Coral Bleaching: Causes & Consequences

• Context (TH): The Great Barrier Reef suffered its most significant annual coral loss in 39 years due to severe, prolonged marine heatwaves and environmental stressors.

Coral Reefs

• Coral reefs are built by tiny marine animals called coral polyps. These organisms extract calcium carbonate from seawater to create hard skeletons, forming reef structures.
• Over thousands of years, these structures have grown into vast and complex habitats that support more than 25% of all marine species, even though they cover less than 1% of the ocean floor.
• Coral reefs are vital ecosystems that support a vast diversity of marine life, provide coastal protection, and contribute to local economies and livelihoods through tourism, fishing, and coastal protection.

What is Coral Bleaching?

• Coral bleaching is when corals become white due to the loss of symbiotic algae and photosynthetic pigments. It occurs when coral polyps expel the zooxanthellae that live inside their tissue, causing the coral to turn white.
• A bleached coral is not necessarily dead, and some corals may survive. However, a bleached coral is under stress, more vulnerable to starvation and disease, and at risk of death.

Coral Bleaching Events

• The 1^st and 2^nd global coral bleaching events occurred in 1998 and 2010, respectively.
• The 3^rd global coral bleaching event, which took place between 2014 and 2017, affected 68.2% of the world’s reef areas with heat stress levels high enough to cause coral bleaching.

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What Triggers Coral Bleaching?

• Climate Change: Rising ocean temperatures, primarily caused by human activities, are the leading cause of coral bleaching. A temperature about 1 °C (or 2 °F) above average can cause bleaching.
• Ocean Acidification: As the atmosphere increases in carbon dioxide, the oceans absorb more CO2, leading to heightened acidity in seawater. This elevated acidity weakens corals’ ability to form their essential calcareous skeletons, threatening their survival.
• Ultraviolet Radiation: Shifts in tropical weather patterns, resulting in reduced cloud cover, expose corals to higher levels of solar and ultraviolet radiation. This increased radiation contributes to coral bleaching and further stresses these fragile ecosystems.
• Human-Induced Threats: Human activities, including overfishing, pollution from agricultural and industrial runoff, coral mining, and industrial development near coral ecosystems, pose significant threats to coral health and contribute to their decline.

Consequences of Coral Bleaching

• Ecosystem Disruption: Coral reefs support over 25% of marine species despite covering <0.1% of the ocean floor. Loss of live coral cover leads to cascading biodiversity loss.
• Economic Impact: Coral reefs contribute an estimated $2.7 trillion annually to the global economy through goods and services like tourism, fisheries, and coastal defence.
  • Coral-dependent fish stocks in Lakshadweep and Gulf of Mannar have fallen by 20–30% post-bleaching events, threatening income for artisanal fishers.
• Climate Feedback Loop: Extensive reef damage—from mortality to cover loss—reduces the ability of reefs to act as carbon sinks, undermining their role in buffering the climate.
• Coastal Protection Loss: Healthy reefs absorb up to 97% of wave energy, protecting shorelines from erosion and storm surges. Bleaching weakens reef structures, making coasts more vulnerable to cyclones and tsunamis.

Way Forward

• Reduce GHG Emissions: Align with the Paris Agreement 1.5°C target to reduce ocean warming and thermal stress on reefs.
• Expand Marine Protected Areas (MPAs) to cover ecologically significant reef areas with strict fishing and pollution controls.
• Sustainable Tourism: Eco-tourism guidelines, reef-safe sunscreens, and coral education can promote responsible travel.
• Restoration Projects: Coral farming and transplantation to revive damaged reefs.