Ocean acidification

Ocean acidification is an effect of rising concentrations of CO2 in the atmosphere, and is not a direct consequence of global warming. The oceans soak up much of the CO2 produced by living organisms, either as dissolved gas, or in the skeletons of tiny marine creatures that fall to the bottom to become chalk or limestone. Oceans currently absorb about one tonne of CO2 per person per year. It is estimated that the oceans have absorbed around half of all CO2 generated by human activities since 1800 (118 ± 19 petagrams of carbon from 1800 to 1994). In water, CO2 becomes a weak carbonic acid, and the increase in the greenhouse gas since the Industrial Revolution has already lowered the average pH (the laboratory measure of acidity) of seawater by 0.1 units, to 8.2. Predicted emissions could lower the pH by a further 0.5 by 2100, to a level probably not seen for hundreds of millennia and, critically, at a rate of change probably 100 times greater than at any time over this period.
There are concerns that increasing acidification could have a particularly detrimental effect on corals (16% of the world's coral reefs have died from bleaching caused by warm water in 1998, which coincidentally was the warmest year ever recorded) and other marine organisms with calcium carbonate shells.