The most realistic models should run indefinitely without producing infeasible conditions, but a great deal more development is needed before they will provide useful predictions over long periods. Besides those parameters actually being investigated, it is instructive to check values of other parameters generated over time by the model for comparisons with real values. In general circulation models feedback loops are very important. This is the way in which variation of a parameter, due to, say, a change in vegetation, can affect the behaviour of the atmospheric model. The two types of feedback loop, positive and negative, are important since they affect the stability of the system.
Positive feedback tends to reinforce the initial process.
This leads to even greater effects and so destabilizes or even destroys the system. Negative feedback opposes the initial process, tending to damp down its effects and so stabilizes the system. With so complex a system as the global circulation of the atmosphere the numerous positive and negative feedback loops may be expected generally to balance each other. Changes in surface vegetation alter several surface parameters and so affect feedback loops. Important examples of land surface parameters are the surface albedo or reflectivity, which determines the fraction of solar radiation absorbed, and the surface moisture availability, which affects the partitioning of energy between thermal and latent heat.