Context: About 14% of the world’s coral has been lost in less than a decade, a study of the health of coral reefs has found under report by Global Coral Reef Monitoring Network (GCRMN).
Issue

• The reason being ocean-acidification, warmer sea temperatures and local stressors such as overfishing, pollution, unsustainable tourism and poor coastal management. 

Note: The report by Global Coral Reef Monitoring Network (GCRMN) was drawn from a global dataset gathered by over 300 members of the network covering four decades from 1978 to 2019.

Key Study
The study establishes a correlation between coral loss and climate change as sharp declines in coral cover corresponded with rapid increases in sea surface temperatures.

• Rise in sea surface temperatures: The study, which analysed 10 regions with coral reefs, showed that coral bleaching events caused by raised sea surface temperatures were the biggest factor behind coral loss. 
• Mass Bleaching: It found that one such mass bleaching event in 1998 led to the loss of 8% of the world’s coral, or about 6,500 sq km, with the biggest impact observed in the Indian Ocean, Japan and the Caribbean.
• Decrease in hard coral cover: There has been a steady decrease in hard coral cover in the last four decades since 1978 when the world lost nine per cent of its corals. The worst-hit are the corals in South Asia, Australia, the Pacific, East Asia, the Western Indian Ocean and the Gulf of Oman. The decrease is disconcerting because live hard coral cover is an indicator of coral reef health.
• Increase in algae on coral reefs: Since 2010, the amount of algae on the world’s coral reefs has increased by about 20 per cent. Algal bloom on coral ridges are a sign of stress on the structures. Prior to this, on average, there was twice as much coral on the world’s reefs as algae.

This transition from live hard coral to algae-dominated reef communities impacts marine habitats, rendering them less biodiverse and also affects the ecosystem services provided by them.
Positive aspects of Study

• Coral reefs in east Asia’s Coral Triangle accounts for more than 30 per cent of the world’s reefs but has been less impacted by rising sea surface temperatures.

Despite a decline in hard coral cover during the last decade, on average, these reefs have more coral today than in 1983, when the first data from this region were collected, the scientists noted.

• In 2019, the world regained 2% of its coral cover in spite of a short interval between mass coral bleaching events in the last decade.
• These instances mean that these critical ecosystems have the capacity to recover if pressure on them eases, the researchers noted. They can even resuscitate to their pre-1998 health in the next ten years, the report mentioned.

Significance

• Though corals occupy less than 1% of the ocean floor but over one billion people benefit directly from the reefs. 
• The value of goods and services provided by coral reefs is estimated to be $2.7 trillion per year including $36 billion from coral reef tourism. 
• As per some reports, the net economic value of the world’s coral reefs could be nearly tens of billions of dollars per year. 

Conclusion

• A clear message from the study is that climate change is the biggest threat to the world’s reefs, and we must all do our part by urgently curbing global greenhouse gas emissions and mitigating local pressures.