Coral reefs are killed by a combination of rising ocean temperatures, changing water chemistry, pollution, disease, and direct human destruction. No single factor acts alone. Heat stress from warming oceans is the largest and most immediate threat, but reefs weakened by pollution, overfishing, or acidification are far less likely to survive even moderate temperature spikes. Understanding each of these pressures explains why reefs worldwide are declining at an accelerating pace.
Heat Stress and Coral Bleaching
Corals live in a tight partnership with microscopic algae embedded in their tissue. These algae provide up to 90% of a coral’s energy through photosynthesis. When water temperatures rise even 1 to 2 degrees Celsius above the normal summer maximum and stay elevated for weeks, the coral expels those algae. That’s bleaching. The coral turns white and, without its energy source, begins to starve.
NOAA tracks heat stress using a metric called Degree Heating Weeks, which measures how much excess warmth has accumulated over a rolling 12-week window. When that value reaches 4°C-weeks, bleaching risk begins. At 8°C-weeks, reef-wide bleaching with death of heat-sensitive species is likely. If stress keeps building past 12°C-weeks, multiple species start dying. Above 20°C-weeks, near-complete mortality of more than 80% of corals becomes likely. Back-to-back hot years are especially devastating because corals need time to recover between bleaching events, and increasingly they aren’t getting it.
Ocean Acidification Weakens Coral Skeletons
The ocean absorbs roughly a quarter of the carbon dioxide humans emit. That CO₂ reacts with seawater to form carbonic acid, gradually lowering the ocean’s pH. This matters for corals because they build their skeletons from calcium carbonate, and lower pH means fewer carbonate ions available for construction.
Research published in PNAS found that acidification doesn’t stop corals from growing outward. They continue extending at roughly the same rate. What changes is density: the skeleton becomes thinner and more porous, like building walls with less concrete. Models project that the skeletal density of major reef-building corals could decline by an average of 12.4% globally by the end of the century from acidification alone, with losses as high as 20.3% in the Coral Triangle region of Southeast Asia, where the largest pH drops are expected. Weaker skeletons make reefs more vulnerable to storm damage and erosion, compounding every other threat.
Nutrient Pollution and Sediment Runoff
Fertilizers, sewage, and agricultural waste wash into coastal waters and deliver excess nitrogen and phosphorus to reef ecosystems. These nutrients don’t kill coral directly in most cases. Instead, they fuel explosive growth of algae that compete with coral for space and light. When nitrogen levels are high relative to phosphorus (ratios above 16:1) and phosphorus concentrations are very low, the symbiotic algae inside coral tissue actually decline in density. That makes the coral more susceptible to bleaching and disease, even at temperatures it might otherwise survive.
Sediment carried by rivers and stormwater poses a more immediately lethal threat. When organic-rich sediment settles on a coral colony, microbes in that sediment consume oxygen rapidly. Within hours, the area beneath the sediment becomes oxygen-depleted and acidic. Coral tissue begins degrading within a single day under organic-rich sediment. As the tissue breaks down, bacteria produce hydrogen sulfide, a potent toxin that spreads to neighboring tissue and accelerates death across the colony. Notably, sediment low in organic material caused no damage even after six days in experiments. The organic enrichment of coastal sediments from agricultural and urban runoff is the key factor that turns normal sedimentation into a reef killer.
Disease Outbreaks
Stony Coral Tissue Loss Disease has been one of the most devastating reef diseases ever documented. First reported off the coast of Florida in 2014, it reached the northern Mesoamerican Reef by 2018 and spread across that entire 450-kilometer reef system in just a few months. The disease affects nearly 30 coral species. Highly susceptible species like maze corals and pillar corals can become infected and die within weeks.
Mortality rates vary dramatically by species, ranging from under 10% to 94%. Most maze corals and brain corals sustained losses above 50%. The disease strips living tissue from the skeleton, leaving behind bare white coral that quickly becomes colonized by sediment and algae. Because it targets so many species and spreads so quickly, SCTLD has fundamentally reshaped reef communities across the Caribbean, removing the large, slow-growing species that form the structural backbone of the reef.
Overfishing and Loss of Herbivores
Healthy reefs depend on herbivorous fish, particularly parrotfish and surgeonfish, to keep algae in check. When these species are overfished, algae grows unchecked and can smother coral or prevent new coral larvae from settling and surviving. The type of herbivore lost matters. Grazers crop short turf algae, which corals can generally coexist with. Browsers remove larger, fleshier macroalgae, which is far more harmful to coral because it inhibits recruitment of new coral, shades existing colonies, and releases chemicals that damage coral tissue.
Modeling research shows that reefs can recover from disturbances when herbivore communities are balanced and diverse. After a bleaching event that reduces coral cover to just 15%, recovery depends heavily on whether both grazers and browsers are present. A reef dominated by grazers but lacking browsers allows macroalgae to take over, and since macroalgae suppresses coral more aggressively than turf algae, this skew is particularly damaging. Browsers are generally considered more important than grazers for recovering reefs that have already shifted to algal dominance. The 1983 die-off of long-spined sea urchins across the Caribbean, which removed the region’s most important invertebrate grazer, is a textbook example: many Caribbean reefs shifted to algal dominance within years and have never fully recovered.
Destructive Fishing Practices
Blast fishing, still practiced in parts of Southeast Asia and East Africa, uses homemade explosives to stun fish. The blasts shatter the coral framework into rubble. Cyanide fishing, used primarily to capture live reef fish for the aquarium trade and luxury food markets, involves squirting sodium cyanide solution into crevices to stun fish. The poison bleaches and kills coral in the surrounding area.
What makes these practices so devastating is the recovery timeline. Surveys of extensively blasted areas in Indonesia found no significant natural recovery within six years, even when healthy reefs nearby provided plenty of coral larvae. The rubble fields left behind are unstable, constantly shifting with currents, which prevents new coral from gaining a foothold. Estimates based on comparable physical damage from severe storms suggest recovery takes 40 to 70 years at minimum. In heavily blasted areas, where the fragments are smaller and the structural foundation is more thoroughly destroyed, recovery may take centuries.
Chemical Pollutants
Sunscreen chemicals, pesticides, and industrial compounds add another layer of stress. Oxybenzone, a UV filter found in many conventional sunscreens, has received particular attention. Lab studies show that coral larvae begin bleaching at increasing rates as oxybenzone concentrations rise, with deformities appearing at concentrations as low as 6.5 micrograms per liter in light conditions. These are not extreme laboratory doses: field sampling in Hawaii and the U.S. Virgin Islands has detected oxybenzone in reef waters at concerning levels, particularly near popular swimming beaches.
Oxybenzone is just one of many chemicals reaching reefs. Herbicides from agricultural runoff inhibit photosynthesis in coral’s symbiotic algae. Microplastics are ingested by coral polyps and can reduce feeding efficiency. Oil spills coat reef surfaces and block light. Individually, each of these chemical stressors might be survivable. In combination with warming temperatures and nutrient pollution, they reduce the margin corals have to cope with any single threat.
Why Multiple Threats Compound
The most important thing to understand about coral reef decline is that these threats rarely act in isolation. A reef stressed by nutrient pollution grows weaker symbiotic algae communities, making it more vulnerable to bleaching during a marine heat wave. A reef stripped of herbivorous fish by overfishing gets overgrown with macroalgae, leaving less space for coral to recolonize after a disease outbreak. A reef whose skeleton has been thinned by acidification is more easily destroyed by a cyclone. Each pressure lowers the reef’s capacity to withstand the next one, creating a downward spiral that accelerates over time.
Local threats like pollution, overfishing, and destructive fishing are the most directly controllable. Reducing these stressors won’t stop ocean warming, but it gives reefs a better chance of surviving heat events and recovering afterward. Reefs with intact fish populations, clean water, and minimal physical damage consistently show higher survival rates during bleaching events than degraded reefs exposed to the same temperatures.