The oceans can store large amounts of CO₂. The reservoir of inorganic carbon in the ocean is roughly 50 times that of the atmosphere (Sabine et al., 2004). Therefore even small changes in the ocean reservoir can have an impact on the atmospheric concentration of CO₂. The ocean also provides an important sink for carbon dioxide released by human activities, the anthropogenic CO₂ (Cant). Currently, an amount of CO₂ equivalent to approximately 30% of the total human emissions of CO₂ to the atmosphere is accumulating in the ocean (Mikaloff-Fletcher et al., 2006; Le Quéré et al., 2010). In this section, observations of change in the ocean uptake of carbon, the inventory of Cant, and ocean acidification are assessed, as well as changes in oxygen and nutrients. Chapter 6 provides a synthesis of the overall carbon cycle, including the ocean, atmosphere and biosphere and considering both past trends and future projections.