In a new article in Nature Climate Change scientists from NOAA's Cooperative Institute for Marine and Atmospheric Studies show maps that illustrate how rising sea temperatures are likely to affect all coral reefs, including those in Polynesia, in the form of annual coral bleaching events under various different emission scenarios. Their results emphasize that without significant reductions in emissions most coral reefs on the planet are at risk for bleaching within the next several decades. (Credit: Thomas Vignaud)

Feb. 25, 2013 — In a study published February 24 in Nature Climate Change researchers used the latest emissions scenarios and climate models to show how varying levels of carbon emissions are likely to result in more frequent and severe coral bleaching events.

Large-scale 'mass' bleaching events on coral reefs are caused by higher-than-normal sea temperatures. High temperatures make light toxic to the algae that reside within the corals. The algae, called 'zooxanthellae', provide food and give corals their bright colors. When the algae are expelled or retained but in low densities, the corals can starve and eventually die. Bleaching events caused a reported 16 percent loss of the world's coral reefs in 1998 according to the Global Coral Reef Monitoring Network.

If carbon emissions stay on the current path most of the world's coral reefs (74 percent) are projected to experience coral bleaching conditions annually by 2045, results of the study show. The study used climate model ensembles from the upcoming Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC).

Around a quarter of coral reefs are likely to experience bleaching events annually five or more years earlier than the median year, and these reefs in northwestern Australia, Papau New Guinea, and some equatorial Pacific islands like Tokelau, may require urgent attention, researchers warn.

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