Confusing climate models with reality

Here.

Excerpt:

Climate models are sophisticated computer programs that incorporate as many details about the complex workings of the environment as possible. Hundreds of dynamic processes, such as ocean currents, cloud formations, vegetation cover and the increase in atmospheric greenhouse gases, are included. The models produce data of the net effects for square-shaped plots over the Earth's surface. The smaller these squares are, the better the resolution the model can provide.

A model must factor in so many changing variables that a full analysis can require months of nonstop computational effort. The Rosen Center for Advanced Computing on Purdue's campus provided the powerful computing required for this study.

So if you don't fully understand a variable, or if you don't fully understand how all the variables interact, or if your input data is flawed, etc etc, you've got problems.