Rapidly changing ocean pH and the increasing transparency of the ocean to sound.

Abstract

The intrinsic sound absorption coefficient (á, dB/km) of seawater is pH dependent with significant effects at 10 kHz and below. Ocean pH is declining from fossil fuel CO2 invasion, from excess nutrient input, and from climate change reducing ocean ventilation. These effects produce reductions in ocean borate and carbonate species such that an ∼18% decrease in á in the upper ocean has occurred today. Reasonable projections based on IPCC scenarios predict changes of 40% or more by mid-century [Hester et al., Geophys. Res. Lett. 35, L19601 (2008)]; larger changes in the sound channel are very likely. The projected increased transparency of the ocean to sound has attracted strong international environmental attention, and also provides an opportunity for acoustic detection and monitoring of such changes over large ocean regions. How strong is the basis for such assertions, can they be tested, and how well can environmental effects be predicted? This tutorial shows that the rate of change in pH in the sound channel has been underestimated, and how the unusual 1970s era reliance on the Soviet Gorshkov atlas as a pH data resource came about. This now presents a challenge for convergence of modern acoustic, environmental, and global change needs.