Coral reefs are disappearing because of a combination of rising ocean temperatures, ocean acidification, pollution, overfishing, and disease. No single cause is responsible. These stressors overlap and amplify each other, pushing reefs past their ability to recover between damaging events. Globally, coral reefs generate an estimated $36 billion a year in economic value through tourism and fisheries, and they reduce wave energy by an average of 97%, making them critical natural barriers against coastal flooding and storm damage.
How Warming Water Triggers Bleaching
The single biggest driver of coral loss is rising sea surface temperature. Corals survive through a partnership with microscopic algae that live inside their tissue. These algae provide up to 90% of the coral’s energy through photosynthesis and give reefs their vivid color. When water temperatures rise even slightly above normal for a sustained period, corals become stressed and expel the algae. Without them, the coral turns white, a process called bleaching.
Bleached coral isn’t dead yet, but it’s starving. If temperatures return to normal quickly enough, the algae can recolonize and the coral recovers. If the heat persists for weeks, the coral dies. Bleaching events used to be rare, but they’ve become frequent enough that reefs no longer have time to bounce back between episodes. The Great Barrier Reef illustrates the trajectory clearly: after a record-breaking marine heatwave in 2024, coral cover on the northern reef dropped by about a quarter in a single year, from 39.8% to 30%. Near Lizard Island, individual reefs lost up to 70% of their coral. Southern reefs, which experienced the highest heat stress ever recorded, lost almost a third of their coral cover.
Extreme cold can also cause bleaching, though it’s far less common. In January 2010, water temperatures in the Florida Keys dropped nearly 7°C below typical winter levels, triggering a bleaching event that killed some corals.
Ocean Acidification Weakens Coral Skeletons
While warming causes the most visible damage, a quieter chemical shift is undermining reefs from within. The ocean absorbs roughly a quarter of the carbon dioxide humans emit. When CO2 dissolves in seawater, it forms carbonic acid, lowering the water’s pH. This reduces the concentration of carbonate ions that corals need to build their hard skeletons.
Coral skeletons are made of aragonite, a form of calcium carbonate. As the water becomes less saturated with the building blocks for aragonite, corals grow more slowly and their existing structures become more fragile. Research in the southern Red Sea has confirmed a significant decline in coral calcification rates linked to these chemical changes. The effect is subtle compared to a bleaching event, but it compounds over time: reefs grow weaker, recover more slowly from storms, and become less able to keep pace with rising sea levels.
Pollution and Runoff Feed Algae Competitors
Agricultural fertilizers contain large amounts of nitrate that leach into groundwater and river systems, eventually reaching coastal waters. These nutrients don’t stay close to shore. Research on the Great Barrier Reef found that nitrate from farm runoff can travel hundreds of kilometers along the coast and across the reef under flood plume conditions, posing a risk to offshore reefs that scientists wouldn’t have predicted just a few years earlier.
The extra nutrients fuel rapid growth of fleshy macroalgae, the seaweed-like organisms that compete directly with corals for space and light. Once algae gain a foothold, they trigger a damaging cascade: the algae release sugars that spur bacterial growth, and those bacteria consume oxygen in the water immediately surrounding the coral, effectively suffocating it. Sediment carried in the same runoff makes things worse by smothering coral polyps and blocking the light their symbiotic algae need for photosynthesis. Together, nutrients and sediment reduce what scientists call reef resilience, the ability of corals to recover after a bleaching event or storm.
Overfishing Removes the Reef’s Cleanup Crew
Healthy reefs depend on herbivorous fish, especially parrotfish, to keep algae in check. Parrotfish graze constantly on turf algae and the sediment trapped within it, clearing space on hard surfaces where new corals can settle and grow. Larger parrotfish species go further: they scrape and excavate dead coral skeletons, a process called bioerosion that recycles old reef material and opens up real estate for living coral.
When overfishing removes these grazers, macroalgae blooms follow. Experimental studies that excluded large herbivores from reef patches found that algae quickly took over, preventing new corals from establishing. The loss of larger parrotfish has the greatest ecological impact because of their unique role in bioerosion and coral removal. Smaller parrotfish species appear more resilient to fishing pressure and can maintain grazing and sediment removal even when moderately exploited, but they can’t fully compensate for the loss of the big excavators. The result is a reef that slowly shifts from coral-dominated to algae-dominated, a transition that’s extremely difficult to reverse.
Disease Is Decimating Caribbean Reefs
Coral disease has reshaped entire reef systems. In the late 1970s, white-band disease killed nearly 80% of the Caribbean’s two major reef-building coral species, causing a dramatic reduction in reef structure that the region still hasn’t recovered from. Now a newer threat, stony coral tissue loss disease (SCTLD), is on track to become the most lethal disturbance ever recorded in the Caribbean.
First identified off Miami in 2014, SCTLD spread across hundreds of kilometers in just a few months. It affects at least 21 coral species, with mortality rates ranging from under 10% to 94% depending on the species. The disease is reshaping what Caribbean reefs look like at a fundamental level. The corals most vulnerable to SCTLD include important reef-building species, and their loss is pushing these ecosystems toward a new baseline with far less structural complexity and lower physical functionality.
Coastal Development and Physical Damage
Dredging for ports and harbors, coastal construction, and land clearing for agriculture all send sediment plumes into nearshore waters. Sediment that settles on coral smothers the polyps and interferes with their ability to feed, grow, and reproduce. Trash, including plastic bags, bottles, and discarded fishing gear, snags on coral structures and blocks sunlight. Boat anchors, groundings, and even recreational divers touching or standing on coral cause direct physical breakage that can take decades to regrow.
These local stressors matter more than they might seem in isolation. A reef weakened by sedimentation and nutrient pollution is far less likely to survive a bleaching event than one in clean, clear water. This is why some of the best-preserved reefs in the world are in remote locations far from human settlement, even though they experience the same ocean temperatures as degraded reefs closer to population centers.
Why These Threats Compound
No single stressor would be enough to cause the global decline scientists are documenting. The real danger is the way these pressures stack. A reef hit by a marine heatwave might recover in a decade if left alone. But if that same reef is also dealing with nutrient pollution, reduced herbivore populations, and acidifying water, recovery stalls or never happens. Each subsequent bleaching event starts from a lower baseline of coral cover, and the intervals between events keep shrinking.
The Great Barrier Reef, despite its recent losses, still has more coral than many other reef systems worldwide. That’s both reassuring and a measure of how degraded reefs elsewhere have become. The trajectory is clear: without reducing greenhouse gas emissions and managing local stressors like runoff and overfishing, coral reefs will continue to shrink in coverage, complexity, and the ecological services they provide to both marine life and the hundreds of millions of people who depend on them.