Mass bleaching can be defined as events in which entire reef tracts, or regions, completely bleach. These events are primarily caused by elevated sea temperatures. Unfortunately, mass bleaching is becoming more common and widely distributed as we experience global climate change.
Favorable Bleaching Conditions
Whether a reef bleaches during “hot water” events depends on a variety of factors, but there are some basic predictors that scientists use. Typically, a temperature rise of 1-2°C above the long term average is enough to cause mass bleaching. However, whether an area bleaches, and the severity of bleaching, depends on the length of time the corals are exposed to elevated water temperatures. Higher temperatures can be destructive over a shorter period of time, while lower temperatures take longer to cause mass bleaching.
In addition to high temperatures, sunlight combines with heat to increase the impacts of temperature induced bleaching. Thus, when the water is warm, and the sky is clear of clouds, conditions are very favorable for bleaching. One of the physical factors that has been identified that likely increases resilience and resistance to bleaching is shading. When shade is present, either due to weather conditions or physical location of a coral, bleaching is less likely to occur.
Recently, scientists and managers have been developing tools to forecast bleaching events in various parts of the world. Because global weather patterns and sea surface temperatures are closely monitored, it is possible to predict with some accuracy when and where bleaching will occur. Using measures such as Degree Heating Weeks (DHW Methodology), scientists can monitor the duration of heat stress that particular geographies may be experiencing, using satellite technology. However, both local and regional factors difficult to monitor on a global scale continue to make highly accurate predictions difficult. At a minimum, information available through the Coral Reef Watch program can be used to raise awareness and ensure that managers are anticipating an event when conditions are favorable. Managers can sign up to receive automated email updates about reefs in their part of the world. Currently about 24 sites are being closely monitored in real time. For more information on being prepared for a bleaching event, see the section on Rapid Response in this toolkit.