There was good news and bad news for the world's coral reefs last week. The good news, announced 19 June, is that the global coral bleaching event that started in 2015 appears to be over, according to the U.S. National Oceanic and Atmospheric Administration (NOAA) in Silver Spring, Maryland. The bad news, released 23 June, is that the 3 successive years of bleaching conditions damaged all but three of the 29 reefs that are or are contained within United Nations Educational, Scientific and Cultural Organization (UNESCO) World Heritage sites. And the prognosis is grim: Without dramatic reductions in greenhouse gas emissions, all these reefs "will cease to host functioning coral reef ecosystems by the end of the century," predicts the report from UNESCO’s World Heritage Center in Paris.
Bleaching occurs when overly warm water leads corals to expel symbiotic algae called zooxanthellae. Without the colorful algae, which use photosynthesis to produce nutrients for themselves and their hosts, the corals turn white, or bleach. If the waters cool soon enough, algae return; if bleaching persists, the corals die. Reefs are ecosystems that support more than a million marine species. And an estimated half billion people around the world rely on reefs for livelihoods from fishing and tourism.
NOAA's Coral Reef Watch uses satellite observations of sea surface temperatures and modeling to monitor and forecast when water temperatures rise enough to cause bleaching. In the most recent case, waters in the Atlantic, Pacific, and Indian ocean basins began rising in mid-2014 and bleaching started in 2015. The 3-year duration of this latest event is unprecedented; previous bouts of global bleaching came and went within a year.