Ocean soundscapes and trends from 2003 to 2021: 10-100 Hz

We analyze ocean ambient sound from a global network of hydrophones installed and maintained by the Comprehensive Nuclear-Test-Ban Treaty Organization. We process acoustic data from nine hydrophones across six hydroacoustic stations distributed across five oceans: North Pacific, South Pacific, South Atlantic, Indian, and Southern oceans, for up to 19 consecutive years between 2003 and 2021. We identify dominant natural and anthropogenic sources for all six stations and observe long-term trends at four of them. Out of 20 statistical tests, 15 identified a significant downward trend in sound pressure level. Possible causes of these decreasing trends include a global recession in 2016, the COVID-19 pandemic in 2020, five major earthquakes between 2004 and 2012 (with none in 2013-2021), and a steadily increasing sea surface temperature, which decreases the sea surface critical angle and hence the contribution from near-surface sources (e.g., shipping) to sound pressure level.