The Ocean in a High-CO2 World: Science highlights from the symposium
Thirty years ago, scientists trying to determine the absorption of low frequency sound in seawater in order to develop new navy sonar systems, discovered, somewhat surprisingly, that this absorption is pH dependent: the lower the pH the less the absorption. Today this discovery has an implication for ocean acidification: as the ocean acidifies it will become noisier! Lower absorption (the pH change predicted in a recent Royal Society Report  would cut the absorption in half) will result in a smaller propagation loss which means that at a given distance from a noise source (such as a ship’s propeller) the sound level will be louder than it previously was. It is presently the subject of legal contention whether noise levels can cause significant distress to marine mammals but if there is a problem ocean acidification could make it worse. As with other predicted effects, the absorption change would have the greatest impact soonest in specific situations. For example underwater sound typically propagates along the axis of a naturally occurring sound channel – in many areas this axis is over 1,000 meters deep which means that it would take a significant time for pH change to work down the water column. In some locations such as the North Pacific Ocean, however, a shallow secondary sound channel exists where the impact should be observed sooner.
For a more detailed explanation of the ocean chemistry see: Hester K. C., E. T. Peltzer, W. J. Kirkwood, P. G. Brewer. “Unanticipated consequences of ocean acidification: A noisier ocean at lower pH”, Geophysical Research Letters, 35: L19601 (1 October 2008).