GCOS is a system that comprises the climate-relevant components of many contributing observing systems and networks. Its mission is to help ensure that these contributing systems, taken as a whole, provide the comprehensive information on the global climate system that is required by users, including individuals, national and international organizations, institutions and agencies. The programme promotes the sustained provision and availability of reliable physical, chemical and biological observations and data records for the total climate system - across the atmospheric, oceanic and terrestrial domains, including the hydrological cycle, the carbon cycle and the cryosphere.