On 12 April 2014 the governments will have approved and adopted the last of three contributions to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. This marks the completion of the scientific assessment − a five year effort of thousands of scientists worldwide. In this Union Session each of the three IPCC Working Groups will provide concise overviews of the most relevant findings. The current knowledge on the extent, causes, impacts, and options for mitigating human-caused climate change will be highlighted from the perspectives of the physical, ecological, human, and economic sciences. The specific challenges as well as the benefits such a comprehensive scientific assessments represent to the scientific communities will be discussed. This will inform the process of defining and shaping future scientific assessments.