To project the effects of increasing levels of greenhouse gases on future average global temperatures, scientists have developed complex mathematical models that simulate our current understanding of interactions among incoming sunlight, clouds, landmasses oceans, ocean currents, concentrations of greenhouse gases and air pollutants, and other factors within the earth's complex climate system. They run these continually improving models on supercomputers and compare the results to known past climate changes, from which they project future changes in the earth's average atmospheric temperature. Figure 15.A gives a greatly simplified summary of some of the key interactions in the global climate system.