Context: The US National Oceanic and Atmospheric Administration (NOAA) declared the onset of the fourth global mass coral bleaching event due to extraordinary ocean temperatures recently.
This event carries significant implications for marine ecosystems and the millions reliant on reefs for sustenance, livelihoods, and coastal defence.
Causes of Elevated Ocean Temperatures:
- Since mid-March 2023, average sea surface temperatures (SST) have been unusually high, reaching a record monthly high of 21.07 degrees Celsius in March.
- Rising emissions of heat-trapping greenhouse gases (GHGs), such as carbon dioxide and methane, are the primary contributors to soaring temperatures.
- Nearly 90% of the additional heat trapped by GHGs has been absorbed by the oceans.
Importance of Corals and Coral Reefs:
- Corals are essentially animals, which are sessile, meaning they permanently attach themselves to the ocean floor.
- They use their tiny tentacle-like hands to catch food from the water and sweep into their mouth.
- Each individual 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 — complex three-dimensional structures built up over thousands of years.
- “Unlike soft corals, hard corals have stony skeletons made out of limestone that are produced by coral polyps.
- When polyps die, their skeletons are left behind and used as foundations for new polyps,” according to NOAA.
- Coral reefs, also referred to as “rainforests of the sea”, have existed on the Earth for nearly 450 million years.
- Australia’s Great Barrier Reef is the largest in the world, stretching across 2,028 kilometres.
Significance of Coral Reefs:
- Coral reefs harbour immense biodiversity, supporting over 400 coral species, 1,500 fish species, and numerous other organisms.
- They provide economic benefits valued at about $375 billion annually and offer coastal protection from storms and floods for over 500 million people worldwide.
- 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.
Understanding 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 colours 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. They rather go under more stress and are subject to mortality.
- Coral bleaching reduces the reproductivity of corals and makes them more vulnerable to fatal diseases. If the bleaching is not too severe, corals have been known to recover.
Incidences of Coral bleaching
- Global mass bleaching of coral reefs is when significant coral bleaching is confirmed in the Atlantic, Indian and Pacific oceans, according to a report published by The Conversation. Such events are a relatively new phenomenon.
- The first one 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).
Current Status of Global Bleaching Event:
- NOAA has confirmed the fourth global bleaching event, with widespread bleaching reported across 54 countries, territories, and localities from Florida, the US, Saudi Arabia to Fiji.
- The Great Barrier Reef is experiencing its most severe bleaching event, with approximately one-third of surveyed reefs showing extreme bleaching.
- Bleaching was also confirmed in the Western Indian Ocean, including Tanzania, Kenya, Mauritius, Seychelles, and off the western coast of Indonesia.
- The key driver behind the current event is higher ocean temperatures. However, the situation has been exacerbated by El Niño, a weather pattern which is associated with warmer oceans.
Impact of the Event:
- The full impact of the ongoing bleaching event is yet to be determined, but scientists anticipate severe consequences.
- With global temperatures rising, such events are expected to become more frequent and prolonged, posing a grave threat to coral reefs worldwide.
Response and Outlook:
- To curb global warming and mitigate the loss of coral reefs, countries must reduce greenhouse gas emissions to net zero by 2050, as outlined in the Paris Agreement.
- However, achieving this goal remains challenging due to continued record levels of GHG emissions.
- The fourth global mass coral bleaching event underscores the urgent need for concerted global action to mitigate climate change and preserve coral reef ecosystems, upon which millions depend for their well-being.