Deadly Worldwide Coral Bleaching Episode Underway--Earth's 3rd on Record

The strength of the current El Nino is increased by global warming.

By: Bob Henson , 12:47 PM GMT on October 09, 2015

Earth is entering its third worldwide coral bleaching event of the last 20 years--a disturbing example of how a warming planet can harm vital ecosystems--NOAA announced on Thursday. NOAA also released an eight-month outlook that projects even more bleaching to come in 2016. The only other global-scale bleachings in the modern era of observations happened in 1998 and 2010. Global bleaching is defined as an event that causes bleaching in each of the planet’s major coral-reef areas. "We may be looking at losing somewhere in the range of 10 to 20 percent of the coral reefs this year," NOAA coral reef watch coordinator Mark Eakin said, in an interview with Associated Press. Florida started getting hit in August. The middle Florida Keys aren't too bad, but in southeast Florida, bleaching has combined with disease to kill corals, Eakin said. It has also hit Cuba, Haiti and the Dominican Republic and is about to hit Puerto Rico and the Virgin Islands, he said, adding, "you kill coral, you destroy reefs, you don't have a place for the fish to live."

The current global bleaching is the culmination of regional problems that began in mid-2014, when very warm conditions emerged in several parts of the tropics. Hawaii is one of those areas: as Jeff Masters reported in July, Hawaii experienced its worst bleaching on record in 2014 when record-warm ocean temperatures caused 50 - 70% of the corals sampled in Northeast Oahu's Kāneʻohe Bay to bleach.

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"Hawaii is getting hit with the worst coral bleaching they have ever seen, right now," Eakin said. "It's severe. It's extensive. And it's on all the islands." In one part of northwestern Hawaii, "the reef just completely bleached and all of the coral is dead and covered with scuzzy algae."

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El Niño isn’t helping
Rising global temperatures are increasing the likelihood of bleaching, but it is often El Niño that pulls the trigger for the most widespread events. A strong El Niño can suppress upwelling and raise sea-surface temperatures across much of the central and eastern tropical Pacific and other low-latitude areas. Because the algae embedded in coral depend on photosynthesis to survive, coral reefs are limited to the uppermost reaches of the ocean, where sunlight can filter through. When the sea surface temperature is 1°C warmer than the highest monthly mean temperature corals usually experience, coral polyps will expel the symbiotic algae that live in their tissues, exposing the white skeleton underneath and resulting in a "bleached" appearance. Death can result if the stress is high and long-lived--for instance, if unusually warm ocean temperatures persist for months.

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Disease fostered by warmer temperatures is a major threat to coral reefs in its own right, as explored in a 2015 study led by Jeffrey Maynard (Cornell University).

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