The carbon cycle is a very complex and delicate system of large and small scale interactions between multiple carbon reservoirs, the largest of which is the oceans. Many hope that oceans will absorb all anthropogenic carbon dioxide, and that this will slow down the progression of climate change. While climate change may be slowed by the ocean, we have recently learned (see Key Idea 3) about how increases in atmospheric CO₂ are decreasing the pH of the oceans.

Ocean acidification is a very complex, multi-faceted topic. Gigatonnes of carbon flow in and out of the ocean each year, due to equilibrium and mixing processes. Oceanic carbon mainly exists as dissolved CO₂ or as a variation of carbonate. As the pH is shifting, so too is the distribution of carbon species: carbonate ions (CO₃^2-) are shifting to hydrogencarbonate ions (HCO₃^-).