium carbonate structures on the ocean floor.

Why are reefs so important, though? They help shelter islands from storms, and act as a nursery for juvenile fish before they explore the world's oceans. Protecting reefs can help keep fisheries afloat, and keep islands and other coastal areas from losing coastline due to storms and erosion.

Large-scale bleaching events have occurred before. In 1998, 16 percent of the world's coral reefs were lost after a mass bleaching due to warmer-than-average temperatures. If carbon emissions continue to stay on the projected path, these bleachings will probably continue and become even more frequent.

The study, which was published in Nature Climate Change, used climate model ensembles from the upcoming Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPPC). The researchers found that if trends were to continue, 74 percent of the world's coral would bleach annually by 2045 and that worldwide coral bleaching would occur in 2056. In addition, about a quarter of coral reefs are likely to experience bleaching events annually five or more years earlier than the median year. These reefs include those in northwestern Australia, Papau New Guinea, and some equatorial Pacific islands such as Tokelau.

These findings show that without significant reductions in emissions, coral reefs may be at a severe risk of rapid decline. A reduction of emissions could, in fact, delay annual events in nearly 23 percent of the world's reefs.

The predictions remain grim, though. The researchers involved in the study agreed that the projects that combine threats posed to reefs by increases in sea temperature and ocean acidification will continue to aid the decline of these reefs.