Answer: It is very strong.
Explanation:
The credibility of a research subject increases based on the number of experts and professional who review the research and confirm its results and credibility.
In this instance, the IPCC integrated research from many scientists and experts from many different countries to come up with the Model so this model must have very strong credibility because it was corroborated by so many experts in the field and it is their jobs and life's work to know these things in the first place.