Freeman Dyson's opinion is one of skepticism, not denial. Unfortunately I don't think he's deeped in the science or the messiness of climate modeling, so I think he's just not an expert here. Bright, even eminent scientists can be wrong, even in their areas of expertise. For example, Ernst Mach was famous for not believing atoms exist, even declaring it loudly after having heard a lecture by Boltzmann. Einstein firmly opposed the nondeterminism underlying quantum mechanics, refusing to believe that physics is not deterministic (this is the source of his famous "God does not play dice" quote.)