Models can be helpful took. If a process can be described mathematically then one has confidence in extrapolating the variables that would either be time consuming or too difficult to explore. However; when a time variable involving the future is included, projections tend to be unreliable.Steelmakers are familiar with the numerous models for scrap pricing. All were historically accurate, but none ever came close to predicting future scrap prices. Some unanticipated factor always ruined the projections. Therefore, to develop a model of the earth's average surface temperature 100 years from now seems ambitious. Most of us would settle for an accurate local weather forecast for next weekend. Yet hundreds of global modelers around the world are trying to develop enormously complex heat-mass transfer models to explain the climatic variations of our planet. Supercomputers are needed to crunch the staggering amount of input data in a reasonable time frame. Worldwide, more than 30 General Circulation Models (GCMs) now exist. They were created to rationalize the change in the average global surface temperature over the last few centuries, and on this basis, to predict future changes. Major variables include greenhouse gas concentrations (CO_2, CH_4), cloud cover (water vapor is the dominant "greenhouse" gas), ocean currents and aerosols.
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