Better software engineering such as archiving releases with version control, writing portable code, publishing documentation and results closely tied to the code improves integrated assessment models' transparency and control. A case study of four climate change policy analysis models found that source code and data was generally available, but for larger models licenses were more restrictive with respect to modification and redistribution. It is suggested that Free software licenses such as the GNU GPL would improve transparency and control. Moreover, opening the source allows opening the development process, a potentially important tool to improve collaboration, data sharing and models integration.