It is classified as conceptual, graphical, physical, or numerical. A conceptual model describes the general relationships among components of a system. A graphical model assembles and displays the data in an organized format that can be readily interpreted. A physical model is a mini version of a real system. A numerical model consists of one or more mathematical equations that approximate the behavior of a system.
Models are only simulations of reality and subject to error.
For example, numerical models concerns the accuracy of their component equations. In meteorology, this is a major problem in numerical model based long range weather forecasting as the accuracy of the forecast declines with longer forecasting period. The earth's atmosphere today is the result of a longtime evaluation process that began about 4.6 billion years ago. The decline in the atmospheric CO2 concentration was evident by about 4 billion years ago as the planet cooled enough to condensate the water vapor into clouds and rainfall. The significance of an atmospheric gas or aerosol is not necessarily related to its relative concentration.