Causes of Mass Bleaching
Various sources of stress can cause individual corals to bleach. However, high water temperatures along with intense sunlight is the primary cause of mass bleaching events.
When coral reefs are exposed to warm water for sustained periods of time, large numbers of corals may bleach. A temperature increase of only 1-2°C above average hot season maximums can be sufficient to cause mass bleaching.
While high water temperature and bright sunlight are the primary triggers of mass bleaching, calm and clear conditions with minimal current can exacerbate the stress and intensify bleaching. Any factor that reduces these stressful conditions, such as cloud cover, strong winds, or tropical storms may be enough to protect the corals from bleaching.
Scientists believe that mass bleaching events have become more severe and widespread due to rising global temperatures linked to climate change.
In the warmest season, water temperatures are higher and bleaching is more likely. Warm seasons often coincide with calm conditions and the lack of wind and current results in less mixing, clearer seas, and deeper penetration of solar radiation, ultraviolet light, and photosynthetically active radiation (PAR).
Average global temperature has been rising. Bleaching has become more frequent and widespread over the past two decades. Predictions of continued warming seas cause concern that bleaching events will also continue to increase in frequency and intensity.