Coral Bleaching

Primary reference(s)

United Nations, 2017. The First Global Integrated Marine Assessment: World Ocean Assessment I. Cambridge University Press. Accessed 14 October 2020.

Additional scientific description

Coral bleaching was a relatively unknown phenomenon until the early 1980s, when a series of local bleaching events occurred, principally in the eastern tropical Pacific and Wider Caribbean regions. Severe, prolonged or repeated bleaching can lead to the death of coral colonies. An increase of only 1°C to 2°C above the normal local seasonal maximum can induce bleaching. Although most coral species are susceptible to bleaching, their thermal tolerance varies. Many heat-stressed or bleached corals subsequently die from coral diseases (United Nations, 2017).

Increasingly frequent severe coral bleaching is among the greatest threats to coral reefs posed by climate change. Global climate models project great spatial variation in the timing of annual severe bleaching conditions; a point at which reefs are certain to change and recovery will be limited (UNEP, 2017).

Warmer water temperatures can result in coral bleaching. 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 (NOAA, no date a).

In 2005, the USA lost half of its coral reefs in the Caribbean in one year due to a massive bleaching event. The warm waters centred around the northern Antilles near the Virgin Islands and Puerto Rico and extended southward. Comparison of satellite data from the previous 20 years confirmed that thermal stress from the 2005 event was greater than the previous 20 years combined (NOAA, no date a).

There is great spatial variation in the timing of annual severe bleaching conditions; a point at which reefs are certain to change and recovery will be limited. The onset of annual severe coral bleaching is defined as the annual exceedance of more than eight degree-heating weeks accumulating during any three-month period. With more than eight weeks with an extra degree of heat it is possible to have confidence that thermal stress will be enough for bleaching to occur (van Hooidonk et al., 2016).

Metrics and numeric limits

Not globally available.

Examples of drivers, outcomes and risk management

The National Oceanic and Atmospheric Administration Coral Reef Watch programme uses satellite data to provide current reef environmental conditions to identify quickly areas at risk for coral bleaching (NOAA, no date b). Coral Reef Watch also offers a modelled outlook that predicts the likelihood of coral bleaching heat stress on a week-by-week basis, up to four months into the future (the typical length of a bleaching season).

Continuous satellite monitoring of sea surface temperature at global scales and modelled projections of approaching bleaching- level heat stress provide resource managers, scientific researchers, and other coral reef ecosystem stakeholders with tools to understand and better manage the complex interactions leading to coral bleaching. When bleaching conditions occur, these tools can be used to trigger bleaching response plans and support appropriate management decisions and communication with the public (NOAA, no date b).