Ocean acidification occurs when the ocean takes up extra CO2 from the atmosphere. When CO2 dissolves in water it increases the amount of hydrogen ions, when then decreased the pH levels. Since the industrial revolution, the ocean's pH level has lowered by 0.1 [Source: Yale]. While this might not seem like a huge drop, the truth is, the pH scale is logarithmic, each step up or down increases the amount of hydrogen ions tenfold. This roughly translates to a 30% decreased in pH levels in the ocean. [Source: Ocean Acidification]. with half of the anthropogenic CO2 stored in the upper 10% of the oceans [Source: NOAA]. Increase in acidity is particularly harmful to organisms that require calcium bicarbonate to build shells. If the water is too acidic, it can dissolve the shells, sometimes faster than organisms can rebuild them. The impact of this change has been seen in the US northwest, most notably Washington, where there has been a significant decrease in oysters along the coast. Other areas that would be highly sensitive to changing pH are the continental shelves since many marine organisms either live or spawn there. In 2008 a University of Chicago report stated that these changes were happening much faster than predicted and that "This increase will have a severe impact on marine food webs and suggests that ocean acidification may be a more urgent issue than previously thought..." [Source: Science Daily]
The growing urgency to halt the change in pH has forced the EPA to step in and "consider ways the states can address rising acidity levels in oceans" [Source: Seattle P.I.] in accordance with the Clean Water Act. This was brought on by a lawsuit from the Center for Biological Diversity, who wanted the EPA to address the acidity level of Washington's ocean. Many view this victory as another way for the federal government to put restrictions on greenhouse gases that also affect the oceans (only CO2 increases the acidity level of the oceans [Source: Treehugger]). There are, of course, organizations that argue that the increase acidity could be beneficial. While mollusks are growing thinner shells, other crustaceans (namely crab, lobster and shrimp) are forming thicker shells. This might sound beneficial, scientists from the Woods Hole Oceanographic Institution, state that "...any possible ramifications are complex. For example, the crab exhibited improved shell-building capacity, and its prey, the clams, showed reduced calcification. 'This may initially suggest that crabs could benefit from this shift in predator-pray dynamics. But without shells, clams may not be able to sustain their populations, and this could ultimately impact crabs in a negative way, as well..." [Source: WHOI].
It is difficult to say what exactly the long-reaching effects will have on all of the life forms in the oceans, however, it has become serious enough that the EPA, NOAA and various other organizations have begun taking action against ocean acidification.