text: Too, we dont have accurate actual measurements of all locations, levels, and components (like temperature, moisture, wind speed, and so on) of the atmosphere, land, ocean (and outer space). The number of measurements we do have is minuscule compared to the actual size of the earth. So it is hard to reconcile the output of models with what the actual state of the earth is, though, of course, this is a necessary step. The process whereby the models are adjusted so that they conform to both the actual measurements and to the scientists expectations is called tuning . All models undergo extensive tuning until the majority of it users are satisfied of the output.