On 15 April 2024, the National Oceanic and Atmospheric Administration (NOAA) declared that a “global coral bleaching event” was taking place and that bleaching had been documented over the past 14 months in every major ocean basin in both the Northern and Southern Hemispheres (Derek Manzello, Ph.D., NOAA CRW Coordinator).
This is the fourth global coral bleaching event on record (following the 1998, 2010, and 2014-2017 global events), and the second in the last 10 years.
Coral bleaching, especially on a widespread scale, impacts economies, livelihoods, food security and more, but it does not necessarily mean corals will die. If the stress driving the bleaching diminishes, corals can recover and reefs can continue to provide the ecosystem services we all rely on.
WHAT'S GOING ON?
Corals have a symbiotic relationship with microscopic algae called symbiodinium, but more popularly known as zooxanthellae. Coral provides the algae with protection in sunlit, shallow seas. The algae produce large amounts of energy through photosynthesis, which the corals use to survive and to build their skeletons. These algae are the coral’s primary food source and give them their color. The stability of this symbiotic relationship is critical to corals' survival.
When corals are stressed by changes in conditions such as temperature, light, or nutrients, they expel the zooxanthellae living in their tissues. Without the algae, coral loses its major source of food, turns white or very pale, and is more susceptible to disease. This is called bleaching. When a coral bleaches, it is not dead. Corals can survive a bleaching event as long as the zooxanthellae return. If the algae loss is prolonged and the stress continues, the coral does eventually die. In 2005, the U.S. lost half of its coral reefs in the Caribbean in one year due to a massive bleaching event.
Increased ocean temperature is the leading cause of coral bleaching. Storm generated precipitation can rapidly dilute ocean water and runoff can carry pollutants — these can bleach near-shore corals. When temperatures are high, solar irradiance contributes to bleaching in shallow-water corals. Exposure to the air during extreme low tides can cause bleaching in shallow corals.
This global event requires global action. The International Coral Reef Initiative (ICRI) which NOAA co-chairs, and its international members are broadly sharing and already applying resilience-based management actions and lessons learned from the 2023 marine heatwaves in Florida and the Caribbean.
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