The world’s oceans are getting warmer, and the colourful coral reefs that once teemed with life are now fading to white. Rising sea temperatures are triggering frequent marine heatwaves that leave corals stressed, bleached, and struggling to survive. Globally, coral cover has fallen by nearly half since 1998. A recent study finds that in India’s Lakshadweep islands, it has dropped from 37 to just 19 percent—a sign of how deeply the crisis runs.
Corals may look like rocks, but they are living animals that build vast underwater cities for fish, crabs, and countless other species. They are the rainforests of the sea, supporting food security, fisheries, tourism, and coastal protection. When corals die, entire ecosystems begin to unravel.
Climate change is heating up the oceans, causing higher sea surface temperatures and more intense marine heatwaves. These heatwaves are stressing corals globally, leading to bleaching and even death.
Corals are a large group of invertebrate animals called Cnidaria that occur in a wide variety of colours, shapes and sizes. They have a simple stomach with a single mouth that is surrounded by stinging tentacles. Each individual animal is called a polyp, and polyps live in groups to form colonies through a process of budding, where the polyps grow copies of themselves.
Corals can be classified into hard and soft corals. There are around 800 species of hard corals, also known as the ‘reef-building’ corals. Hard corals extract calcium from surrounding seawater and use this to create a hard structure around them for protection and growth. Coral reefs are thus created due to millions of tiny polyps forming large carbonate structures that provide a home to hundreds of plant and animal species. The calcium carbonate structure is crucial to maintain the functionality of the corals.
Soft corals, on the other hand include sea fans, sea feathers and sea whips and do not have the calcium carbonate skeleton like hard corals. Soft corals also live in colonies that look like brightly coloured, feathery plants or trees that hang down in order to capture food floating in the currents.
The corals share a symbiotic relationship with single-celled algae, known as zooxanthellae and exchange oxygen and other gases and nutrients to survive. This contributes to the brilliant colours of the corals. Light is crucial for the survival of corals, and this makes corals highly susceptible to environmental stress.
Coral reefs support staggering biodiversity
Coral reefs are extremely diverse and support a variety of life forms, providing a range of ecosystem services from food, shelter and protection
Corals support livelihoods
Coral reefs are vital for fisheries, as they provide nurseries for the ocean's fish, provide hiding spots for predators, are a rich source of protein and generate revenue for local communities.
Corals can cure
Coral reefs are of great medicinal value and can be useful for curing life-threatening diseases such as cardiovascular diseases, ulcers, leukaemia, lymphoma and skin cancer. The unique skeletal structure of corals is useful for making bone-grafting materials.
Corals can build
Coral reefs are rich in limestone, which is often used as a cement substitute in the construction industry. At an industrial level, the coral sand rich in calcium is also a potential raw material for the cement industry.
Coral reefs act as protective barriers
Reefs act as natural barriers and protect coastal cities, communities, harbours and beaches from ocean waves and prevent erosion, property damage and loss of life. Coral reefs also help in maintaining the quality and clarity of shore water, as they are filter feeders and consume particulate matter suspended in the water. Corals control carbon dioxide levels in the ocean by using dissolved carbon dioxide to form new reefs.
Heatwaves don’t hit all coral reefs the same way. Their impact depends on where the reef is, what stressors it faces, and who lives there.
Some reefs can suffer more than others due to:
Geographic differences: Oceans can experience different temperature extremes, currents, and pollution levels at regional and local scales and can affect coral reefs in the area.
Local stressors: Overfishing, sedimentation, and nutrient runoff can weaken coral resilience
Recovery potential: Some reefs bounce back faster due to better water quality, protected status, better ability to resist or recover from stress or due to type and health of coral populations.
The study titled ‘Local environmental filtering and frequency of marine heatwaves influence decadal trends in coral composition’ published in Diversity and Distribution, documents the changes in coral communities over 24 years in 12 sites of three atolls (ring-shaped coral reefs) in the Lakshadweep Archipelago (Indian Ocean) due to marine heatwaves and local environmental factors.
The Lakshadweep Archipelago includes a group of 12 coral atolls in the northern Indian Ocean and has 10 populated islands. It is one of the most densely populated atoll systems globally. Lakshadweep experienced three marine heatwaves associated with the El Niño phase of the ENSO phenomena in 1998, 2010 and 2016 with 2010 being the most severe of the three.
The study monitored reefs at three atolls (Agatti, Kadmat and Kavaratti), and two types of coral reef locations namely, sheltered from water and light (east-facing) and exposed to water and light (west-facing), were selected for the study.
Coral cover showed a decline
The coral cover reduced from an average of 37.24% in 1998 to 19.06% in 2022 reflecting a ~50% reduction in cover. This was mostly because the reefs could not bounce back after bleaching, even though coral deaths were relatively few each time.
Long recovery times were needed to heal coral reefs
Reefs started to recover after 6 years when there was no bleaching. For example, after the 2016 marine heatwave, some sites showed an increase in raw coral cover – the recovery rates being in the range of 2%–5% annually for the Indian Ocean.
Exposure to water made a difference
Coral sites that were exposed to open water recovered significantly better than sheltered sites.
Depth played a role too
Exposed shallow sites showed the highest recovery at 4% ± 10.7 SD after nine years. Exposed deep sites recovered moderately at 18.7% ± 14.5 SD.
Sheltered sites lagged behind with deep ones showing a recovery of 6.5% ± 8.3 SD while shallow at 10.7% ± 2.2 SD.
Recovery rates were not influenced by time
The difference in recovery rates between sheltered and exposed sites remained consistent, regardless of how long the recovery period was.
Thus the physical setting also influenced coral resilience, not just time. Exposure to currents, light, and reduced sedimentation may boost recovery. Shallow exposed reefs act as recovery hotspots and offer opportunities for conservation. Sheltered reefs may need targeted interventions to support recovery, such as reducing local stressors or enhancing water circulation.
Heatwaves reshuffled reef communities
After three successive bleaching events, coral groups were dominated by stress-tolerant species that were better equipped to survive thermal stress. However, they tended to grow slowly; many included those found in deeper waters and existed in naturally low numbers. Their ability to support reef functions like accretion, sediment production, and wave buffering was limited.
These assemblages may just help in holding on till faster-growing corals are able to reestablish, thus preventing total collapse, but they do not help in rebuilding reefs. Without the return of fast-growing, structurally important types of corals, reefs face the risk of remaining in a limbo state, and few types of corals can survive in the long term.
The study found that local conditions exposed certain sites to repeated decline and also helped in differential recovery. The impacts were spatially distributed, with shallow reefs likely to suffer greater bleaching impacts but also demonstrating the greatest recovery potential.
Over the past 24 years, Lakshadweep’s coral reefs have faced repeated thermal stress, with recovery shaped not by resilience alone, but by the intervals between heatwaves and local environmental conditions.
Reduced diversity: Successive bleaching events have narrowed coral species composition, favouring stress-tolerant but functionally limited types.
Diminished architecture: Slower-growing, low-abundance corals offer little in terms of reef-building or habitat complexity.
Compromised ecosystem function: Key services like sediment production, wave attenuation, and fisheries support are at risk.
The findings of this study are very important, as they provide a way forward by identifying the need to conduct an archipelago-wide vulnerability mapping and spatial management to understand the factors affecting the decline in coral populations in the region.
Local interventions alone may not help and can become temporary fixes unless supplemented by scale-appropriate, radical policies and laws that address the drivers of reef degradation.
These can include:
Climate regulation and emissions control
Marine spatial planning that integrates thermal vulnerability
Legal frameworks for reef restoration and protection
Community-led monitoring and adaptive governance
The findings reveal that coral recovery depends on more than time. It depends on where they live and how much stress they face. Protecting reefs now means tackling both global and local pressures: reducing greenhouse gas emissions, curbing coastal pollution, and supporting communities that depend on reef health.
Coral reefs are silent sentinels of climate change. They reveal how warming, pollution, and neglect are reshaping the oceans that sustain us. The story of Lakshadweep is a reminder that resilience requires both science and compassion.
To protect these underwater cities of life, we must rethink how we live on land. Every choice we make, like how we build, travel, consume, or conserve, ripples into the sea. If we listen closely, the corals are not just warning us; they are inviting us to act before the sea turns silent for good.