Future climate can be realistically simulated by computer models of the Earth-atmosphere system. Using known starting conditions, and mathematical approximations to physical processes, models can calculate the present and future response of climate to "forcing" such as changes in the amount of atmospheric carbon dioxide.
Model calculations are performed at "timesteps", say every 5 minutes of simulated time, which take only a few seconds of actual time to run on a fast computer. At the end of each simulated day, the model saves data, such as wind speed, air temperature etc., to hard disk. A typical run for a global climate model may be for 200-300 simulated years and can take several months to complete.
The stylised depiction of a climate model shown below illustrates these features, and summarises the type of physical processes which a model might include. The model has a fully-interactive atmosphere and ocean to emulate the complex flows of energy across the air/sea interface. Such a model would typically have columns of boxes covering the whole globe, not just Western Europe as shown.