The ICOS project will build an infrastructure for co-ordinated, integrated, long-term high-quality observational data of the greenhouse balance of Europe and of adjacent key regions of Siberia and Africa. Consisting of a centre for co-ordination, calibration and data in conjunction with networks of atmospheric and ecosystem observations, ICOS is designed to create the scientific backbone for a better understanding and quantification of greenhouse gas sources and sinks and their feedback with climate change. The overarching objectives of ICOS are: To provide the long-term observations required to improve understanding of the present state and future behavior of the global carbon cycle and greenhouse gas emissions, and the factors that control the changing atmospheric composition in greenhouse gases. To monitor and assess the effectiveness of carbon sequestration and/or greenhouse gases emission reduction activities on global atmospheric composition levels, including attribution of sources and sinks by region and sector at atmospheric and ecosystem level. These objectives will be achieved by: Establishing a central facility, the ICOS-centre, which is responsible for co-ordination, calibration and data handling. Maintaining a co-ordinated, integrated, long-term high-quality network of atmospheric and ecosystem observations. Improving access to existing and future atmospheric and ecosystem data for research, and forpolitical decisions. Improving access to state-of-the-art facilities for ecosystem measurements for the European research community. Providing European terrestrial ground-truth data for the validation of emerging remotely sensed datasets on atmospheric composition and land cover as provided e.g. by GMES. Contributing the European share to a global greenhouse gas observation network under IGCO and UNFCCC. (Total characters: 1843)