Context

The start of the fourth global coral bleaching event in 2023-2024 has been confirmed by the National Oceanic and Atmospheric Administration’s (NOAA) Coral Reef Watch (CRW) of the United States and the International Coral Reef Initiative (ICRI).

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Global Coral Bleaching Event

• Extent of Bleaching: CRW and ICRI have recorded bleaching of corals in 53 countries, territories and local economies across five different ocean/sea basins between February 2023 and April 2024.
• Affected Region: The Report confirms widespread bleaching across parts of the Indian Ocean, including Tanzania, Mauritius, the Seychelles, Tromelin, Mayotte, and off the western coast of Indonesia. 
• Frequency of Events: The earlier event (3rd global bleaching)  had lasted from 2014 to 2017 making the present occurrence second such event in the last 10 years.
  • Previous events of global bleaching occurred first in 1998, then in 2010, and between 2014-2017.
• Future Projection: Climate models suggest that bleaching events may become an annual occurrence for most reefs somewhere around 2040-2050.
• CRW Declaration: The CRW declares a global mass coral bleaching event only when it records or gets inputs from all ocean basins of coral bleaching.

Coral Bleaching

• Coral bleaching occurs when the coral Polyp expels the marine algae ie. zooxanthellae from its   tissue and breaks the symbiotic relation. This results in them loosing their colour and source of nutrients and energy.
• Stress Response: The bleaching event indicates a stress response enabling them to recover, depending on the intensity of heat stress and its duration.
  • Corals can recover back to their original health if other stressors such as marine pollution and ocean acidification are kept under check and certain adaptation measures taken.

Causes of Coral Bleaching

• Temperature: The corals can survive only in a specific temperature range and global sea surface temperature is rising because of global warming stressing the corals.
• Extensive Marine Heatwaves: During a marine heatwave, temperatures in vast areas of sea and ocean surfaces increase and remain high for elongated periods of time.
  • World Meteorological Organization (WMO)’s State of the Global Climate 2023 Report said that 2023  was the hottest on record for the atmosphere and the oceans with nearly one-third of the global ocean gripped by it any day. 
• Subaerial Exposure: Low tides, sea level drops and tectonic uplift can result in sudden exposure of coral to the atmosphere. Subaerial exposure often results in bleaching and consequent coral death due to sudden changes in atmospheric conditions.
• El Nino: Ocean Heating and mass coral bleaching are closely tied to the occurrence of El Nino events since 1997, each of these pairs of El Nino years has also witnessed mass bleaching of corals.
• General Ocean Warming: Ocean warming will pose a serious threat to coral reef ecosystems around the planet, as bleaching events will increase in severity, frequency, and magnitude.
  • Example: Localised coral bleaching events have also been witnessed in recent La Nina years in the Great Barrier Reef, off the coast of Australia, in 2020 and 2022.
• Sedimentation: Human Activities such as coastal construction and mining can result in high rates of erosion, increasing sediment content in water and disrupting the natural process of photosynthesis.
• Inorganic Nutrients: Increases in inorganic nutrients such as ammonia and nitrate cause zooxanthellae to multiply by 2-3 times which can cause secondary adverse effects such as lower coral resistance and increased disease susceptibility.