Eco-morphodynamic stability of low-lying coral reef islands in a changing climate

Coral reef islands are under threat from warming and rising seas, ocean acidification and increased storminess. These islands are low-lying accumulations of sediment derived from a continual supply of shells and skeletons from calcifying reef organisms. Over 200 million people rely on reefs and their islands for their livelihoods, including Small Island Developing States (SIDS). Coastal States and SIDS commonly use coral islands to support and extend their maritime jurisdictions. Coral islands are morphologically active (e.g., erode, accrete, migrate) across broad spatial-temporal scales and have been extensively studied using historical aerial imagery, satellite imagery and satellite derived bathymetry (SDB). The future of coral islands and the reefs that support them is not certain, and it is unclear what eco-morphological tipping points may cause constructive and destructive impacts. A better understanding of coral island stability and evolution to changing conditions is urgently needed. Here we focus on 31 coral islands on 10 offshore coral reefs that extend Australia’s maritime jurisdictions in the Coral Sea (SW Pacific) and NW continental shelf (Timor Sea). We digitised island morphology (e.g., shoreline positions, island area and shape) from imagery and SDB (1976-2022) and compared this to local ocean and climate data (e.g., cyclone tracks, sea surface temperatures (SST), sea-level rise) to identify potential tipping points and processes. Initial results show that since the 1970s a third of the coral islands were stable (n=9; <3% change in area), half increased in size (n=15) and a fifth decreased in size (n=7). As expected, small (<10 Ha) and unvegetated islands were more active when compared to large and vegetated islands. Meanwhile, we suggest observed increases in island size may be a short-lived response to increasing SST and marine heatwaves that can degrade reefs and produce additional sediment, combined with tropical cyclones that have the capacity to transport additional sediments to islands. The question remains whether islands will survive if reefs continue to degrade, and sediment supply is reduced. Any island loss will have serious social, environmental and geo-political consequences.