Coral bleaching


  • Coral bleaching occurs when coral polyps expel algae that live inside their tissues. Normally, coral polyps live in an endosymbiotic relationship with these algae, which are crucial for the health of the coral and the reef.
  • The algae provides up to 90 percent of the coral’s energy. Bleached corals continue to live but begin to starve after bleaching.
  • The leading cause of coral bleaching is rising water temperatures.
  • A temperature about 1 °C (or 2 °F) above average can cause bleaching.
  • According to the United Nations Environment Programme, between 2014 and 2016 the longest recorded global bleaching events killed coral on an unprecedented scale.
  • In 2016, bleaching of coral on the Great Barrier Reef killed between 29 and 50 percent of the reef’s coral.
  • In 2017, the bleaching extended into the central region of the reef.
  • The average interval between bleaching events has halved between 1980 and 2016.


Factors responsible for Coral Bleaching

  • Increased water temperature (most commonly due to global warming), or reduced water temperatures
  • Oxygen starvation caused by an increase in zooplankton levels
  • Increased solar irradiance (photosynthetic active radiation and ultraviolet light)
  • Increased sedimentation (due to silt runoff)
  • Bacterial infections
  • Changes in salinity
  • Herbicides
  • Extreme low tide and exposure
  • Cyanide fishing
  • Pollutants such as oxybenzone, butylparaben, octyl methoxycinnamate, or enzacamene: four common sunscreen ingredients that are nonbiodegradable and can wash off of skin
  • Ocean acidification due to elevated levels of CO2 caused by air pollution
  • Being exposed to Oil or other chemical spills



The future of the Great Barrier Reef

The largest coral reef in the world, the Great Barrier Reef, is home to at least 400 individual species of coral and thousands of different species of fish, mollusks, sea snakes, sea turtles, whales, dolphins, birds and more. As with the other coral reefs of the world, this incredible ecological hotspot is under threat.

A heat wave in 2016 caused a large percentage of the corals in the Great Barrier Reef to undergo severe bleaching and death. A 2018 study in the journal Nature Communications found that in just the northern third of the reef, over 60 percent of the shallow-water corals (those below 49 feet, or 15 meters) experienced some degree of bleaching, and 30 percent of the coral died. The study also found that even in the deeper, less-explored areas of the reef (down to about 131 feet or 40 m), nearly 40 percent of the corals had at least partial bleaching.

Healthy reefs lead to healthy oceans, and healthy oceans are vital to all life on Earth. The destruction facing not only the Great Barrier Reef, but also every reef around the world, can lead to the extinction of thousands of species of marine life. In turn, coastlines currently protected by reefs would more readily flood during storms, some islands and low-lying countries would vanish under the water, and the $30 billion industry that coral reefs provide could collapse.