Global warming is far and away the symptom at the top of the list of indicators that our planet is overloaded with carbon dioxide. Another important, but less considered consequence of the excess CO2 is the effect it has on the world's oceans. The oceans are a natural carbon dioxide sponge, responsible for maintaining the balance of CO2 in the atmosphere by absorbing a measure of the gas in its water. Currently, it is estimated that the ocean is uptaking nearly one-third of all human-produced CO2, which is slowly lowering its overall pH. Put simply: the oceans are becoming acidic. That's bad news for a lot of organisms, but those that produce shells are likely to be the first and hardest hit.