Without the oceans and their vast ability to absorb carbon dioxide, Earth would be warming up much faster than it currently is. The seas take up about 9 billion tonnes of the gas each year — almost one-third of the 30 billion tonnes emitted globally.
Once it enters the ocean, CO2 reacts with water to produce carbonic acid, which releases positively charged hydrogen ions. Acidity is measured in pH, a logarithmic scale on which low numbers mean high acidity; neutral water has a pH of 7, but sea water is naturally alkaline, owing to the salts dissolved in it. Since the mid-nineteenth century, the average pH of ocean surface waters has dropped by 0.1 units, to a current value of about 8.1. Unless nations sharply curb their emissions, atmospheric CO2 is expected to at least double from its preindustrial concentration by sometime in the second half of this century, and scientists project that ocean pH will fall by a further 0.3–0.4 or so units. Sea water could then contain at least 150% more hydrogen ions than it did at the onset of the industrial era.
Quirin Schiermeier, Nature 471, 154-156 (2011) | doi:10.1038/471154a