Low lying Australian coral islands at risk of disappearing

More than a dozen of the coral islands that mark Australia's maritime boundaries are at risk of disappearing, according to a new study in the Science of the Total Environment journal.

Indicating multiple stresses as threatening their existence - including rising sea levels - the study suggests that their disappearance could have implications for the country's maritime borders.

The islands extend the jurisdiction of Australia, with over a million kilometres² of the nation’s territory supported and demarcated by their presence.

Explaining this, study co-author Dr Thomas Fellowes from the School of Geosciences at the University of Sydney, advised “coral islands support and extend coastal countries’ legal maritime jurisdictions across vast sea areas.

“These maritime territories grant sovereign rights over critical resources, including fisheries and mineral exploration, making the stability and resilience of coral islands a matter of strategic importance for Australia’s and the region’s coastal management.

“In Australia, they play a pivotal role supporting approximately 15%, or 1.2 million kilometres², of our maritime zone across the Indian and Pacific oceans and to the north in the Torres Strait.”

The study assessed 56 islands based on factors including how vulnerable they are to heatwaves and being flooded.

The report identified three islands on Western Australia's North West Shelf that were considered at "very high risk" of climate threats while 11 more islands in the Coral Sea off the Queensland coast were classed as facing high risk.

Dr Fellowes noted “islands in the very high-risk category, particularly those on the north-west shelf of Western Australia, are most at risk. This is due to their small size, low elevation, angular shape, lack of vegetation, exposure to lower ocean pH, higher sea-level rise, isolation, and frequent tropical storms and heatwaves.”

The risk assessments were based on the current conditions the islands endure, but also noted that the threat of marine heatwaves and rising oceans will increase due to climate change.

The research introduces an innovative method to evaluate and categorise coral reef islands based on their risk from and resilience to climate change.

The risk system, built using open-access ocean climate data and satellite imagery that could be applied globally, provides decision-makers with a tool to prioritise protection efforts for vulnerable coral islands in the era of rapid climate change.

The Great Barrier Reef has lost more than half of its corals due to climate change, including mass bleaching events - a phenomenon where corals under stress drive out the algae that give them their distinctive colours.

Approximately 25% of the world's marine species are dependent on coral reefs at some point in their life cycle.

Explaining the risk methodology, Dr Fellowes added “using a risk matrix, we’ve classified these islands into five categories, ranging from very low to very high risk.

“Our findings will not only inform and guide decision-makers in implementing adaptive measures but will also contribute to the broader global understanding of the challenges faced by these vital ecosystems.

“Coral islands are home to an estimated 65 million people worldwide and hold crucial environmental, economic, social, and geopolitical significance, especially for our Pacific neighbours on Small Island Developing States (SIDS).”

Dr Fellowes said while rising sea level poses a significant threat, the overall survival of coral islands is dependent on multiple climate threats and is also linked to declining reef health caused by marine heatwaves and coral bleaching.

This is because the living reef supplies sediment that keeps the islands above sea level. These factors collectively amplify the challenges faced by coral islands and SIDS communities.

The study went on to highlight that risks to low lying islands around the world has implications for the communities that live and rely on them, with Dr Fellowes saying that taking steps to reduce fossil fuel usage could help slow the decline of island decay.

Coral islands are low-lying land masses composed of the sediments produced by coral debris.

Image credit: Australian Institute of Marine Science.