This is wrong. Hundreds of experts from all over the world contribute to the main IPCC reports. Although the summaries are written by diplomats, scientists ensure they remain scientifically valid. More ….
Hundreds of experts from all over the world contribute to the preparation of IPCC reports as authors, contributors and reviewers. The composition of the authoring teams reflects a range of views, expertise and geographical representation. Review by governments and experts are seen as essential elements of the preparation of IPCC reports. The IPCC’s work is also supported by the worldwide scientific community. For example, in June 2005, a joint statement of support was issued by the highly respected national science academies of Brazil, Canada, China, France, Germany, India, Italy, Japan, Russia, UK and the USA.
The only document produced by the IPCC which is not written directly by scientists is the Summary for Policymakers (around 30 pages). This is reviewed by diplomats from around the world whilst the lead authors of the report ensure that any changes remain scientifically valid. However the scientists have the final word. The IPCC thereby delivers reports that have formal approval of the world's governments as well as the authority of the scientific authors.