In this activity, learners explore the factors that tend to resist changes in pH of the ocean and why the ocean is becoming more acidic. This lesson guides inquiry into some properties of the ocean’s carbonate buffer system, and how changes in atmospheric carbon dioxide levels may affect ocean pH and biological organisms that depend upon calcification. This lesson was first created for the 2008 Deepwater Coral Expedition: Reefs, Rigs, and Wrecks, but has been updated in 2009 to include recently published information.