An excellent article on how climate change models work and how much trust we ought to put in them: not much.

http://www.americanthinker.com/2007/...grated_ci.html

Numerical Models, Integrated Circuits and Global Warming Theory
By Jerome J. Schmitt
Global warming theory is a prediction based on complex mathematical models developed to explain the dynamics of the atmosphere. These models must account for a myriad of factors, and the resultant equations are so complex they cannot be solved explicitly or "analytically" but rather their solutions must be approximated "numerically" with computers. The mathematics of global warming should not be compared with the explicit calculus used, for example, by Edmund Halley to calculate the orbit of his eponymous comet and predict its return 76 years later.

Although based on scientific "first principles", complex numerical models inevitably require simplifications, judgment calls, and correction factors. These subjective measures may be entirely acceptable so long as the model matches the available data -- acceptable because the model is not intended to be internally consistent with all the laws of physics and chemistry, but rather to serve as an expedient means to anticipate behavior of the system in the future. However, problems can arise when R&D funding mechanisms inevitably "reward" exaggerated and alarming claims for the accuracy and implications of these models.