Even if one has a billion dollar computer, the predictions of the model depend on the assumptions of the model. In computer science it was at one time often said that one could have "garbage in, garbage out." If a model does not take into account the sun, or clouds, or cosmic rays, or changing orbits,or is incomplete in some other way, and only carbon dioxide matters, how confident can one be about the predictions? Maybe it is just wishful thinking. Scientists can do that too, sometimes. When it comes to global warming it appears that many scientists have lost their skepticism, which I always tell my students is essential in science,and physics in particular.