Coral dies from a combination of rising ocean temperatures, chemical changes in seawater, disease, pollution, predators, and direct physical damage. No single factor acts alone. Most coral death today traces back to thermal stress, which triggers bleaching, but the full picture involves at least half a dozen interacting threats that weaken reefs and prevent recovery.
Heat Stress and Bleaching
The single biggest killer of coral worldwide is warming water. Corals survive through a partnership with tiny algae living inside their tissue. These algae provide up to 90% of the coral’s energy through photosynthesis and give reefs their color. When water temperatures rise even slightly above normal summer maximums, this partnership breaks down.
At elevated temperatures, the algae’s photosynthesis malfunctions and starts producing toxic oxygen molecules that damage both the algae and the coral host. The coral essentially begins to perceive its own symbiotic partner as a threat. In response, it expels the algae, turning white in the process. Lab experiments with one common reef species showed that raising water temperature from 28°C to 32°C caused a nearly 80% reduction in algae density within 18 days. That 4°C increase is the difference between a healthy coral and a bleached one.
Bleached coral isn’t immediately dead. It’s starving. If temperatures return to normal within a few weeks, the algae can recolonize and the coral recovers. But if the heat persists, the coral starves to death. NOAA tracks thermal stress using a metric called Degree Heating Weeks, which accumulates the intensity and duration of heat exposure. Even modest warming sustained over several weeks can push reefs past their tipping point. Recent analysis suggests that the traditional thresholds used to predict bleaching may actually be too generous, and that corals begin bleaching at lower heat accumulation than previously assumed.
Ocean Acidification
The ocean absorbs roughly a quarter of the carbon dioxide humans emit, and that CO₂ reacts with seawater to form carbonic acid. As the water becomes more acidic, it contains fewer of the carbonate ions corals need to build their calcium carbonate skeletons. This process, ocean acidification, doesn’t kill coral outright the way bleaching does. Instead, it slows growth and weakens the reef structure over time.
The interaction between temperature and pH matters more than either factor alone. Experimental data shows corals calcified fastest at higher pH and moderate temperatures (around 25°C), while the combination of low pH and the same temperature produced the slowest growth. At pH 7.71 combined with 25°C water, some coral colonies experienced net dissolution, meaning their skeletons were literally dissolving faster than they could build. Current ocean pH averages about 8.1, down from 8.2 before industrialization, and continues to drop.
Nutrient Pollution and Algal Takeover
Nitrogen and phosphorus from agricultural runoff, sewage, and stormwater feed a different kind of coral killer: macroalgae. On a healthy reef, corals and algae compete for space, and corals generally hold their own because nutrient-poor tropical waters limit algal growth. When excess nutrients flood in, the balance tips. Algae grow faster, spread over coral colonies, block light, and eventually smother them.
This process accelerates dramatically when herbivorous fish like parrotfish and surgeonfish are removed by overfishing. These grazers normally keep algae in check. Without them, even moderate nutrient increases can trigger what ecologists call a “phase shift,” where a coral-dominated reef flips to an algae-dominated one. Experiments excluding large herbivorous fish from reef areas showed explosive algal growth that suppressed coral reproduction, recruitment of new coral larvae, and survival of existing colonies. Once a reef shifts to algal dominance, it rarely shifts back on its own.
Disease
Coral diseases have become increasingly common as reefs weaken from other stressors. The most devastating recent example is stony coral tissue loss disease (SCTLD), first identified off the coast of Florida in 2014 and now spreading across the Caribbean. SCTLD causes coral tissue to slough off the skeleton in patches, and affected colonies can experience mortality rates as high as 99%, though survival varies by species.
The disease appears to involve specific bacterial communities that proliferate in damaged tissue. Researchers have identified groups of bacteria in diseased tissue that may produce toxins capable of degrading both the coral cells and their symbiotic algae. SCTLD is particularly alarming because it affects over 20 coral species, spreads through water contact, and has no known cure at reef scale. Warm, polluted water appears to make corals more susceptible.
Predators
Coral has natural predators, and under normal conditions, the reef can handle the grazing. The problem comes with population explosions. The crown-of-thorns starfish is the most destructive coral predator on the planet. A single adult consumes an average of about 200 square centimeters of coral per day in planar area, roughly the size of a large smartphone screen. In three-dimensional terms, accounting for the coral’s branching surface, that figure jumps to nearly 1,000 square centimeters daily.
Outbreaks involving millions of these starfish have devastated sections of the Great Barrier Reef repeatedly since the 1960s. The starfish climbs onto a coral colony, everts its stomach over the surface, and digests the living tissue, leaving behind bare white skeleton. Nutrient runoff is suspected to fuel these outbreaks by boosting the plankton that starfish larvae feed on, linking this threat directly back to land-based pollution.
Sedimentation
Coastal development, deforestation, and dredging send enormous quantities of soil and sediment into nearshore waters. This kills coral through three distinct mechanisms. Suspended particles cloud the water and reduce the light reaching the reef, starving the photosynthetic algae inside coral tissue. Sediment that settles directly on coral smothers it, cutting off both light and gas exchange. Under a layer of fine silt, the tissue beneath becomes oxygen-deprived, and if the sediment isn’t washed away by waves or currents, the tissue dies and lesions form. Fine sediments like silt and clay are far more damaging than coarse sand because they pack together tightly and block light more completely.
Toxic Chemicals
A growing body of evidence links specific chemicals to coral damage at surprisingly low concentrations. Oxybenzone, a UV-filtering ingredient in many sunscreens, causes deformities in coral larvae at concentrations around 50 micrograms per liter in lab conditions. That’s a tiny amount, roughly equivalent to a drop in a large bathtub. Related compounds like dioxybenzone can cause visible bleaching in coral larvae at 250 micrograms per liter. Hawaii and several other jurisdictions have banned certain sunscreen ingredients based on this research.
Sunscreen is just one source. Herbicides, pesticides, and industrial chemicals wash off land and into reef waters. Many of these compounds interfere with coral reproduction or weaken their immune response, making them more vulnerable to disease and bleaching. The effects are often subtle and chronic rather than immediately lethal, which makes them harder to study but no less damaging over time.
How These Threats Interact
What makes the current crisis so severe is that these stressors rarely act in isolation. A reef weakened by nutrient pollution bleaches faster when a heat wave arrives. A coral recovering from bleaching is more susceptible to disease. A reef stripped of herbivorous fish by overfishing can’t bounce back after a cyclone because algae colonize the rubble before new coral can settle. Acidification slows the rebuilding process even when conditions otherwise improve.
The result is a compounding cycle. Each stressor lowers the threshold for the next one to cause damage. Reefs that historically recovered from natural disturbances within a decade are now being hit by bleaching events every few years, faster than they can regrow. An estimated 14% of the world’s coral was lost between 2009 and 2018 alone, and the pace has accelerated since. Local threats like pollution, overfishing, and sedimentation are the ones most directly within human control, and reducing them is the most immediate way to give reefs a better chance of surviving the global threats they can’t escape.