The Southern Ocean plays an important role in the exchange of heat, momentum and gases between the deep ocean and the atmosphere. Deep water comes to the surface south of the Polar Front, before downwelling further north as Antarctic Intermediate Water (AAIW) and Subantarctic Mode Water (SAMW) and close to Antarctica as Antarctic Bottom Water. Sabine etal. (2004) have estimated that AAIW and SAMW had stored 20 Pg C of anthropogenic carbon by 1994 or 5% of the carbon emitted by human activities. Uptake of anthropogenic carbon by AABW is less well understood. Key processes affecting the Southern Ocean CO2 (carbon dioxide) sink are discussed below, notably iron limitation of phytoplankton growth and the role of sea ice. Low iron concentrations limit phytoplankton growth in large parts of the Southern Ocean. Downstream of islands and steep topography iron supply from sediments fuels strong seasonal phytoplankton blooms, thus creating important CO2 sinks. Wintertime upwelling and entrainment of Circumpolar Deep Water promote high CO2 levels below Weddell Gyre sea ice, with the ice cover preventing outgassing. Strong biological carbon uptake and possibly calcium carbonate dissolution rapidly reduce surface water CO2 levels during and upon ice melt, thus creating a sizeable summertime CO2 sink. While process studies are strongly improving our understanding of total CO2 uptake, separating the natural and the anthropogenic CO2 sinks remains difficult.