What Are Coral Reefs?
Coral reefs are among the most biodiverse and productive ecosystems on Earth. Often called the "rainforests of the sea," they cover less than 1% of the ocean floor yet support an estimated quarter of all marine species. Beyond biodiversity, coral reefs protect coastlines from wave erosion, support fisheries that millions of people depend on for food and income, and contribute to tourism economies worldwide.
Corals themselves are tiny animals — polyps — that build hard calcium carbonate skeletons. They live in a symbiotic relationship with microscopic algae called zooxanthellae, which live within coral tissues and provide the coral with up to 90% of its energy through photosynthesis. This relationship is the foundation of reef health — and its vulnerability.
What Is Coral Bleaching?
Coral bleaching occurs when corals are stressed — most commonly by elevated water temperatures, but also by pollution, disease, or changes in salinity. When stressed, corals expel their zooxanthellae algae. Without the algae, the coral loses its colour (turning ghostly white — hence "bleaching") and its primary energy source. A bleached coral is not dead, but it is severely weakened and vulnerable.
If temperatures return to normal quickly enough, corals can reabsorb algae and recover. However, if high temperatures persist for weeks or months, the coral will die, leaving behind a bare white skeleton that is eventually overgrown by algae.
The Role of Climate Change
Ocean temperatures are rising as a direct consequence of global warming. The oceans have absorbed more than 90% of the excess heat trapped by greenhouse gas emissions since the Industrial Revolution. Even small increases in sea surface temperature — as little as 1–2°C above the usual summer maximum — sustained over several weeks can trigger mass bleaching events.
Before widespread climate change, mass bleaching events were rare. Today, the frequency and intensity of bleaching have increased dramatically. The Great Barrier Reef — the world's largest coral reef system — has experienced multiple mass bleaching events in recent years, affecting large portions of the reef in ways previously unseen.
Ocean Acidification: A Compounding Threat
Beyond warming, the oceans are also becoming more acidic as they absorb CO₂ from the atmosphere. This ocean acidification reduces the availability of calcium carbonate, making it harder for corals to build and maintain their skeletons. It's a second front in the climate threat to reef systems, operating alongside — and interacting with — temperature stress.
Why Reef Loss Matters Beyond the Ocean
- Food security: Reef fisheries provide protein for hundreds of millions of people, particularly in tropical island nations and coastal communities.
- Coastal protection: Reefs act as natural breakwaters, reducing wave energy and protecting shorelines from erosion and storm surge.
- Medicine: Reef organisms have been the source of multiple important pharmaceutical compounds, with many more yet to be discovered.
- Biodiversity: The loss of reefs triggers cascading extinctions throughout marine food webs.
What Can Be Done?
Protecting coral reefs requires action at multiple scales:
- Reducing greenhouse gas emissions — the single most important long-term action. Limiting warming to 1.5°C would preserve a far greater fraction of the world's reefs than a 2°C or higher scenario.
- Reducing local stressors — controlling pollution, runoff, and overfishing gives reefs greater resilience to temperature stress.
- Assisted evolution — scientists are researching heat-tolerant coral strains and coral restoration programmes to help reefs adapt.
- Marine protected areas — establishing no-take zones and managed marine areas reduces human pressure on reef ecosystems.
The fate of coral reefs is one of the clearest and most visible indicators of how climate change is reshaping life on Earth. Acting urgently to reduce emissions is the most meaningful contribution we can make to their survival.