SE has an excellent post about Validating Climate Models. It is all good, but I particularly liked
when we ask climate scientists for future projections, we’re asking the question of the scientists, not of their models. The scientists will apply their judgement to select appropriate versions/configurations of the models to use, they will set up the runs, and they will interpret the results in the light of what is known about the models’ strengths and weaknesses and about any gaps between the comptuational models and the current theoretical understanding. And they will add all sorts of caveats to the conclusions they draw from the model runs when they present their results.