Key factors likely to affect the geological features of the Great Barrier Reef are rising sea level, increasing sea temperature and severe weather events.

The geological features of the Great Barrier Reef include:

  • More than 2900 reefs of varying types, sizes, and stages of growth
  • Around 300 coral cays and more than 600 continental islands.
  • More than 2000km of mainland coastline comprised mainly of sandy shorelines and mangrove-lined muddy coasts and estuaries.

These major geological features of the Great Barrier Reef will persist into the future, however many such features will be altered in ways that significantly affect their viability as critical habitat for reef creatures.

The entire Great Barrier Reef and adjacent coast have experienced massive and repeated environmental changes in the recent geological past. Such changes have included sea level rises of over 100 metres since the peak of the last ice age.

The geological structures of the Great Barrier Reef and their living inhabitants are strongly interdependent. Coral reefs and other landforms provide living creatures with the physical habitat structures in which they live, seek food, shelter and reproduce. In turn, structures such as reefs and coral cays are themselves built from the shells and skeletons of vast numbers of marine animals and plants over long periods of time.

Living organisms are also important in modifying, eroding and redistributing geological materials. For this reason, the impacts of climate change on living species will also affect the Reef’s geological features.

Sea level rise

Healthy coral reefs can easily keep pace with sea level rises like those projected over the course of the current century. Sea level rise alone can actually benefit reefs, by freeing up space into which corals can grow.

However, the survival of critical reef-building organisms (especially hard corals) is threatened by climate-driven changes, in combination with a range of pressures from human activities.

The combined effects of coral bleaching, ocean acidification and human pressures will slow reef growth, and some reefs will likely begin to erode over time. Other geological features will also change as sea level rises. For example, many coral cays are predicted to initially increase in size, though most could eventually be inundated if sea level rise continues beyond the end of the current century.

Increasing sea temperature and ocean acidification

The continued building of geological features such as reefs and coral cays will depend in large part on the ongoing capacity of reef organisms to produce calcium carbonate (limestone) material.

Coral bleaching caused by increasing sea temperatures, in combination with ocean acidification, is expected to greatly reduce the production of limestone shells and skeletons over coming decades.

Whether calcium carbonate production by less sensitive organisms can compensate for the expected loss of hard corals remains to be seen. Existing reefs will remain as geological structures, but the probability of continued vigorous reef growth seems low.

Extreme weather events

The Great Barrier Reef has a long history of exposure to severe cyclones and floods but such events are expected to occur more often in the future. Projected increases in severe weather events will affect both the build-up and erosion of different geological features.

If coral reefs are damaged more regularly they may struggle to recover between cyclones – especially when bleaching, ocean acidification or human pressures are already stressing corals. Beaches and coral cays will also be affected in a variety of ways by increased exposure to severe weather. For example, some features may initially be built up more rapidly by the accumulation of rubble and sand.

Other features may be eroded by increased exposure to large waves and storm surge. Such changes will have important consequences for the use of reef features by people and animals (such as nesting turtles and seabirds).