Fig. 3 Results from a coupled climate-economy model that was originally developed by economists at Yale University. The upper figure (a) shows, year by year, the reduction in CO₂ emissions that the model prescribes for optimum economic benefit. The lower figure (b) gives the resulting atmospheric concentrations of CO₂. Results are shown for two different time preference discount rates: 3%/yr in black, and 0 in blue. As in Figure 2, dashed and solid lines distinguish the carbon cycle model that was employed: results from the more rudimentary model in dashed lines, and from a more advanced model as solid lines.