Climate change impacts on corals

Climate change has already impacted coral reefs in the Great Barrier Reef as corals are highly vulnerable to its potential effects. As the back-bone of the Great Barrier Reef ecosystem, corals play a critical role. They form habitats for thousands of species, including fish and molluscs.

Coral biology

Corals have a mutually dependent relationship with minute algae called zooxanthellae. The coral structure provides a place for zooxanthellae to live, while the algae serve as an energy source for corals. Zooxanthellae photosynthesis generates enough food for both the algae and the coral, providing up to 95% of the coral's energy. Zooxanthellae also give coral with their many vibrant colours.

Sea temperature

Rising sea temperatures over the past 30 years have meant corals on the Great Barrier Reef are now at their 'normal' state of thermal thresholds. This means when temperatures increase again, particularly during summer months, corals will exceed these thresholds and move to a state of thermal stress.

Thermal stress is most visible when corals bleach. Too much heat means corals and zooxanthellae separate, the zooxanthellae taking the colour (hence 'bleaching') and energy sources with them. Coral bleaching is not always fatal, but has been one of the main causes of coral death around the world in the last 20 years.

Increased sea temperatures can also affect the growth and reproduction of corals. Experiments have shown a quicker embryo development, reduced egg and sperm production and an incomplete fertilisation cycle in corals due to warm water.

Ocean acidification

Ocean acidification is an additional threat to coral reef survival. It reduces the corals' ability to build their skeletons. This will result in weaker coral structures, thus reducing their ability to withstand physical disturbances from storms, they will have less energy to respond to stress and disease and there will be fewer protective habitats for other marine life.

Sea level rise

Studies conclude the rate of sea level rise is slower than current coral growth and therefore may have limited impact on coral reefs. However, the accumulation of other climate change impacts may slow coral growth, exposing them to the effects of rising sea levels.

Storms and rainfall

Storms, rainfall and floods are all natural, regularly occurring events on the Great Barrier Reef and as such, play a role in the ecosystem.

However, predictions suggest an increase in the frequency of severe weather events which could lead to more serious physical damage, with less time between events to recover.

The most immediate impact is coral breakage, dislocation and degradation from wind and waves. However, the effect doesn't stop there. Heavy rainfall events lead to flood plumes stretching across the Reef, exposing inshore coral reefs to freshwater inundations.

Freshwater run-off reduces salinity levels, sometimes causing bleaching, and brings increased nutrients and sediments, which can lead to disease outbreaks, algae blooms and murky water reducing light reaching corals.

The vulnerability of coral reefs to these impacts is dependent on the effect of other climate change impacts. For example, reefs that are weakened by ocean acidification or stressed from high sea surface temperatures will respond poorly to a major storm or flood event.