Fig. 3.

Longitude × latitude color plots of monthly mean, surface CO₂ abundance (ppm). (A and B) Data for July Year 10 from U1 (A) and G1 (B). The maximum differences (G1 − U1 in ppm) over the 3 source regions are given in B. An extrapolated estimate of the correct answer is calculated from the doubled resolution U2 and quadrupled resolution U4: Uext = 2 × U4 − U2. (C–F) Difference plots for July Year 10 of dU1 = U1 − Uext (C), dG1 = G1 − Uext (D), dU2 = U2 − Uext (E), and the doubled resolution dG2 = G2 − Uext (F). The rms, area-weighted difference (ppm) for each plot is given in brackets.