Underwater sound is a critical variable for measuring changes in physical factors such as wind, waves, rain. and sea ice that are associated with climate change, with biological measures of ocean health, and monitoring threats such as earthquakes and military activities. These applications are so important as to call for a considerable investment in sustained global ocean sound observation networks. The closest we have to a mature system is the hydrophone network of the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO). Continuous recordings from this system make it possible to analyze diurnal, seasonal, and long-term variation in ocean sound. Analysis of CTBTO data from 2003–2021 helps to address concerns that human activities are increasing global ocean sound; this century has actually seen significant downward trends on sound pressure level, with the major sources of sound energy varying between sites. Causes of the decline at different sites include changes in global economic activity (e.g., COVID-19 and oil exploration), changes in earthquake activity, and increases in sea surface temperature, which reduces propagation of sound from near-surface sources. We hope that these examples will help stimulate ocean acoustics communities to work for the further development and maturation of global ocean sound observing systems.
Tyack et al. (2025) studied this question.