Globally, our climate is undergoing fundamental shifts, both in changing average weather conditions and the frequency of extreme weather events. Most notably, the Earth’s climate system is unequivocally warming, and it is extremely likely (at least 95% probability) that these changes are in large part a result of human activities.
Corals inhabiting tropical coral reefs are thermally sensitive, meaning that they can only tolerate small temperature ranges. However, climate change is causing abnormally high sea-surface temperatures, which is causing corals to bleach during summer months (see below for detail). The intensity of coral bleaching increases as temperatures become hotter.
The Great Barrier Reef has experienced two major bleaching events in recent decades, in the summers of 1998 and 2002 when, respectively, 42% and 54% of reefs were affected by bleaching.
The science of coral bleaching
Bleached staghorn with damselfish. Photo by Jodie Rummer.
Zooxanthellae are tiny, colourful marine algae, which live inside corals, providing them with much of their colour and, most importantly, their primary supply of energy. However, if the surrounding sea temperature becomes too warm, the algae die.
The loss of these zooxanthellae is what we refer to as ‘coral bleaching’. Without zooxanthellae coral tissue becomes transparent, revealing the white coral skeleton beneath it. Once this happens, the corals can die if unfavourable conditions persist. If, however, temperatures return to normal levels, corals can regain their zooxanthellae, although the stress is likely to cause a decrease in growth and reproduction.
Building reef resilience
Future bleaching events are inevitable, but there are a number of important steps that we can take, locally, nationally and internationally to give the Great Barrier Reef a fighting chance.
A concerted effort to reduce global carbon emissions will lessen the rise of ocean temperatures and ocean acidification. At the state level, we need to substantially improve the quality of water flowing on to the Reef. Poor water quality is particularly harmful for coral growth, reproduction and the survival of young corals, severely limiting reef recovery potential. Furthermore, research shows that excessive nutrients arriving on the Great Barrier Reef trigger harmful crown-of-thorns outbreaks, which can devastate vast areas of the reef.
How effectively we manage fishing, coastal development, pollution, trawling and shipping will play an important part in determining the future resilience of the Great Barrier Reef.
Coral Bleaching Map
Scientists from the ARC Centre of Excellence for Coral Reef Studies surveyed 83 reefs in March at the height of the 2016 bleaching event. Take a look at our interactive map of the Great Barrier Reef. You can click on photos and videos captured during the aerial surveys and see for yourself the extent of the bleaching.
Map of corals surveyed in the 2020 bleaching event.
Not all data is shown, only reefs at either end of the
bleaching spectrum: Red circles indicate reefs undergoing
most severe bleaching (60% or more of visible corals
bleaching) Green circles indicate reefs with no or only
minimal bleaching (10% or less of corals bleaching).
Composite map of surveyed corals across the 2016 and 2017 back-to-back bleaching events. Not all data is shown, only reefs at either end of the bleaching spectrum: Red circles indicate reefs undergoing most severe bleaching (60% or more of visible corals bleaching) Green circles indicate reefs with no or only minimal bleaching (10% or less of corals bleaching).
Hot news on the 2016, 2017, and 2020 coral bleaching events: