BharatNotes What is Coral Bleaching?
Coral bleaching is the loss of the symbiotic algae called zooxanthellae (genus Symbiodinium) from coral tissues, leaving the otherwise colourful coral a stark white. These algae give corals their colour and supply up to roughly 90% of their nutrition through photosynthesis. A bleached coral is stressed and starving but not necessarily dead — if normal conditions return quickly it can re-acquire algae and recover; prolonged stress leads to mortality.
Why It Happens
The principal trigger is rising sea surface temperature (SST). Corals bleach when water exceeds the long-term mean maximum summer temperature by about 1-2 degC for a sustained period. Heat disrupts the algae's photosynthesis, causing overproduction of toxic reactive oxygen species (free radicals); the coral then expels the algae to limit damage. Other stressors include ocean acidification, excessive solar irradiance, pollution and freshwater run-off, low tides and disease. NOAA's Coral Reef Watch tracks heat stress using Degree Heating Weeks (DHW) and a Bleaching Alert Scale.
Global Status (as of 2025)
The four confirmed global mass bleaching events show a worsening trajectory:
| Event | Period | Share of reefs hit by bleaching-level heat stress |
|---|---|---|
| First | 1998 | ~21% |
| Second | 2010 | ~37% |
| Third | 2014-2017 | ~68% |
| Fourth | 2023-2025 | ~84% |
The fourth global event was confirmed by NOAA and the International Coral Reef Initiative (ICRI) on 15 April 2024. From January 2023 to mid-2025 it impacted about 84% of the world's reef area, with 82 countries and territories reporting damage across all three reef-bearing ocean basins. NOAA's Coral Reef Watch had to add three new levels (Levels 3-5) to its Bleaching Alert Scale to flag mass-mortality risk. NOAA assessed that the event likely ended in mid-2025 — making it the largest on record.
The India Picture
India has four major reef areas: Gulf of Mannar, Gulf of Kachchh, Andaman and Nicobar Islands, and Lakshadweep. The first three are mainly fringing reefs; Lakshadweep is built of atolls. During 2024, Lakshadweep was the worst-hit, recording record heat stress of about 9.2 DHW (versus the earlier peak of ~6.7 DHW in 2010), with shallow-lagoon water near 36 degC in April-May (Down To Earth, 2024). Widespread bleaching was also reported in the Gulf of Mannar and Palk Bay, where live coral cover fell from ~37% to ~27.3% between 2005 and 2021. Major Indian bleaching episodes track global events — 1998, 2010, 2016 and 2024.
UPSC Angle
Examiners use coral bleaching to test the symbiosis mechanism, the SST-warming link, and reef geography. For Mains, connect it to climate change, ocean acidification, the Blue Economy, the Sustainable Development Goal SDG-14 (Life Below Water) and conservation tools such as Marine Protected Areas and reef restoration. Foundation concept — no single direct PYQ on the exact term, but it underpins recurring Prelims and GS3 questions on marine biodiversity, ocean warming and climate impacts.
