model used in climate assessment is the three-dimensional general circulation model.6$ This type of model accounts for both horizontal and vertical motions and includes snow cover, rainfall, and the vertical lapse rate of temperatures as dependent variables. Part of the value of this last type of model lies in the fact that it can predict climate change for specific geographic regions of the earth, not merely for the whole globe or for latitude belts. Table 2.8 compares the predictions of global surfac-e temperature of several different models. Models are generally compared by examining their response to a doubling of atmospheric CO2 levels, which is particularly appropriate because a doubling of CO2 over preindustrial levels is possible by the early 21st century. The models listed in Table 2.8 tend to show a global surface temperature increase of 1° to 3°C for a doubling of CO2 level. The Table 2.8. Predictions by Different Models of Surface Temperature Response to a Doubling of CO2
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