Why are corals bleaching?


Coral Bleaching and Climate:

Since coral bleaching is closely related to changes in ocean temperature, bleaching events clue us in to oscillations in the climate system, both natural and anthropogenically-induced.  

El Niño/La Niña events are part of a large-scale climate phenomenon that occurs every 3-8 years. The events are driven by changes in the Pacific Ocean, but have global climate impacts–and in El Niño years, we often observe coral bleaching events. During El Niño years, a swath of warm ocean water sweeps across the entire Pacific basin from west to east, leaving bleached and dying corals in its wake. The huge 1997-1998 El Niño saw unprecedented coral bleaching events in the eastern tropical Pacific. Bleaching events stretched to Southeast Asia in 1998-1999 during the subsequent strong La Niña, which is characterized by warmer than average temperatures in the western tropical Pacific.  


sstanom_bar Sea surface temperatures in the equatorial Pacific Ocean across the 1997-1998 El Niño/La Niña event. From NASA/Goddard Space Flight Center Scientific Visualization Studio

We don’t know exactly how El Niño dynamics will evolve as the planet warms, but intensity of the cycle may increase: warm events might get warmer. If this intensification occurs, we would likely see widespread coral bleaching events occurring across the tropical oceans. Many central and eastern tropical Pacific reefs could be devastated. 

Global warming is steadily driving increases in ocean temperatures (see figure below). Coral reefs worldwide are vulnerable to bleaching driven by these increases in temperature. Coral reef ecosystems are delicately balanced, and are highly sensitive to changes in their environmental conditions. Incidences of coral bleaching noted by Bleach Patrol surveyors may signal a shift in typical oceanic conditions, and may provide crucial information for climate scientists and biologists trying to understand coral ecology.