Ocean acidification refers to the effect CO2 has on seawater as the oceans take up the gas. The world's oceans absorb roughly 25 percent of the CO2 humans release into the atmosphere each year. That CO2 reacts with seawater to form small but increasing amounts of carbonic acid.
Shellfish as canaries in the coal mine
The change is virtually imperceptible to humans, but it's a different story for some marine animals that build shells. Shell building creatures rely on an excess of dissolved calcium carbonate in the oceans, built up from millenniums of rock erosion on land. More and more of this raw material for shells is being diverted to, in effect, neutralize the acid, leaving less dissolved carbonates for shell-builders to use.
Ocean acidification could harm coral reefs by taking up mineral aragonite in seawater that the small animals use to build the elaborate structures that are home to fish and protect coasts from storm surges.
To be sure, the reaction to rising acidity isn't the same with all species. Researchers at the Woods Hole Oceanographic Institution (WHOI) subjected 18 species of shellfish to varying degrees of CO2-induced acidity in seawater in special tanks. They exposed the water to varying levels of CO2, selected from among the Intergovernmental Panel on Climate Change's emissions scenarios. Unexpectedly seven species, including lobsters, shrimp, and clams, actually built thicker shells.