Changing Planet

Global Coral Bleaching Event puts Reefs at Risk

Researchers announced this month that a massive global coral bleaching event is jeopardizing the health of coral reefs around the world, and the crisis is still heating up. A triple threat of climate change, El Niño and a climate change-induced “warm blob” in the Pacific is causing the ocean to reach unusually high temperatures, stressing the coral animals that build reefs—the cradles of tropical marine life—and causing them to bleach, a stress response that often causes corals to starve, sicken and die. Researchers from the National Oceanic and Atmospheric Administration (NOAA) have gathered evidence suggesting that about 12% of reefs worldwide have already bleached in the last year, and predict that nearly half of those affected (over 12,000 square kilometers, or over 5% of reefs) could disappear forever. Moreover, this warming trend may continue through 2016 making this bleaching event persist over the course of two years—longer than any previously recorded global bleaching episode.

© XL Catlin Seaview Survey
© XL Catlin Seaview Survey

This is only the third global coral bleaching event ever observed, with the first in 1998 decimating 16-19% of the world’s coral reefs and the second in 2010. While coral reefs cover less than one percent of the world’s seafloor, they support approximately one quarter of all life in the ocean. Small but prolific coral polyps grow atop one another to compose vast, complex habitat structures for thousands of fish and invertebrate species that support food webs around the globe. An estimated 500 million people’s livelihoods depend on healthy coral reefs, with worldwide reef-based economies totaling over $30 billion in value. These benefits to humanity are at stake if we allow coral reefs to decline, as is the wellbeing of countless species that depend on reefs for survival.

Bleaching occurs when corals become stressed, commonly from elevated sea temperatures. This stress response leads corals to expel the colorful symbiotic algae called Symbiodinium that live within their tissues and photosynthesize to provide energy to their coral hosts. Upon losing these algae, a coral’s transparent tissues reveal the ghostly white calcium carbonate skeleton within. Bleached corals also become more susceptible to disease and other threats. It is possible for corals to recover from these stressors if they subside quickly, but if high temperatures persist for too long many coral species cannot survive bleaching.

Once a coral dies and its tissues waste away, its bare calcium carbonate skeleton becomes colonized by turfing algae, making it impossible for baby corals to replenish that part of the reef unless algae-grazing fish are able to clear enough bare space for them to settle. Thus, overfishing of herbivorous reef fish exacerbates severe bleaching events by preventing reefs from regenerating. Pollution from sewage and fertilizer runoff further worsens the situation by allowing algae to grow and smother the reef more quickly. “No reefs that experience unusually warm waters are likely to escape unscathed,” coral reef expert Dr. Nancy Knowlton of the Smithsonian Institution told the Washington Post, “but reefs already suffering from overfishing and runoff may have a particularly rough time recovering, based on what we have learned from past bleaching events.”

All in all, the cumulative effects of ocean warming from climate change combined with overfishing and runoff may cause functioning coral reefs to disappear entirely by mid-century, according to Dr. Ove Hoegh-Guldberg, director of the Global Change Institute at the University of Queensland.

Once swaths of bleached corals die it is very difficult for a reef to regain its functionality as a healthy ecosystem. “At best, you’re talking about a recovery time of ten to 20 years,” Mark Eakin, who heads NOAA’s Coral Reef Watch, told the Washington Post. “And most of these places are getting hit once every five years.” Researchers agree that the best chance we could give coral reefs for survival would be to take swift and urgent action to mitigate climate change while eliminating the more tangible threats of overfishing and runoff.

The International Society for Reef Studies—a consortium of coral reef experts including researchers from NOAA, XL Catlin Seaview Survey, The University of Queensland and Reef Check—recently released a consensus statement on coral bleaching and climate change that calls for climate negotiators at the upcoming COP21 climate conference in Paris to “commit to limiting atmospheric carbon dioxide (CO2) concentrations to no more than 450 ppm in the short-term, and reducing them to 350ppm in the long-term.” Enacting such commitments among all nations at COP21 would limit average global warming below 2°C in the short-term and under 1.5°C in the long-term, dampening the effects of climate change to levels that would likely enable coral reefs to persist into the future.

The bleached tip of an Acropora coral. © XL Catlin Seaview Survey
The bleached polyps of an Acropora coral. © XL Catlin Seaview Survey

In addition to agreeing to limit carbon emissions, policymakers could directly enhance the ability of coral reefs—and our ocean as a whole—to be resilient in the face of climate change by developing an international climate plan that includes protecting carbon stores in the ocean. A massive carbon sink, the ocean absorbs so much CO2 that it may soon hit a tipping point and become unable to keep this greenhouse gas from rapidly accumulating in the atmosphere, acidifying the sea and throwing climate change into overdrive. By establishing a global network of marine protected areas—coined as Hope Spots by Mission Blue founder Dr. Sylvia Earle—policymakers could safeguard vast stores of “blue carbon” in the ocean by keeping coastal and marine ecosystems intact.

Please join Mission Blue and pledge to spread awareness about the need to develop an international climate plan that includes protecting carbon stores in the ocean. As legendary oceanographer and Mission Blue founder Dr. Sylvia Earle says, now is the time to act:

“Quite simply, no ocean, no life. No blue, no green. If not for the ocean, there would be no climate to discuss, nor anyone around to debate the issues. We have to represent those who are not at the table. We must give the ocean a voice!”

© XL Catlin Seaview Survey
© XL Catlin Seaview Survey

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Learn more about the current global coral bleaching event at

Courtney is dedicated to sparking public and political support for the protection of marine ecosystems. With a Master of Arts in environmental studies from Brown University, she has a deep understanding of marine conservation science and policy—particularly regarding coral reef ecosystems—and believes in finding creative ways to bring ocean issues above the surface to inspire conservation. Courtney works as Special Projects Consultant for Mission Blue—an ocean conservation organization founded by famed oceanographer Dr. Sylvia Earle—and is also a prominent environmental artist. She was born and raised in San Francisco and lives in Los Angeles.
  • Daniel lopez

    More pictures less words please

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