Climate change impacts on coral reefs

Coral reefs are highly vulnerable to climate change and the impacts will be far reaching.

Coral reefs are complex structures built mainly from the calcium carbonate (limestone) skeletons laid down by hard corals. These reef-building corals are highly vulnerable to rising sea temperatures and ocean acidification. Slowed growth and loss of hard corals will reduce essential habitat for many other reef creatures.

Reef structures themselves will also begin to crumble if reef growth does not keep pace with erosion by animals and storms.

Coral reefs comprise only six per cent of the area of the Great Barrier Reef, yet they provide critical habitat and food for many species in the ecosystem. Healthy coral reefs are also the essential foundation for reef-based tourism and fishing.

They are vitally connected to other Great Barrier Reef habitats including mangroves and salt marshes, seagrass meadows, estuaries, and open water environments. Reefs also act as barriers, protecting inshore habitats and human communities from large waves and storm surges.

Rising sea temperature

Hard corals are highly susceptible to coral bleaching caused by higher-than-normal sea temperatures. Coral bleaching is expected to occur more often and with greater severity in the future, making it difficult for corals to recover between bleaching events. As a result, the abundance of living corals on reefs is likely to decline in coming decades.

Some coral types, such as staghorn corals, are especially sensitive to bleaching, and these will be the most seriously affected. Coral communities will increasingly be dominated by types that are more tolerant to temperature stress.

Large, fleshy seaweeds (macroalgae), which compete with corals for space on the reef, will also benefit from rising temperatures and coral bleaching. Scientists have shown that degrading reefs can be rapidly overgrown by these macroalgae, which in turn impede coral recovery. Reefs dominated by macroalgae and bleaching-resistant corals have less three-dimensional structure than healthy coral reefs. Such reefs provide fewer shelters and refuges for the many animals that rely on the reef for their habitat.

Ocean acidification

Coral reefs are also highly vulnerable to ocean acidification. Hard corals and many other organisms that contribute to reef building, such as coralline algae, make their skeletons from calcium carbonate (limestone).

The rate of skeleton formation, known as calcification, will slow if waters become more acidic and the skeletons of these animals and plants will be weaker. Reefs are continually worn down by storms, and creatures that eat, burrow or dissolve their way through limestone.

For a healthy reef to be maintained, the growth of corals and encrusting algae has to at least keep pace with this erosion. Continuing ocean acidification will ultimately contribute to coral loss, and a weakening and collapse of limestone reef structures.

Extreme weather events

The Great Barrier Reef has adapted to cope with the impacts of cyclones and severe storms. However, many scientists predict that intense cyclones (such as cyclone Hamish and cyclone Yasi) will occur more often due to climate change. Reef recovery from such severe storms is slow, because fewer corals survive to recolonise affected areas. An increase in severe cyclones could therefore contribute to the degradation of reefs structures already weakened by coral bleaching and ocean acidification.