Ocean Acidity Is Rising And It's Turning The Ocean Into One Big Underwater Amplifier
Seawater is changing, and so is the way sound moves through it. As carbon dioxide dissolves into the ocean, pH drops. At low frequencies, that chemical shift reduces how strongly seawater absorbs sound, letting some noises travel farther than before.
Discovery of Sound in the Sea explains that borate ions govern much of this absorption below a few kilohertz—exactly where many whales, winds, and ships speak.
Ocean acidification lowers seawater pH.
The Chemistry That Weakens Sound
Magnesium sulfate helps soak up higher-pitched sound, but its effect doesn’t depend on pH. Borate does. As acidity rises (pH falls), borate’s grip on sound loosens. Modeling summarized by Discovery of Sound in the Sea shows that a ~0.45 unit pH drop could cut absorption by about half below ~1 kHz—so a low-frequency signal would need roughly twice the distance to lose the same energy to absorption.
Will the Ocean Get Much Noisier?
Not everywhere, and not by much—at least relative to the natural ups and downs of ocean noise. Sound levels depend on more than absorption: waves and ships add energy; spreading, reflection, refraction, and scattering bleed it away. Using realistic future pH paths, studies estimate deep-ocean increases under 0.5 dB across 50–2,000 Hz and no observable change in many shallow settings—small compared with everyday variability.
Lower pH weakens low-frequency sound absorption.
Listening to Measure Acidification
A new twist flips the problem into a tool. In the wind-noise band (1–10 kHz), autonomous “Deep Sound” instruments profile how ambient surface noise fades with depth. Because pH subtly shapes absorption below ~3 kHz, the depth-dependence of that fade can be matched to an analytical model to estimate a volume-averaged pH. Reporting on work in the Philippine Sea, the Mariana Trench, and the Tonga Trench, Phys.org notes decade-scale deployments and pH estimates from about 7.74 to 8.18—plus caveats about wind variability, competing noise sources, and model uncertainty.
A companion summary from the ocean-acidification community highlights how two depth-dependent models—one linear for the upper ocean and one nonlinear for the abyss—can turn those spectral slopes into long-term, kilometer-scale acidity monitoring, Ocean Acidification (OA-ICC) reports.
Borate chemistry controls much of this absorption below a few kHz.
The Takeaway for Marine Life
Low-frequency hearing ranges overlap with the band most sensitive to pH. Even modest changes in range or clarity can matter for species that navigate, forage, and find mates by sound. Early research flagged this possibility years ago; laboratory and field analyses published in Geophysical Research Letters tied the absorption shift to known chemical relaxations in seawater. Today, the same physics is helping scientists eavesdrop on a changing ocean—quietly turning noise into data.
