The International Panel on Climate Changes (IPCC), which was created in 1988, works at the interface between scientific research and political decision-makers. Its role is to provide the latter with clear and reliable scientific information and to assess in a transparent and non-prescriptive manner what is relevant for these decision-makers in climatic science. The first three IPCC assessment reports (1990, 1995, 2001) have strongly influenced the decisions made during the Earth Summit in Rio (1992) and the Framework Convention on Climatic Change that ensued, then the draft Kyoto Protocol in 1997 and its enforcement in 2005. The IPCC’s fourth report, published in 2007, confirms and reinforces the conclusions made in previous reports. An intense effort for international cooperation is developing for the enforcement of a climate observation world system, an essential element for the Global Earth Observation System of Systems (GEOSS) and necessary point of support of climate changes mitigation and impact adaptation policies.