I think there is a reasonable argument somewhere between “there is no human contribution to climate change” and “you should “trust” the IPCC because they are operating on the best understanding available”.

IPCC may be operating on the best available information and still make predictions that will ultimately turn out to be inaccurate is the fact of the matter. We understand a lot but likely still have much to learn. The data is too noisy to make near term predictions using our current climate models such that accuracy can’t be assessed based on actual near term climate change. These models are then used to predict 50 years in the future and it is uncertain how accurate they are.

Trust is a bad word to use and has no place in a discussion of science (see Argument from Authority). The best one can do is try to understand the Scientific Method and how it is being applied in the study of climate as it currently stands to get a sense for how the study will be improved over time (it will eventually lead to models with near term prediction capability)