As the level of carbon dioxide in the atmosphere increases, so does the amount of CO2 absorbed by the oceans. Scientists recognize that the resulting increase in seawater acidity can affect corals and other marine organisms.
Now University of Hawaiʻi at Mānoa researchers suggest that ocean acidification also affects acoustical properties of seawater, making it more transparent to low-frequency sound.
Writing in January 2010 issue of Nature Geoscience, School of Ocean and Earth Science Oceanographers Tatiana Ilyina and Richard Zeebe and a mainland colleague report a .1 decline in surface seawater pH level compared to pre-industrial levels.
At current rates of CO2 emissions, pH levels will drop another .6 units by the end of the century. Because pH is measured on a logarithmic scale, each drop of one unit implies a 10-fold increase in acidity, which in turn affects the dissolved chemicals involved in sound absorption.
The projected acidification means the absorption of low frequency sound—such as that generated by rain, waves, marine mammals, construction, shipping and use of sonar systems—will decrease by up to 70 percent, the scientists predict.
Like turning up the bass on your stereo, low frequencies will become louder at distance, says Zeebe. That could affect human activities, such as naval, commercial and scientific applications that use low-frequency sound for to its long-range propagation, as well as marine mammals, which rely on low-frequency sound to find food and mates.
“We don’’t fully understand what the impacts of these changes in ocean acoustics will be,” says Ilyina. Noise levels could increase, perhaps creating acoustic hot spots in areas where noise sources and sound absorption reduction are greatest. On the other hand, increasing transparency of the oceans to low-frequency sounds could enable marine mammals to communicate over longer distances.
The scientists say that further research is needed to address these questions.