The Global Climate Observing System (GCOS) was established in 1992 as an outcome of the Second World Climate Conference, to ensure that the observations and information needed to address climate-related issues are obtained and made available to all potential users. The GCOS is co-sponsored by the World Meteorological Organization (WMO), the Intergovernmental Oceanographic Commission (IOC) of UNESCO, the United Nations Environment Programme (UNEP), and the International Council for Science (ICSU). In order to assess and monitor the adequacy of in-situ observation networks as well as satellite-based observing systems, GCOS regularly reports on the adequacy of the current climate observing system to the United Nations Framework Convention on Climate Change (UNFCCC), and thereby identifies the needs of the current climate observing system. 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. Structure The primary observing systems contributing to the GCOS are the WMO Integrated Global Observing System (WIGOS), the Global Cryosphere Watch (GCW), and the World Hydrological Cycle Observing System (WHYCOS), and the Intergovernmental Oceanographic Commission-led Global Ocean Observing System (GOOS). A number of other domain-based and cross-domain research and operational observing systems also provide important contributions and encompass both in-situ and satellite observations. GCOS is both supported by and supports the international scientific and technical community, and the World Climate Research Programme (WCRP) co-sponsors the expert panels set up by GCOS for the atmospheric, oceanic and terrestrial domains. The composite observing system designated by GCOS serves as the climate-observation component of the broader Global Earth Observation System of Systems (GEOSS), and at the same time a number of specific observing-system initiatives of GEOSS contribute to the GCOS. Essential Climate Variables (ECVs) GCOS has identified 50 Essential Climate Variables (ECVs) considered to be feasible for global climate observation and to have a high impact on the requirements of the UNFCCC and other stakeholders. There is a strong need for sustained observation of these ECVs, as the