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• Passive acoustics detected and quantified offshore natural CO 2 emissions. • Method was developed to quantify gas flux and derive bubble sizes. • CO 2 bubble plume emitting gas at 2.3 L/min was detected up to distances of 8 m. • Quantification of CO 2 was possible using a hydrophone at distances of up to 4 m. • The bubble signal needs to be 10 dB above ambient noise levels. Estimating the range at which an acoustic receiver can detect greenhouse gas (e.g., CO 2) leakage from the sub-seabed is essential for determining whether passive acoustic techniques can be an effective environmental monitoring tool above marine carbon storage sites. Here we report results from a shallow water experiment completed offshore the island of Panarea, Sicily, at a natural CO 2 vent site, where the ability of passive acoustics to detect and quantify gas flux was determined at different distances. Cross-correlation methods determined the time of arrival for different travel paths which were confirmed by acoustic modelling. We develop an approach to quantify vent bubble size and gas flux. Inversion of the acoustic data was completed using the modelled impulse response to provide equivalent propagation ranges rather than physical ranges. The results show that our approach is capable of detecting a CO 2 bubble plume with a gas flux rate of 2.3 L/min at ranges of up to 8 m, and determining gas flux and bubble size accurately at ranges of up to 4 m in shallow water, where the bubble sound pressure is 10 dB above that of the ambient noise. [ABSTRACT FROM AUTHOR]