CASIX, the Centre for observation of Air-Sea Interactions and fluXes, was a NERC Centre of Excellence in Earth Observation. CASIX scientific focus was on advancing the science of air-sea interactions and reducing the errors in the prediction of climate change. The primary goal was to quantify accurately the global air-sea fluxes of carbon dioxide (CO2). CASIX accelerated the exploitation of new Earth Observation satellite data to further the understanding of marine biogeochemistry in the Earth System. CASIX links NERC Centres, university groups and the Met Office to model ocean circulation and the ocean carbon cycle.