Coral bleaching

Coral bleaching is a stress response where corals expel the symbiotic algae living in their tissues, causing them to turn white and become highly vulnerable to disease and starvation. While bleached corals are not dead, they face a high risk of mortality if the underlying environmental stress persists.

The primary driver of mass coral bleaching is rising ocean temperatures associated with climate change. When water temperatures rise by as little as one degree Celsius above the typical seasonal maximum for a period of four weeks, corals often become stressed and expel their algae, known as zooxanthellae. Other environmental stressors, including pollution, excessive sunlight, extreme low tides, and changes in salinity, can also trigger this phenomenon. Corals rely on these microscopic algae as their primary energy source and for their vibrant colors. When the algae are expelled, the coral's transparent tissue reveals its white calcium carbonate skeleton. If the stressful conditions are temporary, corals can sometimes recover by repopulating their algae. However, if the heat stress is prolonged or severe, the corals will likely die. The ecological consequences of coral bleaching are profound, as reefs are among the most biodiverse ecosystems on the planet. When corals die, the complex three-dimensional habitat they provide is lost, leading to a decline in the fish, invertebrates, and other marine organisms that depend on the reef for food, shelter, and breeding. This degradation can cause a collapse in local biodiversity and disrupt the entire reef food web. Beyond the immediate loss of marine life, the degradation of reefs impacts human communities that rely on these ecosystems. Healthy reefs act as natural barriers that absorb wave energy, protecting shorelines from erosion and storm damage. Furthermore, the loss of reef health threatens global food security for hundreds of millions of people who depend on reef fisheries for protein and livelihoods. According to the Intergovernmental Panel on Climate Change, recovery from mass coral mortality events typically requires at least 10 to 15 years. As climate change increases the frequency and intensity of marine heatwaves, many reefs are being subjected to bleaching events more often than they can naturally sustain, leading to long-term ecosystem decline.