When sulphur and nitrogen gases from ship fuel and other local sources of combustion end up in coastal waters, they strongly diminish ocean acidification by carbon dioxide.

Keith Hunter at the University of Otago in Dunedin, New Zealand, and his colleagues used a simple chemical model to compare the effects of sulphur and nitrogen oxides and ammonia on seawater chemistry with that of CO2. Results from three case studies — in the North Sea, the Baltic Sea and the South China Sea — suggest that, after chemical re-equilibration, the net effect of the gases on ocean chemistry is negligible.

The authors conclude that in coastal regions with intense shipping activity, air–sea fluxes of acidic nitrogen and sulphur compounds effectively cancel out expected CO2-induced ocean acidification by buffering reactions involving carbonate and bicarbonate ions in seawater. However, they suspect that such chemical buffering comes at the expense of shutting down much of the ocean uptake of excess atmospheric CO2.

Geophys. Res. Lett. doi:10.1029/2011GL047720 (2011)

Research highlights, Nature 474:423. Article.