Increasingly Acidic Oceans Will Be Noisier

As ocean pH drops due to human activities, low- and mid-frequency sounds will travel farther in water, report researchers from the Monterey Bay Aquarium Research Institute (MBARI), in Moss Landing, Calif. (Geophys. Res. Lett. 2008, 35, L19601). How the noisier seas will affect marine mammals, many of which depend on low-frequency sonar, remains to be seen. It may turn out that the mammals will be able to communicate over longer distances, although they would have more background noise to deal with, the researchers say. Increased levels of human-generated carbon dioxide are the main reason that oceans are acidifying. The United Nations Intergovernmental Panel on Climate Change suggests that ocean pH, approximately 8.1 in surface waters, will fall 0.3 units by 2050. MBARI's Keith C. Hester, Peter G. Brewer, and colleagues estimate that this level of acidification could make sound travel up to 70% farther in some ocean regions. Ocean chemists have known for decades that acidic seawater absorbs less sound, but no one anticipated that the impact could be so significant. The mechanism that dampens molecular vibrations and propels sound through the seas remains unknown, but it stems from complex equilibria involving boric acid and carbonic acid, with possible influence by calcium ions.