The last time this happened, we lost almost a fifth of world reefs.

“Coral reefs cover just 0.1 percent of the ocean floor, but provide habitat to 25 percent of sea-dwelling fish species. That’s why coral scientist C. Mark Eakin, who coordinates the National Oceanic and Atmospheric Administration’s Coral Reef Watch program, is surprised that the warning he has been sounding since last year (PDF)—that the globe’s reefs appear to be on the verge of a mass-scale bleaching event—hasn’t drawn more media attention.

“Bleaching happens when coral loses contact with zooxanthellae, an algae that essentially feeds them nutrients in symbiotic exchange for a stable habitat. The coral/zooxanthellae relationship thrives within a pretty tight range of ocean temperatures, and when water warms above normal levels, coral tends to expel its algal lifeline. In doing so, coral not only loses the brilliant colors zooxanthellae deliver—hence, “bleaching”—but also its main source of food. A bleached coral reef rapidly begins to decline. Coral can reunite with healthy zooxanthellae and recover, Eakin says, but even then they often become diseased and may die. That’s rotten news, because bleaching outbreaks are increasingly common.”  Source: www.motherjones.com