A substantial proportion of the carbon dioxide emitted into the atmosphere by human activities is sequestered in the oceans. Unfortunately, this sink is not without side effects: on entering the ocean, the carbon dioxide reacts with sea water to produce carbonic acid, decreasing the pH of the oceans and potentially threatening the stability of marine ecosystems.
Despite the severity of the implications — the livelihoods of large numbers of people depend on protein from the marine realm — empirical data documenting changes in ocean acidity over time are lacking, and current understanding relies heavily on model projections and laboratory studies. Temperate latitudes, where many of the world's most productive fisheries reside, are particularly under-sampled. J. Timothy Wootton, of the University of Chicago (Proc. Natl Acad. Sci. USA 105, 18848–18853; 2008), and colleagues offer a glimpse from the coastal North Pacific Ocean. Using a high-resolution eight-year record they suggest that the rate of acidification at the sea surface may be greater than current models predict.
The team collected more than 24,000 measurements of surface water pH between 2000 and 2007 at the uninhabited Tatoosh Island, off the coast of Washington State, United States. When they amalgamated the data, they found a decrease in ocean pH over the eight-year period. As expected, this was associated with a rise in atmospheric carbon dioxide concentrations. However, what they hadn't foreseen was the relatively fast rate of pH decline at this site.
The observed consequences for the ecosystem were complex: although some calcareous species such as the California mussel were hit hard, others, such as acorn barnacles, benefited from a lack of competition and increased in abundance.
The unexpected rate of change combined with the complexity of the ecosystem response highlights the need for ocean-based studies. The range of conditions and diversity of species found in the real world simply cannot be replicated in a laboratory.