Coral Bleaching

Latest context: A new study has found that if the trend of declining coral growth continues at the current rate, the world’s coral reefs may cease calcifying around 2054.

Aspirants would find this topic very helpful while preparing for the IAS Exam.


  • Coral polyps are short-lived microscopic organisms that live in colonies. They flourish in shallow, mud-free and warm waters.
  • Coral reefs are the colonies of tiny living creatures that are found in oceans.
  • Coral are bright and colourful because of microscopic algae called zooxanthellae.
  • The zooxanthellae live within the coral in a mutually beneficial relationship, each helping the other survive.
  • The coral and the zooxanthellae share a symbiotic relationship. 90% of the nutrients that are produced by the algae are transferred to the coral hosts.
  • But when the ocean environment changes, for instance, if it gets too hot, the coral stresses out and expels the algae.
  • As the algae leaves, the coral fades until it looks like it’s been bleached.
  • If the temperature stays high, the coral won’t let the algae back, and the coral will die.

Please read in detail about coral reefs in the linked article.

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Causes of Coral Bleaching

  • Extreme climate conditions: High temperature of water leads to the declination of these corals as they cannot survive in high temperatures. As estimated by scientists, most of the coral reefs of the world will soon decline with the increasing rates of ocean warming.
  • Ocean Acidification: Oceans absorb more carbon dioxide due to a rise in CO2 levels. This increases the acidity of ocean water. This inhibits the corals’ ability to create calcareous skeletons that essential for their survival.
  • Pollution: The toxic pollutants which are dumped directly into the ocean can lead to the poisoning of the coral reefs as it increases the nitrogen level of the seawater, leading to an overgrowth of algae.
  • Sedimentation: Construction along the coasts and islands lead to soil erosion, increasing the sediments in the river. As a result, it can smother corals by depriving them of the light needed to survive.
  • Infectious Diseases: Vibrio shiloi is a bacterium that inhibits the photosynthesis of zooxanthellae. This bacterium becomes more potent with an increase in sea temperatures.
  • Coastal development: Development of coastal infrastructure and tourist resorts on or close by these coral reefs causes significant damages.
  • Human activities: Over-fishing, coral mining, development of industrial areas near coral ecosystems etc. impact coral ecosystem.

Global Data on Coral Bleaching

  • Coral reefs are among the most threatened ecosystems on Earth, largely due to unprecedented global warming and climate changes, combined with growing local pressures.
  • Over the last 3 years, reefs worldwide have suffered from mass coral bleaching events due to the increase in global surface temperature caused by anthropogenic greenhouse gas emissions.
  • According to UNESCO, the coral reefs in all 29 reef-containing World Heritage sites would cease to exist by the end of this century if we continue to emit greenhouse gases under a business-as-usual scenario.
  • Limiting global average temperature to well below 2 °C above pre-industrial levels in line with the Paris Agreement provides the only chance for the survival of coral reefs globally.

Global Examples of Coral Bleaching

  • According to the National Oceanic and Atmospheric Association, between 2014 and 2017, around 75% of the world’s tropical coral reefs experienced heat stress severe enough to trigger bleaching. For 30% of the world’s reefs, that heat stress was enough to kill coral.
  • The Great Barrier Reef suffered the worst bleaching event in history in 2016 and in 2017. It was estimated that about half of its coral was killed between the two events.
  • Many reefs were badly hit in the Indian Ocean. A survey in the Maldives found that all reefs there were affected, with 60% – 90% of coral colonies bleached. Christmas Island had almost all its coral bleached (around 85% of its coral died).
  • Jarvis Island, one of the most remote, protected and productive coral reefs in the U.S., saw 98% of its coral died during the 2015-2016 El Niño event.

What can be done?

  • If the Paris Agreement is fully implemented, we will likely see a decrease in carbon concentrations in the atmosphere and improve conditions for the survival of reefs.
  • The Paris Agreement must be reflected in all other global agreements, such as the Sustainable Development Goals.
  • Transform the mainstream economic systems and move towards circular economic practices. These are highlighted in SDG 8 and SDG 12.
  • Economic systems need to rapidly move to the low greenhouse gas emission scenario to enable global temperature to decrease.
  • Restoring coral reefs should be treated as an asset, and long-term investments should be made for their preservation.
  • Investments should also include support for research such as genetic selection of heat-resistant corals that can withstand rising global temperatures.

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