Current coral bleaching event is the longest known | Science News for Students

Current coral bleaching event is the longest known

Heat stress has made corals eject algae that feed them and give them color
Jul 18, 2016 — 8:56 pm EST
coral bleaching

Healthy coral (left) is a riot of color. When stressed, corals eject the colorful algae they depend on for food (right). Such “bleaching events” threaten coral reefs’ survival. Global warming may make long bleaching events more common, scientists say.

Jim Maragos/U.S. Fish and Wildlife Service (CC BY 2.0)

Coral reefs won’t be out of hot water for quite a while. These normally colorful undersea ecosystems are under increasing stress, mostly because of warming oceans. Now, researchers report that a global coral bleaching event began in June 2014. The longest on record, it has sapped the color out of vast areas of coral — and now threatens their health. The reefs affected cover a larger area than ever before. What’s worse, the bleaching shows no signs of ending.

Corals are tiny animals that dwell mainly in tropical oceans. They live in colonies, on structures made up of the hard exoskeletons of dead corals. These are called reefs. Corals depend on algae that live inside their tissues. Those algae are a source of food and give corals their vibrant colors. But when stressed by heat, corals may eject the colorful algae living inside them and turn a ghostly white. With their major food source gone, reef corals are at risk of dying.

Several climate conditions have combined to foster the long bleaching event. That's what scientists at the National Oceanic and Atmospheric Administration, or NOAA, now report. The first of these is a long-term warming of the planet. The second is a short-term period of warmer surface waters in the Pacific Ocean (called an El Niño). Scientists with NOAA, which studies ocean resources and works to preserve them, presented their findings on June 20. They were taking part in the 13th International Coral Reef Symposium in Honolulu, Hawaii.

Since 1979, coral bleachings across distances spanning hundreds of kilometers have occurred now and again. But none lasted for more than “a year or so,” said NOAA’s Mark Eakin. This current event has already dragged on for two years. And it has threatened more than 40 percent of reefs globally. A similar bleaching threatens more than 70 percent of reefs in U.S. coastal waters.  

NOAA scientists aren’t sure what will end this episode. It could extend into 2017, they said. And more frequent events are possible in the future. Computer programs often are used to predict — or model — what Earth’s future climate will look like. Such climate models “suggest that most coral reefs may be seeing bleaching every other year by mid-century,” Eakin notes. “How much worse that gets will depend on how we deal with global warming.”

coral bleaching infographic



Power Words

(for more about Power Words, click here)

algae   Single-celled organisms, once considered plants (they aren’t). As aquatic organisms, they grow in water. Like green plants, they depend on sunlight to make their food.

coral   Marine animals that often produce a hard and stony exoskeleton and tend to live on the exoskeletons of dead corals, called reefs.

coral bleaching    An event in which warming water or other stressors cause coral colonies to expel the algae that normally live inside them. Algae are a food source for coral (animals) and give them their vibrant colors. When algae are expelled, corals lose their color, turning a ghostly white.

ecosystem    A group of interacting living organisms — including microorganisms, plants and animals — and their physical environment within a particular climate. Examples include tropical reefs, rainforests, alpine meadows and polar tundra.

El Niño   Extended periods when the surface water around the equator in the eastern and central Pacific warms.  Scientists declare the arrival of an El Niño when that water warms by at least 0.4 degree Celsius (0.72 degree Fahrenheit) above average for five or more months in a row. El Niños can bring heavy rainfall and flooding to the West Coast of South America. Meanwhile, Australia and Southeast Asia may face a drought and high risk of wildfires. In North America, scientists have linked the arrival of El Niños to unusual weather events — including ice storms, droughts and mudslides.

exoskeleton   A hard, protective outer body covering of many animals that lack a true skeleton, such as an insect, crustacean or mollusk. The exoskeletons of insects and crustaceans are largely made of chitin.

global warming  The gradual increase in the overall temperature of Earth’s atmosphere due to the greenhouse effect. This effect is caused by increased levels of carbon dioxide, chlorofluorocarbons and other gases in the air, many of them released by human activity.

model    A simulation of a real-world event (usually using a computer) that has been developed to predict one or more likely outcomes.

National Oceanic and Atmospheric Administration, or NOAA    A science agency of the U.S. Department of Commerce. Initially established in 1807 under another name (The Survey of the Coast), this agency focuses on understanding and preserving ocean resources, including fisheries, protecting marine mammals (from seals to whales), studying the seafloor and probing the upper atmosphere.

reef    A ridge of rock, coral or sand. It rises up from the seafloor and may come to just above or just under the water’s surface.

stress   (in biology) A factor, such as unusual temperatures, moisture or pollution, that affects the health of a species or ecosystem. (in psychology) A mental, physical, emotional, or behavioral reaction to an event or circumstance, or stressor, that disturbs a person or animal’s usual state of being or places increased demands on a person or animal; psychological stress can be either positive or negative. (in physics) Pressure or tension exerted on a material object.

tissue    Any of the distinct types of material, comprised of cells, which make up animals, plants or fungi. Cells within a tissue work as a unit to perform a particular function in living organisms. Different organs of the human body, for instance, often are made from many different types of tissues. And brain tissue will be very different from bone or heart tissue.

tropical    Anything associated with the tropics, a region near the Earth’s equator where temperatures are generally warm to hot, year-round.


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NOAA. U.S. coral reefs facing warming waters, increased bleaching. Published online June 20, 2016.