Universal spatial properties of coral reefs

Coral reefs are under rapid decline due to human pressures such as climate change. Achieving the Kunming-Montreal Global Bio-diversity Framework goals, which include restoring 30\% of degraded habitats like coral reefs by 2030, requires a comprehensive understanding of their extent and structure, which has been hitherto lacking. We address this limitation based on a recently available unprecedented canonical inventory of coral reefs. We identified a total of 1,579,772 individual reefs globally, extending over a total of 52,423 km^2 of ocean area with mean and median sizes of 3.32 Ha and 0.3 Ha, respectively. The distribution of coral reef area conforms to a universal power-law with an exponent of 1.8, common among all coral reef provinces. Coral reefs develop fractal geometries with surface and perimeter dimensions of D_P=1.3 and D_A=1.6, with larger reefs having higher dimensions by developing complex elongated shapes. Our findings help to constrain coral reef models, which should be able to reproduce the universal patterns reported here, which can also guide management, conservation, and restoration targets and help quantify the effort and resources required.