Fourth Global Mass Coral Bleaching

In News

  • The National Oceanic and Atmospheric Administration (NOAA) revealed the fourth global mass coral bleaching event has started due to ocean temperatures.


  • The first mass bleaching took place in 1998 in which 20% of the world’s reef areas suffered bleaching-level heat stress.
  • The next two global bleaching events occurred in 2010 (35% of reefs affected) and between 2014 and 2017 (56% of reefs affected).
  • The fourth global bleaching event is currently underway. Nearly 54 countries, territories, and local economies — from Florida, the US, Saudi Arabia to Fiji — have confirmed bleaching.

Reason for Rising Temperature & Bleaching

  • The primary reason behind the soaring temperatures is the rising emissions of heat-trapping greenhouse gases (GHGs) such as carbon dioxide and methane in the atmosphere. Nearly 90% of the extra heat trapped by GHGs has been absorbed by the oceans.
  • The temperature rise has been exacerbated by El Niño, a weather pattern that is associated with warmer oceans.
  • Coral Bleaching could have serious consequences for ocean life and millions of people who rely on reefs for food, jobs, and coastal defense.

What are Coral & Coral Reefs?

  • Corals are essentially animals, which are sessile, meaning they permanently attach themselves to the ocean floor.
  • Each coral animal is known as a polyp and it lives in groups of hundreds to thousands of genetically identical polyps that form a ‘colony’.
  • Corals are largely classified as either hard coral or soft coral. It is the hard corals that are the architects of coral reefs.
  • Unlike soft corals, hard corals have stony skeletons made of limestone produced by coral polyps. When polyps die, their skeletons are left behind and used as foundations for new polyps.
  • Geographical Conditions: Temperature: 20°C- 35°C; Salinity: Between 27% to 40%. Coral reefs grow better in shallow water; less than 50 m.
  • Australia’s Great Barrier Reef (World Heritage Site) is the largest in the world, stretching across 2,028 kilometers.
  • Coral reefs in India: Gulf of Kutch, Gulf of Mannar, Andaman & Nicobar, Lakshadweep Island and Malvan.

Coral Bleaching

  • Most corals contain algae called zooxanthellae — they are plant-like organisms — in their tissues. Corals and zooxanthellae have a symbiotic relationship.
  • While corals provide zooxanthellae a safe place to live, zooxanthellae provide oxygen and organic products of photosynthesis that help corals to grow and thrive. Zooxanthellae also give bright and unique colors to corals.
  • Corals are very sensitive to light and temperature and even a small change in their living conditions can stress them. When stressed, they expel zooxanthellae and turn entirely white. This is called coral bleaching.
  • Coral bleaching doesn’t immediately lead to the death of corals. Coral bleaching reduces the reproductivity of corals and makes them more vulnerable to fatal diseases.
  • Global mass bleaching of coral reefs is when significant coral bleaching is confirmed in the Atlantic, Indian, and Pacific oceans.

Importance of Corals

  • Coral reefs, also referred to as “rainforests of the sea”, have existed on the Earth for nearly 450 million years.
  • Thousands of marine species can be found living on one reef.
  • These massive structures also provide economic goods and services worth about $375 billion each year. More than 500 million people across the world depend on coral reefs for food, income, and coastal protection from storms and floods.
  • Coral reefs can absorb up to 97% of the energy from waves, storms, and floods, which prevents loss of life, property damage, and soil erosion.

Way Ahead

  • To curb global warming to no more than 1.5 degrees Celsius, countries need to bring GHG emissions to a net zero by 2050, according to the Paris Agreement.
  • Steps needed in the direction of Climate resiliency (Paris Agreement, SDG 8 & 12).
  • R & D for heat-resistant corals.