A well-known approach to studying the impact of greenhouse gases on climate is computer-simulated general circulation models (GCMs) that integrate current understanding of the various factors influencing climate through a comprehensive mathematical model. In such models, for example, one can input hypothetical CO2 levels for the next 100 years and examine their effect on temperature. There is much current debate over the degree of validity that can be attributed to the output from a GCM since our understanding of the input factors that affect climate is far from perfect.