Climate Models, Calibration, and Confirmation

We argue that concerns about double-counting—using the same evidence both to cali-brate or tune climate models and also to confirm or verify that the models are adequate— deserve more careful scrutiny in climate modelling circles. It is widely held that double-counting is bad and that separate data must be used for calibration and confirm-ation. We show that this is far from obviously true, and that climate scientists may be confusing their targets. Our analysis turns on a Bayesian/relative-likelihood approach to incremental confirmation. According to this approach, double-counting is entirely proper. We go on to discuss plausible difficulties with calibrating climate models, and we distinguish more and less ambitious notions of confirmation. Strong claims of con-firmation may not, in many cases, be warranted, but it would be a mistake to regar