Global Climate Models

Global Climate Models (also called general circulation models) or GCMs provide quantitative estimates of potential climate change by "modelling" the physical climate system. The intent is to deduce climate and climate change from "first principles" such as Newtons laws applied to a fluid, the equations of thermodynamics, the conservation of salt in the ocean and water substance in the atmosphere, and so on. Mathematical equations representing these physical laws are solved in numerical form on a three-dimensional grid of points over the globe encompassing the atmosphere, ocean, and land surface by stepping forward in time. New values of winds, temperatures, clouds, precipitation, ocean currents and many other climate variables are calculated at each grid point every 20 minutes of simulated time (with results stored every 6 to 12 hours). The averages (and other statistics) of these quantities give the three dimensional climate simulated by the model. The calculations are then redone with changing greenhouse gas concentrations and aerosol loadings in the atmosphere in order to simulate potential climate change. A typical climate simulation for the period 1900 to 2100 demonstrates the model's ability to simulate the climate change of the past century and provides projected changes to the end of the current century.