As carbon dioxide builds up in the atmosphere, increasing amounts of carbon are entering the world’s oceans, which is changing the chemical balance of seawater and leading to ocean acidification. Marine chemistry can be studied using four parameters: partial pressure of carbon dioxide in the water; dissolved inorganic carbon; alkalinity; potential of hydrogen (pH). Two of these parameters, along with measurements of salinity and temperature, allow us to understand the complete carbon chemistry of the ocean. Salinity and temperature can be detected from space by their effect on electromagnetic emissions from the ocean surface. ESA’s SMOS mission provides information on ocean salinity – a key piece of the puzzle.