Ocean noise level change in response to ocean acidification.

Abstract

It has been known for several decades that reduction in ocean pH (increase in acidity) would have an impact on ocean sound content and propagation loss. Net flux of carbon dioxide into the ocean has already reduced pH in the upper ocean, and as a result has reduced absorption in those same waters. This trend will likely continue. Based on forecasts of oceanic pH profiles and sound sources, changes in ocean sound content (ambient noise) can be computed. In general, sound energy loss can be partitioned into intrinsic dissipation (the chemical relaxation absorption process) and dissipation in the seafloor, with the boundary loss being analogous to sub-unity reflectivity at the boundaries of a room or concert hall. Here, expected deep ocean noise level changes in various frequency bands from 0.1 to 10 kHz are presented. A model is used that includes basic propagation physics (including the sound channel), depth variability of absorption, loss of sound energy into seafloor, and distributed acoustic sources. At low frequency, little sound is absorbed, so absorption effects are muted. At higher frequencies, sound does not propagate far, also minimizing absorption effects. In the center of the band, there can be sizable effects.