Rising sea levels will be significant because much of the Great Barrier Reef coastline is low-lying. Predictions of a future increase in sea levels are highly variable and range from 0.68m across the Great Barrier Reef Region to a global increase of up to 0.9m by 2100.  

Small changes in sea levels will mean land inundation which will cause significant changes in tidal habitats such as mangroves and saltwater intrusion into low-lying freshwater habitats.

Sea levels on the Great Barrier Reef have already risen by approximately 3mm per year since 1991 due to a combination of thermal expansion (i.e. warmer water occupying a greater volume of space) in the oceans and glaciers melting. Changes in sea levels from temperature increases are time-dependent and uncertain, because they are partly linked to the collapse of the Earth’s great ice shelves.

Since 1959, records of variations in sea levels for Townsville, in north Queensland, show an average increase of 1.2mm per year. The rate of increase may be accelerating. Records of sea levels at Cape Ferguson near Townsville show an average increase of 2.9mm every year between 1991 and 2006.

By the standards of past geological history of the Great Barrier Reef, this current sea level change is very small. However, it is believed that sea levels had been very constant for the past 6000 years, resulting in a well-defined depth profile across virtually all reefs.

Most reefs will probably be able to accommodate a sea level rise of 3mm per year as the maximum rate of reef growth is about twice this. However as the rate of sea level rise increases, the depths at which coral reefs are able to survive may change as will the shape and existence of some cays and islands.