More than half of the world’s coral reefs experienced major bleaching during a global heatwave spanning 2014 to 2017, according to a study published in Nature Ecology & Evolution. The event, the third global coral bleaching event ever documented, surpassed previous episodes in both extent and severity, impacting reefs across the tropics.
Researchers analyzed data from over 15,000 reef surveys worldwide, finding that 80 percent of surveyed reefs experienced moderate or greater bleaching – defined as affecting more than 10 percent of corals – while 35 percent suffered moderate or greater mortality. Predictive models, calibrated from the survey data and incorporating remote sensing of heat stress, estimate that 51 percent of the world’s coral reefs suffered moderate or greater bleaching during the period, with 15 percent experiencing mortality.
The 2014-2017 event differed from previous global bleaching episodes, which typically coincided with El Niño events. This heatwave began before the emergence of El Niño and continued after its dissipation, leading scientists to implicate human-caused global warming as a primary driver. The National Oceanic and Atmospheric Administration (NOAA) responded to the increased severity of heat stress by developing more extreme Bleaching Alert levels, now in use during the ongoing Fourth Global Coral Bleaching Event.
Micronesian reefs were particularly hard hit, experiencing severe heat stress and widespread bleaching in multiple years with limited opportunity for recovery between events. Some reefs, like those near Guam, bleached annually from 2013 to 2017. The event also caused mass bleaching in reefs that had never previously experienced such stress, including areas of the northern Great Barrier Reef.
Coral bleaching occurs when corals, stressed by high temperatures and light levels, expel the symbiotic algae living in their tissues, causing them to turn white. While a brief bleaching event doesn’t necessarily kill coral, prolonged and severe bleaching can lead to disease and starvation. These reefs provide essential ecosystem services, including food for up to a billion people, habitat for economically valuable fish, and protection from storms and waves.
NOAA Coral Reef Watch continues to monitor thermal conditions and provide four-month outlooks for coral bleaching heat stress. As of March 19, 2018, the agency’s data indicated ongoing areas of high heat stress, but a comprehensive assessment of the long-term impacts of the 2014-2017 event remains ongoing.
