Atmospheric general circulation model
The climate models that GISS has developed and operates include several atmospheric general circulation models (AGCMs). Such computer models numerically solve fundamental equations describing the conservation of mass, energy, momentum, etc. 
- A general circulation model (also known as a global climate model, both labels are abbreviated as GCM) uses the same equations of motion as a numerical weather prediction (NWP) model, but the purpose is to numerically simulate changes in climate as a result of slow changes in some boundary conditions (such as the solar constant)or physical parameters (such as the greenhouse gas concentration). Numerical weather prediction (NWP) models are used to predict the weather in the short (1-3 days) and medium (4-10 days) range future. GCM's are run much longer, for years on end, long enough to learn about the climate in a statistical sense (i.e. the means and variability). A good NWP model accurately predicts the movement and evolution of disturbances such as frontal systems and tropical cyclones. A GCM should do this as well, but all types of models err so much after some time (e.g. 2 weeks), that they become useless from a perspective of weather foresight. The quality of a GCM is judged, amongst others, by the quality of the statistics of tropical or extratropical disturbances.