When water is too warm, corals will expel the algae (zooxanthellae) living in their tissues causing the coral to turn completely white. This is called coral bleaching. When a coral bleaches, it is not dead. Corals can survive a bleaching event, but they are under more stress and are subject to mortality.
Coral bleaching matters because it’s not an isolated phenomenon. According to the National Oceanic and Atmospheric Association, between 2014 and 2017 around 75% of the world’s tropical coral reefs experienced heat-stress severe enough to trigger bleaching
Once these corals die, reefs rarely come back. With few corals surviving, they struggle to reproduce, and entire reef ecosystems, on which people and wildlife depend, deteriorate.
What causes Coral Bleaching?
- Change in Ocean temperature: Increased Ocean temperature caused by climate change is the leading cause.
- Runoff and Pollution: Storm-generated precipitation can rapidly dilute ocean water and runoff can carry pollutants. These can bleach near shore corals.
- Overexposure to Sunlight: when temperature is high, high solar irradiance contributes to bleaching in shallow-water corals.
- Extreme low tides: Exposure to the air during extreme low tides can cause bleaching.