What is coral bleaching?

There are many environmental stressors to the Great Barrier Reef, but of particular concern is rising sea temperature, which causes coral bleaching. Most species of coral live in a symbiotic relationship with a tiny, colourful algae known as 'zooxanthellae', which live in the coral's tissue, providing the coral's beautiful and striking colour, as well as up to 90% of the coral's food needed to grow and reproduce. 

When water temperature rises above that to which the corals are adapted, the algae are forced to leave the coral, taking away the coral's primary source of food and removing the colour from their tissue, exposing the white skeletons. The loss of the algae leaves the coral in a vulnerable state as most struggle to provide enough food for themselves and starve. They are also left more susceptible to other ocean stressors such as poor water quality, attack from crown of thorns starfish, and ocean acidification, which hinder the coral recovering. 

Bleaching such as that which recently occurred in 2016 is especially concerning as it was most severe in northern regions of the reef, which until now remained the most pristine. With the more recent coral bleaching event in early 2017, occurring so soon after the last, there is grave concern about the Reef's survival. Two thirds of the Great Barrier Reef have now been severely bleached in just the last 2 years, 29% of the reef’s corals died in 2016, while the figure is not yet available for 2017.  

Bleaching occurs predominantly over just a few months in summer and more often during El Nino years, when sea surface temperatures off North East Australia are especially warm. We can expect bleaching events in the Great Barrier Reef to become more frequent, more extensive, and of longer duration as climate change progresses, with devastating consequences for the reef.

The Australian Institute of Marine Science estimates that under current trends, 50% of what today remains of the Great Barrier Reef will be lost by just 2021. 

Great Barrier Reef at 'terminal stage': scientists despair at latest coral bleaching data. Read more