Fig. 2 Upper figure (a): Two hypothetical tracks of year-to-year CO₂ emission in Gt C/yr, each of which depletes the entire fossil fuel inventory of 5000 Gt C. Lower figure (b): the projected atmospheric concentrations of CO₂ in parts per million that would result in each case, when two different carbon cycle models are used. The dashed curves in the lower figure are based on a rudimentary carbon cycle model that assumes a fixed 120-year lifetime for anthropogenic CO₂ and a constant airborne fraction of 0.64. The solid curves employ an improved model with a more realistic representation of ocean processes. Blue and black colors in the lower figure identify the emission track in the upper figure on which each modeled calculation was based.