Coral reefs are dying primarily because ocean temperatures are rising, but warming water is far from the only threat. Pollution, overfishing, coastal development, and chemical runoff all compound the damage, weakening reefs so they can’t recover between heat events. Between January 2023 and September 2025 alone, bleaching-level heat stress affected roughly 84% of the world’s coral reef area, making this the most severe global bleaching event ever recorded.
How Warming Water Triggers Bleaching
Corals get most of their energy and their color from tiny algae that live inside their tissue. The relationship is mutually beneficial: the coral provides shelter, and the algae produce food through photosynthesis. But when water temperatures rise even 1 to 2 degrees Celsius above normal summer highs, corals become stressed and expel those algae. Without them, the coral turns ghostly white. That’s bleaching.
Bleached coral isn’t immediately dead. If temperatures drop within a few weeks, the algae can recolonize and the coral can survive. But prolonged heat is lethal. NOAA Coral Reef Watch uses a metric called Degree Heating Weeks to predict outcomes: when accumulated heat stress reaches 4°C-weeks, substantial bleaching is expected. At 8°C-weeks, severe bleaching with significant mortality becomes likely. Many reefs in recent years have blown past both thresholds.
Marine heatwaves are becoming longer and more frequent as global ocean temperatures climb. What makes this especially damaging is the pace. Reefs typically need a decade or more to recover from a major bleaching event, but back-to-back heat events are now hitting reefs every few years, leaving no time for regrowth.
Nutrient Pollution From Land
Agricultural runoff carries nitrogen and phosphorus from fertilizers into coastal waters. These nutrients trigger a cascade of problems. Excess nitrogen increases oxidative stress inside coral cells, disrupts the balance between the coral and its symbiotic algae, and impairs the coral’s ability to build its calcium carbonate skeleton. It also lowers heat resistance, meaning nutrient-polluted reefs bleach at lower temperatures than clean ones would.
The nutrients fuel explosive growth of seaweed and other algae on the reef surface, which smothers coral and blocks the light it depends on. Combined nitrogen and phosphorus enrichment also increases the frequency and severity of coral diseases. The result is a reef that’s simultaneously under attack from above (heat) and below (poor water quality), with fewer defenses against either.
Overfishing Removes the Reef’s Cleanup Crew
Healthy reefs depend on herbivorous fish, especially parrotfish and surgeonfish, to graze algae off coral surfaces. This constant grazing keeps algae in check and creates open space for young corals to settle and grow. When those fish are removed by overfishing, algae spread unchecked. Once algae gain a foothold, the reef can undergo what ecologists call a “regime shift,” flipping from a coral-dominated system to one dominated by seaweed. These shifts are notoriously difficult to reverse.
Protecting herbivorous fish populations has become a central strategy in reef management worldwide, precisely because their grazing is one of the few natural forces that can hold algae back while corals recover from bleaching. But recent research shows that even intact fish populations may struggle during extreme heat events, when some herbivorous species abandon overheated reef areas. That means the safety net has limits.
Plastic Debris and Disease
Plastic waste on reefs does more than look unsightly. A study published in Science surveyed over 124,000 corals across the Asia-Pacific and found that the likelihood of disease jumped from about 4% to 89% when corals were in direct contact with plastic. That’s a 20-fold increase. Plastic creates wounds in coral tissue, blocks light, and creates low-oxygen pockets where harmful bacteria thrive. The most common diseases found on plastic-draped corals were skeletal eroding band disease, white syndromes, and black band disease, all of which can kill coral tissue rapidly.
The volume of plastic entering the ocean continues to grow, and much of it accumulates in the shallow, warm waters where reefs live. Fishing nets, bags, bottles, and microplastic fragments all contribute. For reefs already weakened by heat stress, the added burden of disease from plastic contact can be the tipping point.
Sedimentation From Coastal Development
Construction, dredging, and deforestation along coastlines send massive volumes of sediment into nearshore waters. This fine sediment does several things at once: it clouds the water, reducing the light that reaches corals and cutting off the energy supply from their symbiotic algae. It settles directly on coral surfaces, suffocating living tissue. And fine sediments often carry attached pollutants, including heavy metals and pesticides, that damage coral cells on contact.
Sediment is particularly deadly for coral larvae and young colonies. Baby corals need clean, hard surfaces to attach to and enough light to grow. In sediment-heavy water, fewer larvae survive, which means the reef loses its ability to replenish itself. Even if adult corals survive a stressor, the reef can still decline over decades simply because new corals can’t establish.
Chemical Pollutants in the Water
Sunscreen chemicals, herbicides, and industrial compounds wash into reef waters from shorelines and rivers. Oxybenzone, a UV filter found in many sunscreens, is toxic to coral larvae at concentrations as low as a few parts per billion, causing deformities that prevent them from developing normally. Hawaii and several Pacific island nations have banned sunscreens containing oxybenzone for this reason.
Herbicides from agricultural land, particularly those designed to inhibit photosynthesis in weeds, have the same effect on the photosynthetic algae inside coral. Pesticides and industrial chemicals add to the toxic load. These pollutants rarely kill a reef outright on their own, but they weaken corals in ways that make every other stressor more dangerous.
Why These Threats Compound Each Other
No single threat is responsible for the global decline of coral reefs. What makes the crisis so severe is that these stressors stack. A reef receiving nutrient runoff bleaches at a lower temperature. A bleached reef can’t fight off disease from plastic contact. A reef stripped of herbivorous fish gets overgrown with algae before new corals can settle. Sediment clouds the water so recovering corals can’t photosynthesize enough to rebuild.
Climate change is the overarching driver because it affects every reef on the planet simultaneously, regardless of local conditions. But reefs with clean water, intact fish populations, and minimal coastal development consistently survive heat events better and recover faster than degraded ones. That’s why local management still matters even in the face of a global problem. Reducing runoff, enforcing fishing regulations, and limiting coastal construction give reefs their best chance of outlasting the heat.